JXB Advance Access originally published online on November 16, 2006
Journal of Experimental Botany 2006 57(15):4099-4109; doi:10.1093/jxb/erl186
RESEARCH PAPER |
Comparison of environmental and mutational variation in flowering time in Arabidopsis
Laboratoire de Biologie Cellulaire, INRA, Route de Saint-Cyr, F-78026 Versailles cedex, France
* To whom correspondence should be addresssed. E-mail: sylvie.pouteau{at}versailles.inra.fr
Received 9 May 2006; Accepted 4 September 2006
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Abstract |
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Developmental dynamics can be influenced by external and endogenous factors in a more or less analogous manner. To compare the phenotypic effects of (i) environmental [i.e. standard (stPhP) and extended (exPhP) photoperiods] changes in Arabidopsis wild types and (ii) endogenous genetic variation in eav1–eav61 early flowering mutants, two temporal indicators were analysed, the time to bolting (DtB) and the number of leaves (TLN). It was found that DtB and TLN are differentially affected in different environmental and genetic contexts, and some factors of dynamic convergence were identified. The quantitative response to photoperiod is markedly contingent on the phototrophic input for DtB, but less so for TLN. To discriminate the light quantity and period components in DtB, two novel temporal indicators were determined, LtB (photosynthetic time to bolting) and PChron (DtB h–1 of photoperiod), respectively. The use of PChron results in a coincidence of the variation profiles across stPhP and exPhP, interpreted as a buffering of the trophic response. Unlike natural accessions and later flowering mutants, the variation profiles across stPhP and eav mutants are significantly divergent, pointing to differences in environmental and genetic variation in flowering time. Yet, phenocopy effects and dynamic convergence between wild-type and mutant profiles are detected by using exPhP and the LtB indicator. Additional analyses of the cauline leaf number (CLN) show that the apical and basal boundaries of the primary inflorescence vary co-ordinately. The finding that the correlativity between CLN and TLN changes across photoperiods suggests that different states of intra-connectedness are involved in ontogenetic specification of flowering time and embodied in the primary inflorescence.
Key words: Arabidopsis, bolting, correlativity, developmental dynamics, flowering time, early flowering mutant, phase change, phenocopy, phenotypic plasticity, photoperiodic response
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary dataReferences |
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Because plants are sessile organisms characterized by an extended and sequential mode of development, the adjustment of their endogenous developmental dynamics and ontogenetic trajectory to seasonal time is crucial for their adaptation to the environment (Roberts and Summerfield, 1987; Battey, 2000). This is achieved through phenotypic plasticity, the capacity to express different predictable phenotypes in reaction to external changes, i.e. norms of reaction. This capacity to vary relies on the constitutive mode of variation in living organisms, i.e. growth forces that allow ontogenetic transformations and development (Pigliucci, 1996; Debat and David, 2001; Sultan, 2004). The ontogenetic trajectory can be defined as a progression through a series of growth phases, each one characterized by the production of morphological structures with constant or gradually changing features, interspaced with discrete, critical phases, or phase changes, in which new morphological organization occurs (Haughn et al., 1995; Diggle, 1999; Poethig, 2003). Besides phenotypic plasticity in response to environmental changes, variation in ontogenetic trajectory and in the timing of developmental processes, i.e. heterochrony, may also be the consequence of endogenous changes at the chromatin or DNA level, i.e. epigenetic or genetic changes, that in turn may lead to altered reactions to the environment (Freeling et al., 1992; Wiltshire et al., 1994; Haughn et al., 1995; van Tienderen et al., 1996; Diggle, 1999; Finnegan, 2001; Sung and Amasino, 2004).
There is evidence that environmental and mutational variation may converge into analogous phenotypes. Indeed, mutant phenocopies can often be obtained in the wild type by applying adequate external constraints (Waddington, 1942; Mitchell and Lipps, 1978). Phenocopy and the functional versatility reported for many genes suggest that mutant phenotypes reflect not only specific functional defects but also distortions in wild-type intra-connectedness of network biological systems (Amzallag, 2000, 2001; Edelman and Gally, 2001; Finnegan, 2001; Greenspan, 2001; Espinosa-Soto et al., 2004; Luscombe et al., 2004; Wagner, 2005). An important question is to understand how this intra-connectedness is linked to the regulation of endogenous dynamics and eventually to the adaptability to the environment. One possible approach to address this question is to compare the phenotypic impact of environmental and mutational ontogenetic variation.
Central in plant ontogenetic dynamics, flowering time is a key life history trait that is both extremely plastic and sensitive to genetic variation (Zhang and Lechowicz, 1994; Clarke et al., 1995; Kuittinen et al., 1997; Searle and Coupland, 2004; Sung and Amasino, 2004; Bernier and Périlleux, 2005). Photoperiod, the only fully reliable seasonal signal, is one essential factor of variation in flowering time which contributed to crop domestication and species adaptation, or acclimation to different habitats (Garner and Allard, 1920; Roberts and Summerfield, 1987; Searle and Coupland, 2004; Borchert et al., 2005). Commonly described as a quantitative long day (LD) species, Arabidopsis flowers earlier under LD than under short days (SD), in agreement with its latitudinal distribution, mostly in temperate areas (Koornneef et al., 2004). However, natural accessions isolated at low latitudes show only a weak quantitative response to photoperiod (Alonso-Blanco et al., 1998). Most flowering time mutants, whether late or early, are characterized by a modified SD to LD flowering time ratio, and several of them have been described as day-neutral (Koornneef et al., 1991; Gaudin et al., 2001; Pouteau et al., 2004).
The transition to flowering, or floral switch, is a most critical phase change in plant ontogeny leading from vegetative to reproductive growth. For practical reasons, its actual timing is rarely determined as such. Flowering time is thus usually recorded at later stages based on macroscopic morphogenetic changes. It can be measured by direct temporal indicators such as the time to first floral bud opening or the number of days to bolting (DtB) in rosette species such as Arabidopsis. In the latter case, bolting is commonly used to divide the vegetative phase into subphases, rosette and primary inflorescence bearing cauline leaves (Haughn et al., 1995). Flowering time can also be estimated by indirect morphometric indicators such as the total number of nodes bearing leaves (TLN), i.e. the sum total of rosette leaf number (RLN) and cauline leaf number (CLN) below the secondary inflorescence bearing flowers without bracts.
The variation in the two types of temporal indicators seems essentially correlated across natural accessions of Arabidopsis and late-flowering mutants (Koornneef, 1991; Bagnall, 1993; Karlsson et al., 1993; Clarke et al., 1995; Kuittinen et al., 1997; Stratton, 1998), suggesting that the ontogenetic timing is tightly regulated possibly due to biophysical and/or physiological constraints on the rate of growth. However, in an extensive survey of early-flowering mutants, possible uncoupling between DtB and TLN suggested that they are not surrogates of each other but correspond to differentially regulated temporal components of plant ontogeny. The notion that earliness may impose or reveal greater developmental constraints than delayed flowering was supported by the finding that early flowering mutants exhibit a high level of pleiotropy and multiple changes in phenotypic plasticity (Pouteau et al., 2004).
To approach the question of how character intra-connectedness and ontogenetic dynamics may be related, it was of interest to compare different causes of variation in flowering time. In this work, the following issues were examined. First, what is the respective impact of photoperiodic and mutational variation in flowering time on the relationship between TLN and DtB? Secondly, can the evaluation of light quantity and period components in the wild-type response to photoperiod reveal similarity with mutant dynamic features? Thirdly, can an ontogenetic basis for the relationship between DtB and TLN be found by analysing changes in character correlativity in the primary inflorescence?
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Materials and methods |
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Plant material
The natural accessions Wassilewskija (Ws) and Columbia (Col-0) were used. The 61 T-DNA insertion lines in the Ws background, eav1–eav61, were obtained from the Versailles collection, INRA, France (Bechtold et al., 1993; Pouteau et al., 2001, 2004).
Growth conditions for flowering time assays
Mutant and wild-type seeds were sown on soil (Stender A240, Bluemendenwerk Stender GmbH, Germany) and grown in Sanyo Gallenkamp SGC660 growth cabinets at 20±0.2 °C and 70±2% relative humidity. The soil was kept moist by application of nutrient solution three times a week. Under the standard photoperiod (stPhP), the light during the whole day period was provided with mixed fluorescent and incandescent tubes, and the photon flux density measured at soil level was 230±20 and 2±0.2 µE m–2 s–1, respectively. Under the extended photoperiod (exPhP), the photosynthetically active light quantity was maintained at a constant level by providing mixed fluorescent and incandescent light during an 8 h period and incandescent light only during extension periods promoting no photosynthetic activity. Developmental uniformity was obtained by selecting the 10 most uniform plants on average 
12 d after sowing, bringing the plant density to one plant per pot, and rotating the trays three times a week.
Measurement of flowering time indicators
DtB was measured as the number of days from sowing to the first elongation of the floral stem at 0.1 cm height. The number of true leaves (RLN, CLN, TLN) produced by the apical meristem was recorded on bolted plants. No major variation was observed in 2–4 independent repeats for the mutants. The photosynthetic time to bolting (LtB) and PChron (‘photochron’; DtB h–1 of photoperiod) conversions of DtB were calculated as follows: LtB (days)=DtBxh under photosynthetically active light/24; PChron (days h–1)=DtB/photoperiod. Where appropriate, linear regressions of the relative variation of TLN with DtB were determined. The R dynamic index (d–1) corresponding to the slope of the linear regression was derived from the corresponding equation: TLN=a+RxDtB. R indices were analysed by linear regression slope comparison based on a t-test. Influential points were sought by calculating Cook's distances with the SAS software package (SAS Institute, 2000). Values of Cook's distance were <0.5 except for one case at 1.04 (see Table 1). For each independent regression across one variation factor (photoperiod in Ws, mutants, or natural accessions), low outlying values when present were usually distributed among the latest samples.
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Measurement of ontogenetic correlativity
Variability and character correlation were measured by transforming coefficients into quantitative variables according to Amzallag (2001). The quantity of variability was calculated by the coefficient of variation (CV=standard deviation/average). The coefficients of correlation (r'-values) were normalized with respect to the median degree of freedom (df) at all photoperiods: r2=(r')2x(median df/df). The quantity of correlation or connectance was estimated by transforming non-normally distributed r-values into normally distributed z-values: z=0.5xLn[(1+|r|)/(1–|r|)].
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Results |
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Analysis of phototrophic and true photoperiodic variation in flowering time in Ws
Seasonal changes in photoperiod affect two factors simultaneously: the light period and the quantity of light available for photosynthesis. These two factors are also affected by increasing photoperiod under constant light intensity (stPhP) in controlled environments. Under these conditions, the variation in DtB in the Ws natural accession of Arabidopsis is approximately linear for photoperiods ranging from 6 h to 14 h (Fig. 1A). The conversion of DtB into the corresponding number of days with photosynthetically active lights on (LtB) shows little variation across this range of photoperiod (Fig. 1A), suggesting that DtB is mainly determined by the quantity of light available for photosynthesis. Above a photoperiod of
14 h, DtB reaches a minimum and is not influenced by further increases in light quantity (Fig. 1A). This is commonly referred to as the critical photoperiod (Pc; Roberts and Summerfield, 1987), i.e. the photoperiod below which flowering is delayed. The variation in flowering time as measured by RLN and CLN reveals similarities and discrepancies compared with DtB (Fig. 1B). Under LD, the variation in both RLN and CLN parallels that of DtB, with a Pc lying between 14 h and 16 h. Below the Pc, however, the variation in RLN and CLN, unlike that of DtB, is linear only within a limited photoperiod window and a saturation is observed below a photoperiod of 8 h and between 12 h and 14 h, respectively. This is defined as the ceiling photoperiod (Pce; Roberts and Summerfield, 1987), i.e. the photoperiod at and above which the greatest delay in flowering occurs.
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To circumvent the additional photosynthetic effects of stPhP, the photoperiod can be artificially increased without modifying the total quantity of light available for photosynthesis by extending a reference photoperiod of 8 h with periods at low light intensity (extended photoperiods or exPhP; Martinez-Zapater and Somerville, 1990; Karlsson et al., 1993; Bagnall et al., 1995; Lee and Amasino, 1995). However, a side-effect of the exPhP conditions in Arabidopsis is that they trigger a typical shade avoidance response characterized by elongated hypocotyls, petioles, and limbs, indicating that light signalling is modified (data not shown; Karlsson et al., 1993; Lee and Amasino, 1995; Smith and Whitelam, 1997). Besides this shade-avoidance response, a number of quantitative changes are observed in the flowering response of Ws to exPhP compared with stPhP (Figs 1, 2). First, the variation in DtB below 14 h is non-linear and shows a gradual saturation with a Pce at
10 h. Secondly, the DtB response curve is shifted toward longer photoperiods and a shift interval of 2 h is observed for the Pc of all temporal indicators and for the Pce of CLN. Thirdly, the maximum level of CLN and minimum level of RLN are modified: under 10 h and 12 h exPhP, CLN exhibits a significant increase above the highest level under short stPhP (t=5.75 and 5.35, P <<0.1
) and under long exPhP, RLN decreases below the minimum value under stPhP (t=21.76, P <<0.1). The flowering responses under stPhP and exPhP also share common features: the minimum DtB and CLN under LD and the RLN response curve between 8 h and 16 h are little altered.
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The use of the LtB conversion reveals a differential requirement for photosynthetically active light across exPhP, unlike stPhP, with apparent Pc and Pce at 16 h and 10 h, respectively. This difference may account for true photoperiodic contribution to flowering below a mean baseline of 15.7 d of photosynthesis. By using an additional time indicator, the ‘photochron’ (PChron), a more similar response can be obtained for the two types of photoperiodic conditions (Fig. 2A). This suggests that PChron can be useful to discriminate the period versus light quantity component in the variation of DtB with photoperiod. Interestingly, a linear representation can be tentatively obtained with the reciprocal of PChron (Fig. 2B) and could be used for the prediction of flowering time in Ws at different photoperiods.
Analysis of the photoperiodic variation in the relative progression to flowering in Ws
The variation in the progression to flowering was visualized by plotting TLN against DtB (Fig. 3). Variation profiles were tentatively compared by calculating a linear regression where appropriate and by using the regression slope as an estimate of the dynamic variation, hereafter called the R dynamic index (Table 1). The results show differential dynamic variation across photoperiods. In Ws across stPhP, the relative variation of TLN with DtB is linear within a large photoperiod window (Fig. 3A) and the corresponding R index is not significantly different from the one determined across exPhP (Table 1; see supplementary Table T1 available at JXB online). Yet, the variation across exPhP appears continuously linear whilst the DtB and TLN temporal indicators are gradually uncoupled under short stPhP, leading to a plateau above 50 DtB (corresponding to an 8 h stPhP). Similar results were observed in the Col-0 natural accession (data not shown), indicating that they are not contingent on the deficiency in phytochrome D and light signalling in Ws (Auckerman et al., 1997).
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The LtB and PChron conversions provide further indications on the respective contribution of light quantity and period to the dynamic variation in Ws (Fig. 4). For obvious reasons, the LtB conversion does not modify the profile across exPhP. In contrast, the profile across stPhP is Z-shaped, with ceiling and basal plateaus below 10 h and above 16 h, respectively (Fig. 4A). This may point to the need for a higher photosynthetic input for flowering under stPhP than under exPhP, especially in LD. In contrast to the LtB conversion, the PChron conversion results in an almost complete coincidence of the profiles across stPhP and exPhP (Fig. 4B). Therefore, PChron may be a useful tool to examine the true photoperiodic response. The results also indicate that period effects on the relative progression to flowering are essentially unaltered by additional light quantity and signalling effects associated with the stPhP and exPhP conditions, respectively.
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Comparison of mutational and photoperiodic variation in the progression to flowering
Heterochrony in the eav1–eav61 early flowering mutants reflects common alterations in endogenous connectedness expressed at the organism level of organization. For this reason, although these mutants probably exhibit different molecular genetic alterations at a lower level of organization, it is considered that they can be analysed together as a coherent population. Their variation profiles under SD and LD were compared with that of Ws across photoperiods. Under SD, the eav mutant profile is separate from that of Ws across the stPhP (Fig. 3A; Pouteau et al., 2004), whilst under LD a consistent overlap is observed (Fig. 3B). Conversely, many mutants under SD localize near the variation profile of Ws across the exPhP (Fig. 3A) whilst under LD the overlap is only marginal (Fig. 3B). Yet, the corresponding R indices are significantly different in all cases (Table 1; see supplementary Table T1 available at JXB online).
To explore further to what extent some mutants may phenocopy the dynamic behaviour of the wild type in different environments, the variation of TLN with LtB and PChron was compared in Ws and the mutants (Fig. 4). The LtB and PChron conversions have contrasting impacts on the overlap between mutant and Ws distributions compared with the DtB response, and point to the importance of light response changes in the mutant dynamics. Under LD, the overall distribution of the mutants is expanded in the LtB conversion whilst it is compressed near the Ws values in the PChron conversion, indicating that trophic effects on the true photoperiodic response may be buffered in the mutant as in Ws (see above). Under SD, the LtB conversion results in a coincidence between the least precocious mutants and Ws at a 12 h stPhP (Fig. 4A) whilst the PChron responses of the mutants and Ws across stPhP and exPhP remain separate (Fig. 4B), suggesting that the mutant variation involves both light response phenocopy effects and period perception changes.
To examine the potential impact of different sources of genetic variation, the variation of TLN with DtB was analysed in a large collection of natural accessions and in a set of later flowering mutants based on data recently published by Lempe et al. (2005; Table 1; see supplementary Figs S1 and S2, and supplementary Table T1 available at JXB online). This study shows that the R index across natural accessions is conserved under different temperatures in LD but is significantly different in SD compared with LD (Supplementary Fig. S1 and Supplementary Table T1 available at JXB online). Interestingly, the variation profiles across natural accessions and later flowering mutants overlap, and the R index is conserved under SD. Similarities are also found with the R index in Ws across photoperiods but not with the R index across eav mutants (see supplementary Fig. S2 and supplementary Table T1 available at JXB online). These comparisons can only be provisional since the environmental conditions used in the work by Lempe et al. (2005) and the present study differ in a number of factors, in particular temperature. However, they seem to support the notion that early and late flowering mutants have a different dynamic behaviour.
Link between flowering time indicators and the specification of the primary inflorescence
The bolting node marks the beginning of the primary inflorescence characterized by the presence of cauline leaves subtending secondary inflorescences, or co-florescences, so that the size of the primary inflorescence, or cauline leaf zone, visualizes to some extent the relationship between the DtB and TLN temporal indicators. The analysis of the variation of CLN with DtB and TLN in Ws across photoperiods and in mutants under SD and LD (Fig. 5) reveals a number of important features.
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First, the variation of CLN with DtB in Ws across stPhP shows a saturation >35 DtB (corresponding to the Pce of CLN at 12 h), whilst the saturation is more limited for the variation of CLN with TLN (Fig. 5). These results raise the possibility that the measure of flowering time by the DtB indicator is overestimated for late flowering times and that TLN is potentially a more accurate temporal indicator. Secondly, under exPhP, the saturation in the variation of CLN with DtB is less pronounced, and more cauline leaves are produced with short extension periods (2 h and 4 h) than under an 8 h stPhP (Fig. 5A). In addition, the CLN-to-TLN ratio is globally higher across exPhP than across stPhP (Fig. 5B). This indicates that the global light regime and not only the photoperiod or flowering time per se influences the size of the cauline leaf zone. Thirdly, except for a few cases, the variation of CLN seems loosely connected to DtB in the mutants under both SD and LD compared with Ws (Fig. 5A), suggesting that no strong ontogenetic correlation exists between the DtB indicator and the specification of the cauline leaf zone. By contrast, the variation in the CLN- to-TLN ratio is grossly conserved between the mutants under SD and Ws across stPhP (Fig. 5B). Even under LD, most mutants localize in the continuity of the distribution observed for Ws across photoperiods. These results may indicate that the leaf ratio reflects an intrinsic developmental correlativity.
Changes in ontogenetic correlativity across photoperiods
Because the transition to flowering involves drastic morphogenetic changes, including the cessation of leaf production and start of flower organogenesis, the question was addressed as to whether extensive variation in this transition across photoperiods in Ws is accompanied by changes in ontogenetic correlativity. This can be estimated by measuring actual correlations between characters based on their r coefficient of correlation and z quantity of correlation or connectance (see Materials and methods). In addition, the quantity of character fluctuations, i.e. variability measured by the CV, can also provide an estimate of a loose or tight connection with other characters (Amzallag, 2001).
On average, the CV within independent experiments shows no significant variation for DtB across photoperiods. However, a significant increase is observed for RLN and CLN from 10 h to 12 h and from 12 h to 14 h, respectively (t=3.88 and t=3.37, P <1.5; Fig. 6A) resulting in a higher mean CV under LD than under SD. The CV between independent experiments is more variable, especially under intermediate photoperiods, with a peak at 12 h and a trough at 14 h which can be discriminated by variance comparison (risk=1%; Fig. 6B). These findings indicate that the susceptibility of RLN and CLN to changes in initial conditions and/or experimental microvariation is highest under photoperiods inducing the largest variation in flowering time (Fig. 1). Strikingly, the correlation between RLN and CLN is low at all photoperiods except for a major peak (r=0.69) at 12 h (Fig. 6C), corresponding to a high connectance (z=0.86). The coincidence of this peak with the major peak of variability suggests that stronger correlativity for the leaf ratio is associated with increased susceptibility to the environment and/or reduced connection with other characters. Conversely, a relaxed leaf correlativity under SD and LD, in particular at 14 h, seems to coincide with lesser environmental influence and/or tighter connection with other characters. An interpretation of the apparently higher variability above 16 h could be that an excess of light supply generates additional developmental instability.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary dataReferences |
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Based on extensive comparison between mutational and environmental changes in flowering time, a number of conclusions on photoperiodic regulation in Arabidopsis wild types and early flowering mutants can be discussed.
Discrimination of light period and phototrophic effects on flowering time
In modelling for the prediction of flowering time in annual crops under natural and therefore variable environments, the daily contribution of photoperiod to flowering time can be treated as additive increments, i.e. photoperiodic time, in the same way as thermal time (Roberts and Summerfield, 1987). Yet, the photoperiodic time is a measure not only of true photoperiodic response but also of phototrophic effects. Here it is shown that under stPhPs, the DtB of the Ws natural accesssion is a linear function of the light sum total available for plant photosynthesis below the critical photoperiod rather than the photoperiod per se. In contrast, the use of exPhPs that maintain a constant phototrophic input leads to a typical quantitative variation in the DtB function characterized by critical (Pc) and ceiling (Pce) photoperiods. Yet, plants also exhibit a strong shade avoidance response (Karlsson et al., 1993; Lee and Amasino, 1995; Smith and Whitelam, 1997; this work) unlike other species with an SD habit such as Impatiens (Battey and Lyndon, 1984; Pouteau et al., 1997). In addition, the Pc and Pce values, hence the notions of SD and LD, prove contingent not only upon temperature as reported for various species (Roberts and Summerfield, 1987), but also upon the phototrophic input. Other natural accessions of Arabidopsis probably share this phototrophic contingency, for example, Landsberg erecta and Col-0 under different irradiances (Corbesier et al., 1996).
Although both stPhP and exPhP present caveats, photosynthetic and light signalling modifications, respectively, that may interfere with the interpretation of flowering time, the present results suggest two ways to address true photoperiodic responses. First, the RLN and TLN norms of reaction appear more robust against environmental variation in the phototrophic input. It may thus be concluded that the leaf number indicator is a more operational measurement of true photoperiodic responses in Ws than the bolting date. Secondly, two new temporal indicators were derived from DtB, a photochron index corresponding to the DtB h–1 of photoperiod (PChron) and the photosynthetic time to bolting (LtB), that prove useful to discriminate the period and trophic components of photoperiod. Indeed, LtB shows little variation across stPhP, whilst the reciprocal of PChron is a linear function of photoperiod. Most strikingly, the variations of DtB with TLN across stPhP and exPhP, which are parallel but distinct, coincide when using the PChron conversion. Despite the fact that sugars are an important component of a multifactorial, mobile inductive signal for flowering (Bernier et al., 1993), it can be concluded that the trophic input necessary for flowering does not interfere with light signalling in a true photoperiodic response. This confirms the conclusions reached in analyses of flowering responses to modified photoassimilate and phytochrome levels (King and Evans, 1991; Bagnall et al., 1995). Similarly, Roberts and Summerfield (1987) showed that the responses to both temperature and photoperiod are linear and without interaction, thus allowing simplification of the prediction of photothermal responses based on the measurement of a small number of genetic coefficients.
Differential impact of photoperiodic and mutational changes on ontogenetic dynamics
The analysis of the variation profiles of DtB with TLN suggests that the dynamic impacts of environmental (external) and genetic (endogenous) changes operate through distinct processes. Indeed, the variation profiles across photoperiods and early flowering mutants are different. Yet, the use of exPhP contributes to reduce these differences, leading to a response profile that phenocopies to a large extent the mutant variation under SD. In addition, the LtB conversion also reveals convergent dynamic behaviour between Ws under a 12 h stPhP and some mutants under SD. These results and the various alterations in hypocotyl elongation observed under LD and/or SD and in the dark (Pouteau et al., 2004) suggest that changes in light quantity perception and signalling play an important part in the heterochronic modifications exhibited by the early mutant population. The finding that the mutant and Ws distributions remain separate with the PChron conversion also points to more profound ontogenetic changes, possibly associated with perturbations in global correlativity of developmental networks and explaining the high level of pleiotropy observed in the eav1–eav61 mutants (Pouteau et al., 2004).
The previous observations of a conserved linear variation between DtB and TLN in natural accessions and in mostly late-flowering mutants were re-examined based on a large set of data recently reported by Lempe et al. (2005). In spite of a significant difference in the variation profiles under LD, the behaviour of these mutants was globally similar to that of the natural accessions. Insofar as the different environmental conditions used by Lempe et al. and in the present work can be compared, this analysis thus brings support for the notion that the ontogenetic dynamics of early- and late- flowering mutants are different. Heterochronic changes in the timely onset, duration, and dynamics of the different ontogenetic phases (Wiltshire et al., 1994; Diggle, 1999) seem more likely to impose uncoupling between morphogenetic events and physiological processes in early than in late-flowering mutants.
Co-ordinate specification of the primary inflorescence boundaries
The analysis of the primary inflorescence, or cauline leaf zone, in Ws across photoperiods and in mutants provides further indication of the link between the two types of temporal indicators, DtB and TLN. Indeed, the basal and apical boundaries of the cauline leaf zone are determined by bolting and cessation of leaf differentiation, respectively. The present results suggest that the two boundaries vary co-ordinately in wild types and mutants, possibly due to biophysical and/or homeostatic constraints. Their timely specification, which is conserved in stPhP and exPhP, is altered in mutants. This heterochrony may result from the shortening of developmental phases with a different growth rate and/or photoperiod sensitivity. Indeed, early stages in Arabidopsis are characterized by a slower growth rate (Groot and Meicenheimer, 2000). It was also shown in soybean that more sensitive genotypes exhibit longer phases of photoperiod insensitivity (Upadhyay et al., 1994).
Alternatively, uncoupling between the bolting node and time may be a consequence of variation in leaf and flower specification relative to the time and nodal position at which the floral switch occurs. Indeed, due to flexible organ specification, the position of the apical boundary proves contingent on the potency of the inductive treatment since it can coincide with the switch node (Hempel and Feldman, 1994) or be specified at a lower node under more potent inductive conditions (Hempel et al., 1998). Likewise, in Impatiens, the first axillary flowers can be moved above or below the switch node by applying inductive conditions at early or late developmental time, respectively (Pouteau et al., 1998). It is thus likely that the position of the apical boundary fluctuates relative to the switch node due to environmental and genetic variation in the quantity and/or diffusion of the floral inductive signal and in plant age or ontogenetic stage when the floral switch occurs.
Similarly, internode specification may be gradual and susceptible to modification in the course of differentiation in response to environmental and/or genetic conditions and lead to fluctuations in the position of the bolting node. Internode elongation is first detected 52 h after the floral switch in Arabidopsis meristem microscopic analyses (Jacqmard et al., 2003) and becomes macroscopically visible at a much later stage. This implies that the basal boundary is specified either at the same time or after the apical boundary. The morphological discontinuity introduced by bolting, leading to the adoption of an erect bearing common to most flowering plants, is mediated by phytochromes and gibberellins (Koornneef et al., 1995; Devlin et al., 1998; King et al., 2001). Yet, the specification of the bolting node itself is still poorly understood. It is shown here that this ontogenetic specification is differentially regulated across photoperiods in Ws. The correlation between RLN and CLN appears highest under the strongest photoperiodic influence, i.e. an intermediate photoperiod of 12 h, whilst it is low under SD and LD. Because a high variability is also observed under a 12 h photoperiod and possibly reflects a reduced correlativity with other characters, this correlation peak may be interpreted as an endogenous compensation to maintain ontogenetic integrity. In conclusion, it is proposed that the cauline leaf zone may represent an important mediating zone in which environmental and endogenous fluctuations are ontogenetically integrated and possibly buffered through variation in its boundaries. This raises new questions for future work as to how this transition zone is established and whether it may be involved in developmental correlativity and possibly more robust to mutational perturbations.
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Supplementary data |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary dataReferences |
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Supplementary data consisting of two figures S1 and S2 and one table T1 can be found at JXB online.
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Acknowledgements |
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We thank Hervé Ferry for technical assistance with plant culture, and Yves Chupeau and Herman Höfte for their support. We are grateful to Nissim Gérard Amzallag, Jean-Pierre Rospars, and Fabien Chardon for discussion and helpful comments on the manuscript. This work and VF were partly supported by grant no. BI04-CT97-2340 from the European Union.
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CLN, number of cauline leaves; CV, coefficient of variation; DtB, days to bolting; exPhP, extended photoperiod; LD, long day; LtB, photosynthetic time to bolting; Pc, critical photoperiod; Pce, ceiling photoperiod; PChron, ‘photochron’, DtB h–1 of photoperiod; RLN, number of rosette leaves; SD, short day; stPhP, standard photoperiod; TLN, total number of leaves.
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TopAbstractIntroductionMaterials and methodsResultsDiscussionSupplementary dataReferences |
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