JXB Advance Access originally published online on June 19, 2007
Journal of Experimental Botany 2007 58(9):2329-2338; doi:10.1093/jxb/erm114
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RESEARCH PAPER |
Signalling mechanisms underlying the morphological responses of the root system to nitrogen in Arabidopsis thaliana
Institute of Integrative and Comparative Biology, LC Miau Building, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK
* To whom correspondence should be addressed. E-mail: bgyhz{at}leeds.ac.uk
Received 23 February 2007; Revised 13 April 2007 Accepted 16 April 2007
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Abstract |
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 Top Abstract IntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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Plants display considerable developmental plasticity in response to changing environmental conditions. The adaptations of the root system to variations in N supply are an excellent example of such developmental plasticity. In Arabidopsis, four morphological adaptations to the N supply have been characterized: (i) a localized stimulatory effect of external nitrate on lateral root elongation; (ii) a systemic inhibitory effect of high tissue nitrate concentrations on the activation of lateral root meristems; (iii) a suppression of lateral root initiation by high C:N ratios, and (iv) an inhibition of primary root growth and stimulation of root branching by external L-glutamate. These responses have provided valuable experimental systems for the study of N signalling in plants. This article will highlight some recent progress made in this direction from studies using the Arabidopsis root system. One recent development of note has been the emerging evidence of a regulatory role of nitrate transporters in some of the responses. It has been reported that the AtNRT1.1 (CHL1) dual-affinity nitrate transporter acts upstream of the ANR1 MADS box gene in mediating the stimulatory effect of a localized nitrate supply on lateral root proliferation. The AtNRT2.1 high-affinity nitrate transporter seems to be involved in the repression of lateral root initiation by high C:N ratios. The systemic inhibitory effect of high nitrate supply on lateral root development, which is mediated by abscisic acid (ABA), may be linked to the recently identified ABA receptor, FCA. The newly discovered root architectural response to external L-glutamate potentially offers a valuable experimental tool for studying the biological function of plant glutamate receptors and amino acid signalling.
Key words: Arabidopsis, morphological plasticity, nitrogen, root system
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Introduction |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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Plants have the ability to modify their morphology according to environmental conditions. This developmental plasticity is an integral part of a strategy for overcoming their immobility and allows plants to place their resource-capturing organs in close contact with vital resources that vary significantly in both quantity and spatial distribution in natural environments. An excellent example of developmental plasticity was elegantly illustrated by Drew and colleagues in their classical experiments in the 1970s (Drew et al., 1973; Drew, 1975; Drew and Saker, 1975, 1978). They showed that when the roots of barley (Hordeum sativum L.) seedlings were exposed to a locally concentrated supply of certain mineral nutrients (nitrate, ammonium, or inorganic phosphorus), lateral root (LR) proliferation in the nutrient-rich zone was stimulated. This growth response occurs in many different plants and represents a common adaptation phenomenon (Robinson, 1994). Like other forms of adaptive response, morphological changes require the plant both to sense constantly changing environmental signals and to convert such signals into appropriate actions.
Nitrogen (N) is one of the most important nutrients for plants and its availability is a major limiting factor for plant growth. Because of its importance various mechanisms have evolved in plants for efficient capture of N nutrients. Indeed, recent studies in the model plant Arabidopsis have revealed complex effects on root growth and development in response to changing N supplies (Zhang et al., 2000; Forde, 2002a, b; Malamy, 2005; Walch-Liu et al., 2006a). Detailed characterization of these morphological responses in the past few years has provided valuable insights about how N signals are sensed and integrated into developmental programmes in plants. In this article, recent progress in our understanding of the morphological responses of the Arabidopsis root system to variations in the N supply will be reviewed, with particular emphasis on the signalling mechanisms underlying these responses.
In Arabidopsis, four principal effects of the N supply on root growth and development have been identified (Fig. 1). These include: (i) a localized stimulatory effect of external nitrate on lateral root elongation (Zhang and Forde, 1998; Zhang et al., 1999, 2000; Linkohr et al., 2002); (ii) a systemic inhibitory effect of high tissue nitrate concentrations on the activation of lateral root meristems (Zhang et al., 1999, 2000; Remans et al., 2006a); (iii) a suppression of lateral root initiation by high C:N ratios (Malamy and Ryan, 2001), and (iv) an inhibition of primary root growth and stimulation of root branching by external L-glutamate (Walch-Liu et al., 2006b). Recent progress in each of these four areas will be considered.
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The localized stimulatory effect of nitrate on LR growth and the underlying mechanisms |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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The ability of plants to proliferate LRs preferentially in nutrient-rich patches has been well documented in the literature, but the signalling mechanisms behind this response are not well understood. In an attempt to understand this problem, Zhang and Forde (1998) examined the response in the model plant Arabidopsis with the intention of exploiting the accumulating molecular and genetic information from this species. Using a segmented agar plate system, they first established that the Arabidopsis root system responds to a localized nitrate supply by proliferating LRs preferentially in the nitrate-rich segment, a response similar to that observed by Drew and colleagues in barley. However, there are two noticeable differences between the responses in Arabidopsis and barley. Firstly, the response in barley involves an increase in both the number and the length of LRs. In the (shorter term) experiments done on Arabidopsis the predominant effect was on LR length (Zhang and Forde, 1998; Linkohr et al., 2002). Secondly, a localized ammonium supply also stimulated LR proliferation in barley (Drew et al., 1973; Drew, 1975), but failed to do so in Arabidopsis (Zhang and Forde, 1998; Zhang et al., 1999). The Arabidopsis experiments were done under sterile conditions, but in the case of the barley experiments it is possible that some of the ammonium was converted to nitrate by nitrifying bacteria so that ammonium itself was not the ion responsible for growth stimulation. It was established that the stimulatory effect of the localized nitrate supply on LR growth in Arabidopsis (subsequently referred to as ‘the localized stimulatory effect’) was due to the signalling property of the NO
ion itself (rather than its downstream metabolites). This conclusion was based on the observation that a mutant deficient in nitrate reductase showed a wild-type response and on the inability of alternative N sources (glutamine or NH
) to induce a similar response (Zhang and Forde, 1998). An important breakthrough in understanding the signalling mechanisms underlying the localized stimulatory effect was made when a component of this signalling pathway, the ANR1 MADS box transcription factor, was identified (Zhang and Forde, 1998). Plants in which ANR1 expression is down-regulated (by antisense or cosuppression) were either unable to respond or were significantly less sensitive to the localized stimulatory effect of nitrate (Zhang and Forde, 1998).
The identification of ANR1 provides a strong indication of the existence of a specific signalling pathway for the localized morphological response to nitrate. Another significant development in understanding this signalling pathway comes from a recent report that a nitrate transporter, AtNRT1.1 (also known as CHL1: Tsay et al., 1993), may act upstream of ANR1 in the localized stimulatory response (Remans et al., 2006a). These authors investigated the stimulatory effect of a localized high nitrate supply in mutants defective in AtNRT1.1, a previously characterized dual affinity nitrate transporter with nitrate uptake activities at both low and high external nitrate concentrations (Tsay et al., 1993; Wang et al., 1998; Liu et al., 1999). They found that the AtNRT1.1-defective mutants display a strongly decreased response to a localized nitrate supply, a phenotype similar to that observed in the ANR1 down-regulated lines. Apparently, this reduced responsiveness was not due to a decrease in nitrate uptake, as the specific nitrate uptake activity in the mutants was not reduced. Nor was it due to a reduced accumulation of N metabolites, as supplementation with an alternative N source in the nitrate-rich segment could not overcome the reduced sensitivity to the localized nitrate supply. Most importantly, they demonstrated that the reduced responsiveness of the mutants to the localized stimulatory effect was accompanied by reduced abundance of ANR1 transcript, suggesting that AtNTR1.1 may act upstream of ANR1 and could be involved in the transcriptional regulation of ANR1. The significance of this finding is that it links ANR1 directly with an upstream component which not only has the nitrate-binding property but also has a regulatory role in some nitrate-regulated processes (Munos et al., 2004; Alboresi et al., 2005). It was proposed that AtNTR1.1 could either act as a nitrate sensor in this signalling pathway or play a specific role in facilitating the access of nitrate to the nitrate sensor in the root tip (Remans et al., 2006a). Other evidence supporting a role of AtNRT1.1 in N signalling comes from studies of the regulation of the AtNRT2.1 gene in AtNRT1.1-defective mutants. AtNRT2.1 is normally induced by nitrate and repressed by N metabolites, but this regulation is dramatically altered in AtNRT1.1-defective mutants (Munos et al., 2004).
There is considerable evidence that transport proteins can also act as nutrient sensors (Lalonde et al., 1999), and as long ago as 1988 Behl and colleagues suggested that what they then identified as a low capacity, constitutive uptake system in barley could act as a sensor for nitrate in soil (Behl et al., 1988). In yeast, several identified glucose sensors share high sequence homology with glucose transporters (Özcan et al., 1996). Similar homology has also been found between amino acid sensors and amino acid transporters (Didion et al., 1998; Iraqui et al., 1999). There is also evidence that some dual function transporter/sensor proteins are involved in morphological responses. For example, under ammonium limitation, diploid yeast cells differentiate into a filamentous, pseudohyphal growth form (Marini et al., 1997). This morphological adaptation requires a dual functional ammonium transporter/sensor, MEP2 (Lorenz and Heitman, 1998).
If AtNRT1.1 is indeed a nitrate sensor, the linkup between AtNRT1.1 and ANR1 would be a very significant and exciting development towards the unveiling of the mechanism of this important morphological adaptation. However, there are still many unresolved questions about how the AtNRT1.1-ANR1 signalling pathway works. The first obvious one is how the nitrate signal is transmitted from AtNRT1.1 to ANR1. Although the observation of Remans et al. (2006a) indicates that AtNRT1.1 plays a role in regulating the abundance of ANR1 transcript, it is not clear how this is achieved at the molecular level. Theoretically, the reduced ANR1 mRNA aboundance in the AtNRT1.1-defective mutants could be due to a change in either the transcriptional rate or the stability of ANR1 transcript. Understanding the precise mode of action of this regulation could provide vital clues about the signalling passage between AtNRT1.1 and ANR1. Another key question is about the precise role of ANR1 in nitrate signalling. As stated previously, there is convincing evidence that ANR1 plays an important role in nitrate-induced LR proliferation and that the signal for this effect is NO itself. However, how the nitrate signal is converted to the growth response has not yet been established. The early model that nitrate might act by up-regulating ANR1 at the transcriptional level (Zhang and Forde, 1998) seems unlikely to be true as recent experimental evidence shows that, in intact plants, nitrate has no stimulatory effect on the abundance of ANR1 transcripts (Gan et al., 2005). Nevertheless, the observation of Remans et al. (2006a) suggests that transcriptional regulation of ANR1 must be part of the mechanisms involved in the response to the localized stimulatory effect of nitrate. It is possible that sustained LR proliferation may require the ANR1 transcript to be maintained at or above a certain threshold level and that a nitrate signal is needed for this maintenance. Alternatively, the nitrate signal could be transduced through post-translational modifications or protein–protein interactions (Walch-Liu et al., 2006a). In that context, Gan et al. (2005) examined the expression pattern of 11 root-expressed MADS-box genes in response to different N-treatments (N starvation, nitrate-resupply) and identified seven of the MADs box genes (AGL16, AGL21, AGL14, AGL19, SOC1, AGL26, and AGL56) with a similar response pattern to ANR1. Interestingly, three of these genes (AGL16, AGL21, and SOC1) were found in a separate study to encode proteins that interacted with ANR1 in a yeast two-hybrid assay (de Folter et al., 2005). It would be interesting to know if any of these genes play a role in the response to the localized stimulatory effect of nitrate.
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High-nitrate-induced lateral root inhibition and the underlying mechanisms |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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In addition to the stimulatory effect on LR growth, nitrate also has an inhibitory effect on LR development (Zhang and Forde, 1998; Zhang et al., 1999, 2000). This effect increases with the overall nitrate supply and is most apparent when the external nitrate concentration is above 10 mM. The two opposing responses to the same nutrient reflect the sophistication of the mechanism whereby plants adapt to nitrate supply and quantify their responses precisely according to its abundance. A low overall nitrate supply favours the stimulatory effect and promotes root proliferation and a high overall nitrate supply favours the inhibitory effect and suppresses root growth. Morphological analyses show that the high nitrate-induced LR inhibition occurs at a specific stage of LR development, immediately after the emergence of the LR primordia from the parent root but before the activation of the LR meristem, resulting in the accumulation of stunted LRs (Zhang et al., 1999, 2000) and is reversible. The inhibition is released within 24 h when the seedlings are transferred to media with low nitrate supplies. The specific stage and reversibility of the inhibition are well suited to the need of plants to optimize the root system quickly by activating quiescent root meristems in response to rapidly changing nutrient supplies in the environment.
Experimental evidence indicates that the signal responsible for the suppression of LR development by high nitrate is the accumulation of nitrate inside plant tissues, rather than the external nitrate concentration or the accumulation of N metabolites inside the plant. This conclusion is based on the observation that the nia1nia2 mutant of Arabidopsis, which is defective in both nitrate reductases, is more sensitive to the high nitrate-induced inhibition (Zhang et al., 1999). It has been hypothesized that the nitrate signal is perceived in the shoots and that the inhibition probably involves long-distance signalling between the roots and shoots (Zhang et al., 1999; Forde, 2002a). Walch-Liu et al. (2006a) tested this hypothesis and examined a possible role of auxin in the long-distance signalling. They measured IAA contents in the shoots and roots of seedlings either grown continuously on a high nitrate supply (50 mM KNO3) or grown initially on the high nitrate supply but then transferred to a medium with a low nitrate supply (1.0 mM KNO3) for 24 h and found that the seedlings that had been transferred to the low nitrate supply had a significantly higher IAA content in the root and a lower IAA content in the shoot compared with the seedlings kept on the high nitrate supply continuously. A similar observation has also been reported in soybean (Caba et al., 2000), where it was found that plants grown on 8 mM KNO3 had 4-fold less IAA in the root tissues than plants grown on 1 mM KNO3. These results suggest that high nitrate supply may either suppress auxin biosynthesis or hamper auxin translocation from the shoots to the roots and that this effect is widespread in plants. However, it has been observed that exogenously applied auxin was unable to overcome the inhibitory effect of high nitrate in Arabidopsis seedlings (H Rong, H Zhang, unpublished results), indicating that the observed effect on auxin content may be independent of the inhibitory effect, or that external auxin is unable to substitute for auxin delivered via the phloem.
It has been established that another plant hormone, abscisic acid (ABA), plays a vital role in mediating the inhibition by high nitrate, as this inhibition is found to be significantly lower in known ABA-deficient mutants (aba1-1, aba2-3, aba2-4, and aba3-2) and in some ABA responsive mutants (abi4-1, abi4-2, and abi5-1) (Signora et al., 2001). Whilst ABA concentrations in split-root barley plants grown under steady-state conditions were shown to be unrelated to the rate of supply of nitrate, there were rapid but transient changes in ABA concentrations after short-term changes in nitrate supply (Brewitz et al., 1995). Roots receiving an elevated nitrate supply sensed this increase and gave rise to whole plant changes in ABA levels.
It has also been demonstrated that exogenously supplied ABA mimics the high nitrate-induced inhibitory effect. Under this study's conditions, inhibition by ABA and high nitrate both appear to occur at the same developmental step; immediately after the activation of LR meristems (De Smet et al., 2003). This novel ABA function in LR development fits well with the well-documented general ‘dormancy-promoting’ property of this hormone in seeds and buds and has led to the concept of ‘lateral root dormancy’ (De Smet et al., 2006). However, it appears that the role of ABA in promoting dormancy in LR growth is not mediated by the same mechanism involved in seed dormancy, as mutants in genes such as ABI1, ABI2, and ABI3 that are crucial in seed dormancy regulation do not show significant defects in their response to the inhibitory effect of ABA in LRs (Signora et al., 2001). Genetic evidence has been obtained that the inhibition by both ABA and high nitrate is mediated by the same signalling mechanism; mutants that are identified based on their ability to produce visible LRs in the presence of 0.5 µM ABA (designated as labi for lateral root ABA-insensitive) are also less sensitive to the high-nitrate-induced inhibition on LRs (Fig. 2). Identification of the LABI genes will provide valuable information about the mechanisms underlying this inhibition. Surprisingly, all the labi mutants also have a shorter primary root phenotype, indicating that the regulation of LR development is probably intrinsically linked to primary root growth. In fact, evidence has been obtained that the presence of the primary root meristem is required for the high-nitrate and ABA-induced inhibition: removal of the primary roots abolishes the inhibition (Fig. 3).
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An interesting, and potentially exciting, development in our understanding of the mechanisms involved in the high-nitrate/ABA-induced LR inhibition is that a recently identified putative ABA receptor, FCA, could be involved (Razem et al., 2006). FCA is a key regulator of flowering in Arabidopsis thaliana (Macknight et al., 1997; Simpson et al., 2003). Mutants defective in FCA flower late. Razem et al. (2006) reported that FCA could act as an ABA receptor and that a loss-of-function fca mutation showed a reduced sensitivity to the inhibitory effect of ABA on LRs, indicating that FCA could be a component of a signalling transduction pathway involved in the high-nitrate/ABA induced inhibition. It would be interesting to establish if fca mutants are also less sensitive to the effects of high nitrate, as has been predicted for them to be.
Is it possible that LR inhibition by high nitrate is due to an osmotic effect of the high potassium nitrate concentrations (Malamy, 2005). Deak and Malamy (2005) found that high osmotic potential repressed LR development and mimicked the inhibitory effect of high nitrate and that the osmotic repression also involved ABA, as it was much reduced in two ABA-deficient mutants, aba2 and aba3. However, there is a distinct difference between the repression caused purely by osmotic potential and the inhibition induced by high-nitrate/ABA as osmotic repression can be overridden by auxin (Deak and Malamy, 2005), whilst the inhibition by high nitrate or ABA is auxin-independent (Signora et al., 2001; De Smet et al., 2003). This difference suggests that the two forms of inhibition, although morphologically similar, are not mediated by the same mechanism. It also raises the question whether osmotic repression is mediated directly by ABA or by an ABA-independent stress response mechanism. Nevertheless, it is possible that high nitrate could affect LR development through both osmotic repression (which can be overridden by auxin) and osmotic-independent, nitrate/ABA-specific inhibition (which is not auxin reversible). Although the exact developmental stage of the osmotic repression is not clear, it is possible that it may occur during the emergence step, based on the antagonism between osmotic repression and auxin and the observation that the emergence of LR primordia is auxin-dependent (Bhalerao et al., 2002).
Interestingly, the inhibition of LR development by high nitrate was found to be influenced by soil bacteria. It was reported recently that the inhibitory effect can be overridden by inoculation with a plant growth-promoting rhizobacterium (PGPR), Phyllobacterium strain STM196 (Mantelin et al., 2006). The impairment in the morphological response to high nitrate supply was accompanied by an altered expression of several putative nitrate transporter genes including AtNRT1.1, AtNRT2.1, AtNRT2.5, and AtNRT2.6, indicating a general effect of the rhizobacterium on nitrate signalling. Although it is not clear how the rhizobacterium influences nitrate signalling, the existence of such cross-talk between nitrate and rhizobacteria in LR regulation provides evidence that the morphological adaptation of the root system involves mechanisms for integrating different biotic and abiotic signals.
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Mechanisms underlying the suppression of LR initiation induced by high sucrose:nitrogen ratio |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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Malamy and Ryan (2001) investigated the effect of sugar to N ratio on the development of the Arabidopsis root system and found that when Arabidopsis plants were grown on high ratios of sucrose to nitrogen (4.5% sucrose to 0.01 mM NH4NO3), lateral root initiation was dramatically suppressed. Initial analysis suggests that the signal for this suppression is the high C:N ratio, as altering this ratio by either reducing the sucrose concentration while maintaining the low nitrogen concentration, or increasing the N concentration while maintaining the high sucrose concentration, restored lateral root initiation to the levels seen with high levels of nitrogen. However, it is also possible that the signal could be the low N (NO
or NH) status as the high level of sucrose supply could speed-up the N depletion rate and thereby promote N shortage. How the signal for the C:N ratio is sensed and converted into a repression of LR initiation is not fully understood. There is evidence that the suppression of LR initiation could be due to a block of auxin movement from the shoot to the root, as the repression was accompanied by enhanced auxin accumulation/response in the hypocotyls indicated by an increased expression of the DR5-GUS reporter (Malamy and Ryan, 2001). In an effort to dissect the underlying mechanisms of the C:N ratio-induced inhibition on LR initiation, these authors carried out a genetic screen and obtained a mutant, lin1, which is able to initiate large numbers of lateral roots when grown on media with a high sucrose:nitrogen ratio (Malamy and Ryan, 2001). The existence of the mutant led the authors to speculate that repression of lateral root initiation was not caused by inadequate nutrients to complete the essential cell divisions, but by a signalling process that negatively regulates the LR initiation programme, and that LIN1 is an important component of this signalling pathway. Surprisingly, LIN1 was later found to encode the high affinity nitrate transporter AtNRT2.1 (Little et al., 2005).
Evidence in support of this putative role of AtNRT2.1 includes the finding that the lin1 mutant contains a missense mutation in AtNRT2.1 (Little et al., 2005) and that allelic mutants, including one in which the NRT2.1 gene is completely deleted, have similar phenotypes to lin1 and fail to complement lin1. In addition, it has also been confirmed that the lin1 mutant has a reduced nitrate uptake capacity at low enternal nitrate concentrations. The results indicate that NRT2.1 is a repressor of LR initiation. There are a number of other interesting questions that follow from this. First, how does AtNRT2.1 repress LR initiation? As increasing nitrate in the medium actually releases LR repression, the enhanced LR initiation phenotype of lin1 cannot be caused by reduced nitrate uptake or by defective transporter activity. This leads to the conclusion that the role of AtNRT2.1 in the repression of LR initiation is more likely to be a regulatory one. The apparent nitrate-binding property of AtNRT2.1 has led to further speculation that AtNRT2.1 could even act as a nitrate sensor (Little et al., 2005). This leads to the second question: what is the signal for the LR repression? As stated before, initial analysis indicates that the signal is probably the high C:N ratio. However, the finding that LIN1 is a high affinity nitrate transporter has raised the possibility that nitrate itself may be sensed and that the lack of nitrate could be the repressory signal. Under high sucrose and low N conditions, nitrate could be rapidly depleted due to the metabolic activities driven by the high sugar supply. The decreased nitrate pool, probably sensed via AtNRT2.1, would, in turn, lead to a repression of LR initiation. This hypothesis could explain why decreasing sugar concentration or increasing nitrate concentration can both reduce the repression, i.e. by slowing down the depletion rate of nitrate in the tissues or by increasing nitrate level, respectively. The hypothesis is also consistent with the observation that plants grown on a high sucrose and low N medium were severely stunted, a symptom of N starvation (Malamy and Ryan, 2001; Little et al., 2005). However, there is currently very little direct experimental evidence to support the above hypothesis. It is perfectly possible that both C and N signals play a part in the repression of LR initiation. For example, since it is known that AtNRT2.1 is regulated at the transcriptional level by both C and N signals (Orsel et al., 2002; Lejay et al., 1999), AtNRT2.1 itself could act as the integrator of these two signals via its transcriptional regulation in regulating the LR initiation step (Little et al., 2005).
The above hypothesis is based on the understanding that AtNRT2.1 plays a repressive role in LR initiation. However, a recent report by Remans et al. (2006b) suggests that AtNRT2.1 may play a different role in this developmental process under different physiological conditions. It was found that, under N-limiting conditions, mutations in AtNRT2.1 led to a decreased number of LR primordia (Remans et al., 2006b), an observation exactly opposite to that of Little et al. (2005). It is not clear how the same protein could play two completely opposite roles in the same developmental process. One possible explanation is that the plants used in the two studies had different levels of N deficiency and that the role of AtNRT2.1 in LR initiation may depend on the extent of shortfall of N supply. As plants used by Little et al. (2005) were grown continuously on a medium with a high sucrose:low N ratio and those used by Remans et al. (2006b) were exposed to 10 mM NO for 6 d prior to the low NO treatment, one would expect that the plants used by Little et al. (2005) probably experienced a higher degree of N deficiency than those used by Remans et al. (2006b). The different degree of N deficiency may determine the effect of low NO on LR initiation and also the roles of AtNRT2.1 in this process. Consistent with this interpretation, Remans et al. (2006b) observed contrasting effects of low NO on root branching depending on the history of NO treatment. They found that low NO stimulated the emergence of LRs in the basal primary root zone which had been exposed to high NO previously, but suppressed LR emergence in the primary root zone which had no previous exposure to high NO. It is also possible that the different roles of AtNRT2.1 in LR initiation observed by the two groups may also be linked to the different levels of sugar supply in the media.
The three morphological adaptations of the root system discussed so far are linked to one form of mineral N nutrient, nitrate. Recent studies have demonstrated that root architecture is also responsive to an organic form of N, namely glutamate (Sivaguru et al., 2003; Filleur et al., 2005; Walch-Liu et al., 2006b). The presence of low concentrations (0.05–0.5 mM) of L-glutamate can lead to a marked inhibitory effect on primary root growth (Walch-Liu et al., 2006b), a stimulatory effect on both LR initiation and LR outgrowth (Walch-Liu et al., 2006b), resulting in a shorter, more branched root system not dissimilar to the phenotype seen when Arabidopsis seedlings are grown on a limiting supply of P (Williamson et al., 2001).
The mechanisms underlying the effect of glutamate on root growth are not understood. One possibility is that it involves sensing by plant homologues of mammalian ionotropic glutamate receptors (iGluRs) (Sivaguru et al., 2003; Walch-Liu et al., 2006a). Sivaguru and colleagues found that the inhibition of root growth by L-glutamate was accompanied by rapid cytological changes, such as depolymerization of cortical microtubules and membrane depolarization (Sivaguru et al., 2003) and that these cytological changes could be blocked with an iGluR antagonist, AP-5, suggesting that activities of the GluRs are required for the glutamate-induced cytological changes. The existence of a large family of GluR-related genes in plants is consistent with this interpretation (Lacombe et al., 2001; Chiu et al., 2002). However, there is no direct experimental evidence to link GluRs with the morphological adaptations of root system to glutamate.
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Physiological and ecological significance of root morphological adaptations to the N supply |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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As nitrate is freely water soluble it readily moves in soil water, so it can be transported to the root surface by mass flow in field-grown crops. At above 7 mM nitrate in the soil, mass flow moves enough nitrate to the root system for plant growth and the rate of uptake is dependent purely on the rate of uptake across the root surface (Engels and Marschner, 1995). At lower concentrations, diffusion becomes increasingly important, and the rate at which this occurs limits the rate of uptake. The rate of diffusion is dependent on the difference in nitrate concentration between the bulk soil and at the root plane, the water content of the soil and the root surface area, and it appears to become limiting for the growth of crops in the field below 0.4 mM in soil with a 20% water content (Engels and Marschner, 1995). Root length density, root hair density, root hair length, and root diameter therefore become important for nitrate transport to the root surface and nitrate uptake at low concentrations of nitrate (Engels and Marschner, 1995; Robinson, 1994, 1996, 2001), but at high concentrations the occurrence of transpiration, and hence the resultant mass flow, are more important. As ammonium has a much lower diffusion coefficient than nitrate the role of root parameters in determining the rate of uptake would become apparent at higher concentrations.
In experimental systems such as hydroponics or agar the relative importance of mass flow and diffusion, and the nitrate concentration at which diffusion becomes limiting, differ considerably from the situation in soil. However, if plants are subjected to nitrate concentrations that are low or high relative to their requirements, root morphology should adapt in the same manner as it would in soil even if the threshold concentrations at which responses are observed may differ. This still leaves open the question as to what advantage plants might gain from root proliferation in patches of high nitrate concentration, when the solubility of the ion makes it freely available without such proliferation (Robinson, 1996). It seems likely that root proliferation under such circumstances would give one plant a competitive advantage over another in terms of its ability to acquire nitrogen in a mixed stand (Hodge et al., 1999). Why would the high nitrate-induced inhibition of lateral root growth be an advantage to a plant? One possible answer is that with high nitrate in the soil the plant has adequate N supply, so its resources would be better used to increase shoot growth because extra rooting would not enhance the acquisition of nitrogen. Furthermore, water uptake into any roots growing in a high-nitrate patch would actually be more difficult than into other parts of the root system because of the lower water potential in the patch.
The physiological and ecological significance of the adaptation of root system morphology in response to supply of L-glutamate is still unclear. Although amino acids are present in soils (Lipson and Nasholm, 2001; Jones et al., 2005), until recently they were not considered to be a significant source of N for plants due to intense microbial competition. However, if that is the case, one would ask why plants have conserved a large number of amino acid uptake systems with differing substrate specificities. The existence of amino acid uptake systems across different plant species suggests that plants are able to absorb amino acids directly from the soil (Fischer et al., 1998; Lipson and Nasholm, 2001) and it is now believed that plants in a wide range of different ecoysystems acquire a significant proportion of their N directly from the organic N pool (Jones et al., 2005). In addition, it has also been suggested that plants could access organic N via mycorrhizal fungi (Tibbett et al., 1998). The ability to access organic N directly is considered to be advantageous to plants because they do not have to rely on microbial mineralization to produce inorganic N, which is generally considered to be a bottleneck in the N cycle in soils (Neff et al., 2003).
Walch-Liu et al. (2006a, b) have proposed that the root architectural response to glutamate could enhance a plant's ability to compete for organic N by increasing the root density within the organic N-rich patch. It is within such patches that amino acid concentrations will be highest and where plants are most likely to be able to compete effectively with micro-organisms for this valuable source of N.
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Concluding remarks |
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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Although the physiological and ecological significance of the adaptations of the root system to N supply are still under debate, their value as models for studying N signalling mechanisms is becoming increasingly evident. Figure 4 summarizes our current understanding of the mechanisms underlying the morphological adaptations of the root system to N supply. One of the most significant findings of the past few years has been the involvement of nitrate transporters in two of the adaptations: the localized stimulation of LR growth by nitrate and the suppression of LR initiation by high C:N ratios. These studies have added further weight to a long-standing hypothesis that nitrate transporters could act as nitrate sensors. Further investigation of their precise role in the morphological adaptations may help to establish whether the transporters are indeed nitrate sensors and also to uncover the downstream signalling components.
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The important role of ABA in the adaptation to high nitrate supply and the potential link to the recently identified ABA receptor FCA, together with the isolation of novel mutants insensitive to LR inhibition by high nitrate/ABA, present an exciting opportunity for using this morphological adaptation to study ABA and nitrate signalling and nutrient/hormonal cross-talks. Furthermore the root architectural response to glutamate may provide a valuable experimental system to study glutamate signalling in plants and to elucidate the possible role of glutamate receptors.
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Acknowledgements |
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The authors wish to acknowledge the Biotechnology and Biological Sciences Research Council (BBSRC) (Research grant 24/P16557) and The Royal Society (Research grants 2005/R4 & RSRG24665) for research funding.
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TopAbstractIntroductionThe localized stimulatory effect...High-nitrate-induced lateral...Mechanisms underlying the...Physiological and ecological...Concluding remarksReferences |
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Alboresi A, Gestin C, Leydecker MT, Bedu M, Meyer C, Truong H-N. Nitrate, a signal relieving seed dormancy in Arabidopsis. Plant. Cell and Environment (2005) 28:500–512.[CrossRef]
Behl R, Tischner R, Raschke K. Induction of a high-capacity nitrate-uptake mechanism in barley roots prompted by nitrate uptake through a constitutive low-capacity mechanism. Planta (1988) 176:235–240.[CrossRef][ISI]
Bhalerao RP, Eklof J, Ljung K, Marchant A, Bennett M, Sandberg G. Shoot-derived auxin is essential for early lateral root emergence in Arabidopsis seedlings. The Plant Journal (2002) 29:325–332.[CrossRef][ISI][Medline]
Brewitz E, Larsson C-M, Larsson M. Influence of nitrogen supply on concentrations and translocation of abscisic acid in barley (Hordeum vulgare). Physiologia Plantarum (1995) 95:499–506.[CrossRef]
Caba JM, Centeno ML, Fernandez B, Gresshoff PM, Ligero F. Inoculation and nitrate alter phytohormone levels in soybean roots: differences between a supernodulating mutant and the wild type. Planta (2000) 211:98–104.[CrossRef][ISI][Medline]
Chiu JC, Brenner ED, DeSalle R, Nitabach MN, Holmes TC, Coruzzi GM. Phylogenic and expression analysis of the glutamate-receptor-like gene family in Arabidopsis thaliana. Molecular Biology and Evolution (2002) 19:1066–1082.
de Folter S, Immink RGH, Kieffer M, et al. Comprehensive interaction map of the Arabidopsis MADS box transcription factors. The Plant Cell (2005) 17:1424–1433.
De Smet I, Signora L, Beeckman T, Foyer CH, Zhang H. An abscisic acid-sensitive checkpoint in lateral root development of Arabidopsis. The Plant Journal (2003) 33:543–555.[CrossRef][ISI][Medline]
De Smet I, Zhang H, Inzé D, Beeckman T. Abscisic acid makes lateral roots dormant. Trends in Plant Sciences (2006) 11:434–439.[CrossRef]
Deak KI, Malamy J. Osmotic regulation of root system architecture. The Plant Journal (2005) 43:17–28.[CrossRef][ISI][Medline]
Didion T, Regenberg B, Jorgensen MU, Kielland-Brandt MC, Andersen HA. The permease homologue Ssy1p controls the expression of amino acid and peptide transporter genes in Saccharomyces cerevisiae. Molecular Microbiology (1998) 27:643–650.[CrossRef][ISI][Medline]
Drew MC. Comparison of the effects of a localized supply of phosphate, nitrate, ammonium and potassium on the growth of the seminal root system, and the shoot, in barley. New Phytologist (1975) 75:479–490.[CrossRef][ISI]
Drew MC, Saker LR. Nutrient supply and the growth of the seminal root system of barley. II. Localized, compensatory increases in lateral root growth and rates of nitrate uptake when nitrate supply is restricted to only part of the root system. Journal of Experimental Botany (1975) 26:79–90.
Drew MC, Saker LR. Nutrient supply and the growth of the seminal root system in barley. Journal of Experimental Botany (1978) 29:435–451.
Drew MC, Saker LR, Ashley TW. Nutrient supply and the growth of the seminal root system in barley. Journal of Experimental Botany (1973) 24:1189–1202.
Engels C, Marschner H. Plant uptake and utilization of nitrogen. In: Nitrogen fertilization in the environment—Bacon PE, ed. (1995) New York: Marcel Dekker. 41–81.
Filleur S, Walch-Liu P, Gan Y, Forde BG. Nitrate and glutamate sensing by plant roots. Biochemical Society Transactions (2005) 33:283–286.[CrossRef][ISI][Medline]
Fischer WN, André B, Rentsch D, Krolkiewicz S, Tegeder M, Breitkreuz K, Frommer WB. Amino acid transport in plants. Trends in Plant Sciences (1998) 3:188–195.[CrossRef]
Forde BG. Local and long-range signalling pathways regulating plant responses to nitrate. Annual Review of Plant Biology (2002a) 53:203–224.[CrossRef][Medline]
Forde BG. The role of long-distance signalling in plant responses to nitrate and other nutrients. Journal of Experimental Botany (2002b) 53:39–43.
Gan Y, Filleur S, Rahman A, Gotensparre S, Forde BG. Nutritional regulation of ANR1 and other root-expressed MADS-box genes in Arabidopsis thaliana. Planta (2005) 222:730–742.[CrossRef][ISI][Medline]
Hodge A, Robinson D, Griffiths BS, Fitter AH. Why plants bother: root proliferation results in increased nitrogen capture from an organic patch when two grasses compete. Plant. Cell and Environment (1999) 22:811–820.[CrossRef]
Iraqui I, Vissers S, Bernard F, De Craene J-O, Boles E, Urrestarazu A, André B. Amino acid signaling in Saccharomyces cerevisiae: a permease-like sensor of external amino acids and the F-box protein Grr1p are required for transcriptional induction of the AGP1 gene encoding a broad-specificity amino acid permease. Molecular and Cellular Biology (1999) 19:989–1001.
Jones DL, Healey JR, Willett VB, Farrar JF, Hodge A. Dissolved organic nitrogen uptake by plants: an important N uptake pathway? Soil Biology and Biochemistry (2005) 37:413–423.[CrossRef]
Lacombe B, Becker D, Hedrich R, et al. The identity of plant glutamate receptors. Science (2001) 292:1486–1487.[ISI][Medline]
Lalonde S, Boles E, Hellmann H, Barker L, Patrick JW, Frommer WB, Ward JM. The dual function of sugar carriers. Transport and sugar sensing. The Plant Cell (1999) 11:707–726.
Lejay L, Tillard P, Lepetit M, Olive F, Filleur S, Daniel-Vedele F, Gojon A. Molecular and functional regulation of two NO uptake systems by N- and C-status of Arabidopsis plants. The Plant Journal (1999) 18:509–519.[CrossRef][ISI][Medline]
Linkohr B, Williamson L, Fitter A, Leyser O. Nitrate and phosphate availability and distribution have different effects on root system architecture of Arabidopsis. The Plant Journal (2002) 29:751–760.[CrossRef][ISI][Medline]
Lipson D, Nasholm T. The unexpected versatility of plants: organic nitrogen use and availability in terrestrial ecosystems. Oecologia (2001) 128:305–316.[CrossRef][ISI]
Little YD, Rao H, Oliva S, Daniel-Vedel F, Krapp A, Malamy JE. The putative high-affinity nitrate transporter NRT2.1 represses lateral root initiation in response to nutritional cues. Proceedings of the National Academy Sciences, USA (2005) 102:13693–13698.
Liu KH, Huang CY, Tsay YF. CHL1 is a dual-affinity nitrate transporter of Arabidopsis involved in multiple phases of nitrate uptake. The Plant Cell (1999) 11:865–874.
Lorenz MC, Heitman J. Regulators of pseudohyphal differentiation in Saccharomyces cerevisiae identified through multicopy suppressor analysis in ammonium permease mutant strains. Genetics (1998) 150:1443–1457.
Macknight R, Bancroft I, Page T, et al. FCA, a gene controlling flowering time in Arabidopsis, encodes a protein containing RNA-binding domains. Cell (1997) 89:737–745.[CrossRef][ISI][Medline]
Malamy J, Ryan K. Environmental regulation of lateral root initiation in Arabidopsis. Plant Physiology (2001) 127:899–909.
Malamy JE. Intrinsic and environmental response pathways that regulate root system architecture. Plant. Cell and Environment (2005) 28:67–77.[CrossRef]
Mantelin S, Desbrosses G, Larcher M, Tranbarger TJ, Cleyet-Marel JC, Touraine B. Nitrate-dependent control of root architecture and N nutrition are altered by a plant growth-promoting Phyllobacterium sp. Planta (2006) 223:591–603.[CrossRef][ISI][Medline]
Marini AM, Soussi-Boudekou S, Vissers S, André B. A family of ammonium transporters in Saccharomyces cerevisiae. Molecular and Cellular Biolology (1997) 17:4282–4293.
Munos S, Cazettes C, Fizames C, Gaymard F, Tillard P, Lepetit M, Lejay L, Gojon A. Transcript profiling in the chl1-5 mutant of Arabidopsis reveals a role of the nitrate transporter NRT1.1 in the regulation of another nitrate transporter, NRT2.1. The Plant Cell (2004) 6:2433–2447.
Neff JC, Chapin FS, Vitousek PM. Breaks in the cycle: dissolved organic nitrogen in terrestrial ecosystems. Frontiers in Ecology and the Environment (2003) 1:205–211.[CrossRef][ISI]
Orsel M, Krapp A, Daniel-Vedele F. Analysis of the NRT2 nitrate transporter family in Arabidopsis. structure and gene expression. Plant Physiology (2002) 129:886–896.
Özcan S, Dover J, Rosenwald AG, Woelfl S, Johnston M. Two glucose transporters in S. cerevisiae are glucose sensors that generate a signal for induction of gene expression. Proceedings of the National Academy Sciences, USA (1996) 93:12428–12432.
Razem FA, El-Kereamy A, Abrams SR, Hill RD. The RNA-binding protein FCA is an abscisic acid receptor. Nature (2006) 439:290–294.[CrossRef][Medline]
Remans T, Nacry P, Pervent M, Filleur S, Diatloff E, Mounier E, Tillard P, Forde BG, Gojon A. The Arabidopsis NRT1.1 transporter participates in the signaling pathway triggering root colonization of nitrate-rich patches. Proceedings of the National Academy of Sciences, USA (2006a) 103:19206–19211.
Remans T, Nacry P, Pervent M, Girin T, Tillard P, Lepetit M, Gojon A. A central role for the nitrate transporter NRT2.1 in the integrated morphological and physiological responses of the root system to nitrogen limitation in Arabidopsis. Plant Physiology (2006b) 140:909–921.
Robinson D. The responses of plants to non-uniform supplies of nutrients. New Phytologist (1994) 127:635–674.[CrossRef][ISI]
Robinson D. Resource capture by localized root proliferation: why do plants bother? Annals of Botany (1996) 77:179–185.
Robinson D. Root proliferation, nitrate inflow and their carbon costs during nitrogen capture by competing plants in patchy soil. Plant and Soil (2001) 232:41–50.[CrossRef][ISI]
Signora L, De Smet I, Foyer CH, Zhang H. ABA plays a central role in mediating the regulatory effects of nitrate on root branching in Arabidopsis. The Plant Journal (2001) 28:655–662.[CrossRef][ISI][Medline]
Simpson GG, Dijkwel PP, Quesada V, Henderson I, Dean C. FY is an RNA 3’ end-processing factor that interacts with FCA to control the Arabidopsis floral transition. Cell (2003) 113:777–787.[CrossRef][ISI][Medline]
Sivaguru M, Pike S, Gassmann W, Baskin TI. Aluminum rapidly depolymerizes cortical microtubules and depolarizes the plasma membrane: evidence that these responses are mediated by a glutamate receptor. Plant Cell Physiology (2003) 44:67–75.
Tibbett M, Sanders FE, Minto SJ, Dowell M, Cairney JWG. Utilization of organic nitrogen of ectomycorrhizal fungi (Hebeloma spp.) of arctic and temperate origins. Mycological Research (1998) 102:1525–1532.[CrossRef][ISI]
Tsay YF, Schroeder JI, Feldmann KA, Crawford NM. The herbicide sensitivity gene CHL1 of Arabidopsis encodes a nitrate-inducible nitrate transporter. Cell (1993) 72:705–713.[CrossRef][ISI][Medline]
Walch-Liu P, Ivanov II, Filleur S, Gan Y, Remans T, Forde BG. Nitrogen regulation of root branching. Annals of Botany (2006a) 97:875–881.
Walch-Liu P, Liu LH, Remans T, Tester M, Forde BG. Evidence that L-glutamate can act as an exogenous signal to modulate root growth and branching in Arabidopsis thaliana. Plant Cell Physiology (2006b) 47:1045–1057.
Wang R, Liu D, Crawford NM. The Arabidopsis CHL1 protein plays a major role in high-affinity nitrate uptake. Proceedings of the National Academy of Sciences, USA (1998) 95:15134–15139.
Williamson LC, Ribrioux S, Fitter AH, Leyser HMO. Phosphate availability regulates root system architecture in Arabidopsis. Plant Physiology (2001) 126:875–882.
Zhang H, Forde BG. An Arabidopsis MADS box gene that controls nutrient-induced changes in root architecture. Science (1998) 279:407–409.
Zhang H, Jennings A, Barlow PW, Forde BG. Dual pathways for regulation of root branching by nitrate. Proceedings of the National Academy of Sciences, USA (1999) 96:6529–6534.
Zhang H, Jennings AJ, Forde BG. Regulation of Arabidopsis root development by nitrate availability. Journal of Experimental Botany (2000) 51:51–59.
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