Journal of Experimental Botany, Vol. 52, No. 354, pp. 153-159,
January 2001
© 2001 Oxford University Press
Original Papers |
Leaf ureide degradation and N2 fixation tolerance to water deficit in soybean1
USDA-ARS, Agronomy Department, Agronomy Physiology Laboratory, IFAS Building No. 350, 2005 SW 23rd Street, University of Florida, PO Box 110965, Gainesville, FL 32611-0965, USA
Received 10 April 2000; Accepted 29 August 2000
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Abstract |
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Accumulation of ureides in leaves is associated with the sensitivity of N2 fixation in soybean to soil water deficit. Consequently, ureide degradation in leaves may be a key to increasing soybean tolerance to dry soils. Previous research indicated that allantoic acid degradation is catalysed by different enzymes in cultivars Maple Arrow and Williams. The enzyme found in Williams requires manganese as a cofactor. The first objective of this study was to determine if the two degradation pathways were associated with differences in N2 sensitivity to soil water deficits. N2 fixation of Williams grown on low-Mn soil was sensitive to stress, but it was relatively tolerant when grown on soil amended with Mn. N2 fixation in Maple Arrow was relatively tolerant of soil drying regardless of the Mn treatment. The second objective of this study was to expand the study of the degradation pathway to nine additional genotypes. Based on ureide degradation in the presence and absence of Mn, these genotypes also segregated for the two degradation pathways. Those genotypes with the Mn-dependent pathway tended to have drought-sensitive N2 fixation, but there was one exception. The genotypes not requiring Mn for ureide degradation were drought-tolerant except for one genotype. These results demonstrated the possibility for increasing N2 fixation tolerance to soil water deficits in soybean by selection of lines with high ureide degradation rates, which were commonly associated with the Mn-independent pathway.
Key words: N2 fixation, soybeans, soil water deficit, ureide degradation, tolerance.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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Symbiotic N2 fixation in soybean is very sensitive to soil drying with a loss in activity occurring earlier in the drying cycle than apparently any other observed physiological process (Serraj et al., 1999a
). As a result of this loss in activity, yield losses appear to be a common feature of soybean production. While variations among soybean genotypes in the sensitivity of N2 fixation to soil drying have been identified (Sinclair et al., 2000), the physiological basis for this variation is unexplored.
This study on differences in N2 fixation sensitivity among selected soybean genotypes focuses on the possible involvement of ureides (allantoin and allantoic acid) levels in leaves because they are associated with decreased N2 fixation activity (Serraj et al., 1999b). It has been speculated that ureides are closely involved in a feedback mechanism between decreased catabolism of ureides in the shoot and inhibition of N2 fixation in the nodules (Serraj et al., 1999a). Furthermore, the great sensitivity of N2 fixation to developing soil water deficits is associated with large increases in leaf ureide concentration at the time that N2 fixation activity is decreasing (Serraj and Sinclair, 1997; Purcell et al., 1998).
The rate of ureide degradation in leaves may be critical for ureide accumulation in leaves when plants are subjected to water deficits. Research to elucidate the pathway for ureide degradation in soybean resulted in the possibility of two pathways in soybean for the breakdown of allantoic acid. Allantoate amidinohydrolase (EC 3.5.3.4) was identified first as the enzyme for the catabolism of allantoic acid leading to the production of urea, CO2 and NH4 (Shelp and Ireland, 1985). Subsequently, evidence was offered that the critical enzyme in allantoic acid degradation was allantoate amidohydrolase (EC 3.5.3.9), which did not produce urea (Winkler et al., 1987). It was shown that allantoate amidohydrolase required manganese as a cofactor for active allantoic acid degradation (Lukaszewski et al., 1992). Recently, however, recognition that these studies were done using differing genotypes of soybean resulted in evidence that both pathways exist within the soybean germplasm (Vadez and Sinclair, 2000).
The existence of two possible pathways for ureide degradation within soybean leads to the possibility of differences among genotypes in degradation capability that might be associated with ureide accumulation and N2 fixation sensitivity to soil water deficits. One important point of difference between the pathways is the involvement of Mn as a cofactor in one pathway, but is not required in the other pathway. Increased Mn availability to intact soybean plants decreases N2 fixation sensitivity to drought (Purcell et al., 2000). It was found that Mn application to the cultivar Biloxi increased leaf ureide degradation rate and lowered ureide accumulation under drought (Vadez et al., 2000). The objective of this study, therefore, was to examine the possible consequences of different ureide degradation characteristics among soybean genotypes in the response of N2 fixation to developing soil water deficits.
There were three components of this study. First, the two cultivars that were originally identified as having differing allantoic acid degradation pathways (Williams and Maple Arrow), were studied in a soil-drying experiment to examine possible differences between these cultivars in the sensitivity of N2 fixation to water deficits. The second component was to study in a soil-drying experiment the response of nine soybean genotypes that had been previously selected in a field screen for N2 fixation tolerance to soil water deficits. This experiment included measurements of ureide concentrations in the plant and ureide degradation rates in the leaves. The third aspect of this study was to investigate the possible differences in ureide degradation pathways in these plants by observing responses to Mn levels in hydroponic solutions.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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Comparison of Williams and Maple Arrow
Two seeds per pot were sown into 20 pots each for soybean cultivars Williams and Maple Arrow, the two cultivars used in the original studies of ureide degradation in soybean. The pots were constructed from PVC tubing and were 10 cm in diameter and 30 cm tall, and designed to allow in situ measures of acetylene reduction activity (ARA). The pots were each filled with about 2 kg of a 2:1 mixture (v/v) of potting soil (Vitagreen, Clermont, FL) and a vegetable plug mix (WR Grace & Co., Cambridge, MA). The soil was recovered from previous experiments so that the soil available Mn was low. A commercial preparation of Bradyrhizobium japonicum (Nitragin, Milwaukee, WI) was applied to the soil as the inoculant.
At plant emergence, a solution containing the following macro and micro elements was added to each pot: CaCl2 (3.3 mM), MgSO4 (2.05 mM), K2SO4 (1.25 mM), KH2PO4 (0.35 mM), H3BO3 (4 µM), ZnSO4 (1.55 µM), CuSO4 (1.55 µM), NaMoO4 (0.12 µM), and FeEDTA (40 µM) (Kalia and Drevon, 1985). Half the pots of each genotype were maintained with no added Mn while half of the pots were given a total of 6.9 mg Mn kg-1 soil in three weekly applications to each pot of 200 ml of 250 µM MnSO4.
About 1 week after emergence, the plants in each pot were thinned to one plant. The plants were grown in a greenhouse under well-watered conditions for 4 weeks before initiating a dry-down experiment. The day/night temperature in the greenhouse was approximately 28/20 °C, and the photoperiod was extended to 14 h with supplemental light using 60 W incandescent bulbs to prevent flowering.
At the beginning of the dry-down experiment a two-piece lid was attached to the top of each pot to facilitate the measurement of acetylene reduction activity (ARA) as described below. The pots were fully watered, allowed to drain overnight, and weighed for initial pot weight. Half the pots of each genotype and Mn treatment were maintained under well-watered conditions by adding water to the pots daily so that they were only 200 g less than the initial weight. The soil in the remaining pots was allowed to dry steadily as a result of transpirational water loss. To avoid rapid dessication of the soil of the drying pots, water was added to these pots as necessary to maintain the net daily water loss to 70 g.
The experiment was continued until daily transpiration rate of the plants on drying soil was less than 10% of the rate of the well-watered plants. For most plants the duration of the dry-down experiment was 2 weeks. The difference in weight between the initial and final pot weight was defined as the total transpirable soil water of each pot (Sinclair and Ludlow, 1986). From the pot weights measured each day, the fraction of transpirable soil water (FTSW) of each pot on each day of the experiment was calculated.
ARA was measured each afternoon on each plant during the experiment by the techniques described previously (Serraj and Sinclair, 1996a). Pots were flushed with air for about 2 h prior to measurement of ARA. A 9:1 (v:v) mixture of air:acetylene was passed through the inlet port at the bottom of the PVC pots at a flow rate of 1.0 l min-1. Equilibration time of 10 min was allowed to reach steady-state production of ethylene. The ethylene concentration of the exit gas was analysed using a gas chromatograph equipped with a flame ionization detector. Plants were flushed at least 45 min after taking the gas samples to remove acetylene and ethylene from the pots.
Within a Mn and genotype treatment, ARA was normalized for each water-deficit plant by dividing the daily ARA of individual plants by the daily mean of well-watered plants of the same Mn and genotype combination. A second normalization of ARA was performed by dividing each normalized value by the mean of normalized values of each individual plant for the first 4 d of the experiment. This was done to minimize plant-to-plant variations.
There was little change in the normalized ARA value until the FTSW had decreased to a threshold below which normalized ARA decreased nearly linearly with FTSW. Therefore, a basis of comparison for the sensitivity of N2 fixation to drying soil was the FTSW threshold for the initiation of the decline in ARA. The FTSW threshold for ARA decline was calculated using a plateau regression technique (Ray and Sinclair, 1997).
At the end of the dry-down experiment, ureide degradation rate in leaves was measured in nine replicate leaves harvested from well-watered plants of each Mn treatment and genotype. (The ureide degradation measurement is described later.) Subsequently, all plants were harvested, and shoot and nodules were separated, and all tissue was dried for 48 h at 70 °C before measuring manganese and ureide concentrations. Manganese concentrations were measured following ashing using atomic absorption spectrophotometry. Ureide concentrations were measured using a colorimetric method (Trijbels and Vogel, 1966).
Comparison of nine genotypes
Eight genotypes were selected for this study from the large screen of soybean germplasm in 1995 described previously (Sinclair et al., 2000). The original population was approximately 1200 plant introduction lines that were screened for low petiole ureide levels. Eighty lines were selected from this population for the estimation of N2 fixation activity under water-deficit conditions in the field. The eight plant introduction lines selected for this study had sustained N2 fixation activity under dry conditions. Sustained N2 fixation activity under these field conditions, however, does not necessarily mean that N2 fixation itself is tolerant of drying soil. For example, deeper rooting that results in superior plant water status can result in sustained N2 fixation in the field test even if there was no advantage specifically in N2 fixation tolerance of water deficit.
To examine directly the tolerance of N2 fixation to water deficit in these eight selected genotypes, a dry-down experiment was done using the experimental protocol with pots as described previously. Biloxi was included in this test because the great sensitivity of N2 fixation to water deficit in this cultivar is well documented (Serraj and Sinclair, 1997). Five replicate plants each for the well-watered and water-deficit treatments were used for each genotype to determine the FTSW threshold for the decline in ARA. In addition, the plants were harvested at the end of the experiment to measure leaf Mn concentration, and shoot and nodule ureide concentration. These analyses were done as described previously.
Ureide degradation capacity in the nine genotypes was measured in leaves harvested from plants grown on hydroponic solution. To highlight the potential difference in the two degradation pathways, this experiment was done by growing the plants on a solution that did not contain Mn. The seeds were inoculated with a commercial inoculant (Nitragin, Milwaukee, WI), germinated in soil, and after 1 week the seedlings were transplanted to 1.0 l Erlenmeyer flasks containing the N-free nutrient solution described previously. Urea (1 mM) was also added to the nutrient solution for the first 2 weeks only following transplanting. The nutrient solution was replaced at 2 weeks after transplanting and thereafter each week. The pH of the solution was maintained close to 7.0 by adding 0.2 g l-1 CaCO3 and air was continuously bubbled through the solution at a flow rate of 2.0 l min-1 (Serraj and Sinclair, 1996b). The volume of the nutrient solution was maintained at c. 500 ml so that most of the nodules developed above the nutrient solution. Plants were grown in a greenhouse with day/night temperatures of about 28/20 °C and a 14 h photoperiod.
The method used to measure in situ ureide degradation was that developed previously (Vadez and Sinclair, 2000). Three weeks following the transplantation of plants to the nutrient solution, the upper most fully expanded leaves were severed at the stem for use in the degradation test. The leaves were initially incubated on a 7.5 mM allantoic acid solution for 13 h under a combination of sodium and metal halide lamps (
500 µmol m-2 s-1; Sun-brella, Environmental Growth Chambers, Chagrin, Ohio) to elevate the ureide content of the leaves. Following the incubation, six leaf samples, each consisting of three 1.6 cm diameter leaf discs obtained by taking a single disc from each blade of the trifoliolate leaf, were incubated in 2 ml extraction vials. At regular intervals during this second incubation period (0, 1.25, 2.5, 3.75, 5.0, and 6.75 h), a vial would be taken for immediate extraction of ureide. The ureide concentration in the leaf was regressed over time to obtain an estimate of ureide degradation rate (µmol h-1 g-1 FW).
The initial ureide concentration in leaves after ureide feeding, Ur0, was found to have considerable variation among leaves (between 13 and 31 µmol g-1 FW). There was a trend for a positive correlation across all genotypes between ureide degradation rate and Ur0 (rate=0.021 Ur0+0.72; r=0.36, n=77, P
0.001). To facilitate comparisons among genotypes, the degradation of each individual leaf was adjusted to a rate at a common Ur0 set equal to 15 µmol g-1 FW (i.e. adjusted rate=observed rate-0.021[Ur0-15]).
Finally, to assess the overall growth of the nine soybean genotypes on no Mn in comparison to an adequate supply of Mn, a second hydroponic experiment was established. The experimental protocol was identical to the first experiment except for the added treatment of adequate Mn (6.6 µM). The plants were harvested after 7 weeks of growth on the hydroponic solutions and measurements were made on shoot and nodule weight, and leaf and nodule ureide concentration.
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Results and discussion |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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Comparison of Williams and Maple Arrow
Soybean cultivars Williams and Maple Arrow have different pathways for the degradation of allantoic acid in the leaves (Vadez and Sinclair, 2000
). These cultivars were subjected to drying soil in this experiment to determine if differences in degradation pathway were associated with tolerance of N2 fixation to water deficits. A difference in the sensitivity of N2 fixation to soil drying was found between the two cultivars when no Mn was added to the soil. The threshold for the decline in ARA expressed as the FTSW obtained from regression analysis was at a much higher soil water content in Williams than in Maple Arrow (Table 1). These results indicated that allantoate aminohydrolase, which exists in Maple Arrow, is advantageous for sustained N2 fixation with soil drying under low Mn conditions as compared to the allantoate amidohydrolase pathway in Williams. N2 fixation sensitivity to soil drying in Maple Arrow was not responsive to the addition of Mn to the soil, which is consistent with the presence of allantoate amidinohydrolase and no requirement for Mn.
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Addition of Mn to the soil caused the threshold FTSW for ARA decline to decrease in Williams to threshold values observed in Maple Arrow (Table 1
). Such sensitivity of the FTSW threshold decline in Williams to the addition of Mn is consistent with the allantoate amidohydrolase pathway and its requirement for Mn as a cofactor (Vadez and Sinclair, 2000). This observed decrease in the threshold with the addition of Mn to the soil is consistent with previous observations with the cultivar Biloxi (Vadez et al., 2000). The overall leaf Mn concentration in the two Mn treatments with Williams, however, were not statistically different (Table 1), although the Mn concentration in the 0 Mn treatment of Williams was below the optimum range identified earlier (Vadez et al., 2000). These results with Williams indicated that tolerance of N2 fixation to drought in some cultivars of soybean might be enhanced by ensuring an adequate supply of Mn to the plants.
Since ureide levels in the shoots of well-watered plants have been correlated with sensitivity of N2 fixation to soil drying (Serraj and Sinclair, 1997), differences in ureide levels were examined. No differences in nodule ureide concentration were found between cultivars or Mn treatments. On the other hand, shoot ureide concentrations in well-watered Maple Arrow plants were more than 3-fold of those in Williams (Table 1). This difference in ureide levels for these soil-grown plants was similar to those observed when these cultivars were compared after growing on hydroponic solutions (Vadez and Sinclair, 2000). Since the FTSW threshold for ARA decline was low in Maple Arrow, it appears there is an adaptation in this cultivar that avoids the triggering of N2 fixation decline associated with high shoot ureide concentrations.
Consistent with the difference between cultivars in shoot ureide concentration, the relative ureide degradation rates in the leaves of Williams was greater than in Maple Arrow (Table 1). The ureide degradation rate was roughly 50% greater in Williams than in Maple Arrow. The Mn treatment had no influence on ureide degradation rates in either cultivar.
The results for Williams challenge previous conclusions (Serraj et al., 1999) concerning the utility of leaf ureide concentrations and ureide degradation rates in indicating sensitivity of N2 fixation to water deficits. Differing soil Mn treatments clearly resulted in varying sensitivities of ARA decline to drying soils, but the Mn treatments resulted in no difference in shoot ureide levels or ureide degradation rate. In comparing the Mn treatments it may be necessary to examine the dynamic response of ureide concentrations and degradation rates as the soil dries. Shoot ureide concentrations in Williams increased substantially under the water-deficit treatment (Table 1), but the level of soil drying at which ureide concentrations increased may have differed between the Mn treatments.
Comparison of nine genotypes
The difference between Williams and Maple Arrow in the response to Mn opened the possibility that other genotypes may differ in leaf ureide levels, ureide degradation, and N2 fixation response to soil drying. The eight plant introduction lines identified in a broad-based field screen of soybean germplasm as potentially having N2 fixation tolerant of water deficits offered a resource for further study. As reported previously (Sinclair et al., 2001), the results of a soil-drying experiment in pots showed five of these lines had N2 fixation tolerance to water deficit, with three of the five having especially low FTSW thresholds for the decline in ARA (Table 2). The remaining three lines plus the cultivar Biloxi had relatively high FTSW threshold values.
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Among the nine genotypes tested there was also considerable variation in the level of shoot ureide ranging from 4.3 µmol g-1 DW for PI 222547 to 15.5 µmol g-1 DW for Biloxi. There was a tendency for lower values of shoot ureide to be associated with lower FTSW thresholds for the decline in ARA (Fig. 1
). Such a trend among genotypes has been reported previously (Serraj and Sinclair, 1997) between petiole ureide levels and the threshold of ARA decline. On the other hand, there was no consistent relationship with shoot dry weight for either the FTSW threshold or ureide concentrations (Table 2). Also, there was no significant difference in the well-watered ARA values among tolerant and sensitive genotypes (Table 2) indicating that low shoot ureide concentrations were not simply a result of poor N2 fixing rates.
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The ureide concentration in the nodules was considerably greater than in the shoots and there was a large range among the genotypes (Table 2
). Among the well-watered plants, however, there was no correlation between the nodule ureide concentration and the response of N2 fixation to the drying soil. Among the drought-stressed plants, nodule concentration was greatly increased but the high variability among genotypes was again not correlated with the response of N2 fixation to soil drying. The shoot ureide concentrations of stressed plants at the end of the soil drying experiment were much greater than the well-watered plants but all genotypes had equivalent concentrations (Table 2).
The nine genotypes were grown on a Mn-free hydroponic solution to provide leaf material for evaluation of ureide degradation rates with low Mn levels in the leaves. Variability among genotypes in the adjusted degradation rates was observed with values ranging from 0.58 µmol h-1 g-1 FW for Biloxi to 1.41 µmol h-1 g-1 FW for PI 222547. There was a significant correlation between higher degradation rates and lower FTSW thresholds for decline in ARA (Fig. 2). These results are consistent with the possibility of high degradation rates under low Mn conditions being associated with a tolerance of N2 fixation to soil drying.
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Finally, the nine genotypes were studied for growth when grown on hydroponic solutions containing either 0 or 6.6 µM Mn. A treatment that supplied no Mn was anticipated to be severe but expected to result in decreased activity of allantoate amidohydrolase, which requires Mn as a cofactor. Indeed, the lack of Mn in the nutrient solution decreased shoot and nodule growth in all but one of the genotypes (Table 3
). PI 429328 had no decrease in either shoot or nodule growth when grown with no Mn as compared to the normal Mn treatment. Although this genotype did have the poorest growth of all nine genotypes under normal Mn, this growth was not decreased further by inadequate Mn. It may be that this genotype has unique properties in several physiological processes that substantially diminish or eliminate the essentiality of Mn. On the other hand, shoot and nodule growth of PI 507039 were especially decreased in the treatment with no Mn. No obvious pattern in growth after 7 weeks between genotypes segregated by N2 fixation sensitivity to soil drying was observed.
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The ureide concentration in the leaves of the genotypes after growth on the adequate Mn hydroponic solution ranged between 4.6 µmol g-1 DW for PI 423886 to 11.3 µmol g-1 DW for PI 507039 (Table 3
). There was no clear distinction between the N2 fixation-tolerant and -sensitive genotypes for leaf ureide levels in the normal Mn treatment.
On the other hand, based on leaf ureide concentration for the 0 Mn treatment the genotypes segregated into two groups. One group of genotypes had ureide concentrations of 15.6 µmol g-1 DW or less and a second group had concentrations of 20.3 µmol g-1 DW or greater (Table 3). While this segregation did not align perfectly with ARA response to soil drying, most of the tolerant lines had low leaf ureide concentration and most of the sensitive lines had high ureide concentration. The segregation based on leaf ureide concentration was even more apparent when the difference in ureide concentrations between the 0 Mn and 6.6 µM Mn treatment was calculated. In this case, one group had a difference in ureide concentration between treatments of only 5.3 µmol g-1 DW or less and was generally associated with tolerance, while the second group had a difference of 12.8 µmol g-1 DW or more and was generally associated with sensitivity (Table 3).
The results of this last hydroponic experiment can be interpreted as segregating genotypes based on the ureide degradation pathway. Those genotypes that showed large increases in leaf ureide in the absence of Mn are likely to rely on the allantoate amidohydrolase pathway, which requires Mn as a cofactor in ureide catabolism. Without Mn, allantoic acid degradation is decreased causing an increase in leaf ureide. The group of genotypes with only small increases in leaf ureides under 0 Mn may have allantoate amidinohydrolase for allantoic acid degradation and do not require Mn for ureide breakdown. Four out of the five genotypes identified as having N2 fixation tolerant on drying soil appeared to have the degradation pathway not requiring Mn. This is consistent with the fact that Maple Arrow demonstrated N2 fixation tolerance to soil drying relative to Williams under low soil Mn conditions.
PI 507039 was the single genotype classified as having drought-tolerant N2 fixation that appeared not to have the allantoate amidinohydrolase pathway. This genotype was found to have the most tolerant N2 fixation to soil drying (Table 2) and also having plant growth that was the most sensitive to a complete lack of Mn in the hydroponic solution. The increase in leaf ureide in this genotype, in fact, may be an artefact of the 0 Mn treatment because the overall growth of this genotype was so negatively influenced by the lack of Mn. That is, other physiological processes that were very sensitive to the lack of Mn in PI 507039 may have caused an accumulation of nitrogenous compounds in the leaf when they were not consumed in growth. The modest Mn supply from the soil in the dry-down experiment allowed reasonable plant growth and low levels of ureides (Table 2).
On the other hand, PI 441355 appeared to have the allantoic acid degradation pathway not requiring Mn but it was classified as having N2 fixation sensitive to drought. Next to Biloxi, however, PI 441355 was measured to have the lowest ureide degradation rate by a fairly large extent. These results imply that there is variation in the activity of the allantoate amidinohydrolase pathway among genotypes and that it is necessary to have this pathway operating at a high level to confer N2 fixation drought tolerance.
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Conclusions |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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This study was undertaken to examine the influence of the two pathways for allantoic acid degradation on N2 fixation sensitivity to water deficits among several genotypes. The initial test was to compare the response of N2 fixation to drying soil between the cultivar Williams, which uses allantoate amidohydrolase and requires Mn as a cofactor, and Maple Arrow, which uses allantoate amidinohydrolase and does not require Mn. A drying experiment with these two cultivars and differing levels of soil Mn showed a clear difference. N2 fixation in Maple Arrow was relatively tolerant of the drying soil and there was no difference in response to Mn availability. On the other hand, N2 fixation in Williams was drought-sensitive under low Mn. With the addition of Mn to the soil, Williams exhibited drought-tolerance similar to Maple Arrow.
A comparison of nine additional genotypes offered additional differences among genotypes in N2 fixation sensitivity to water deficits. There was a general trend of increasing tolerance to water deficits being associated with decreased levels of shoot ureides in well-watered plants (Fig. 1) and with increased rates of ureide degradation by leaves (Fig. 2). An interesting comparison among these genotypes was the accumulation of ureides in the leaves of plants grown on hydroponic solutions with and without Mn. Four of the five genotypes that were classified as drought-tolerant had little additional ureide accumulation in the absence of Mn indicating the presence of the allantoate amidinohydrolase pathway. The fifth had large amounts of ureide accumulation, but the growth of this genotype was extremely poor on the no-Mn treatment and this may well have confounded the results. An additional plant introduction line had little additional ureide accumulation in the absence of Mn, but it was identified as having drought-sensitive N2 fixation. The very low ureide degradation rates, even if the allantoate amidinohydrolase pathway was being used, may have resulted in ureide accumulation and a feedback on nodule activity.
This research indicated that the pathway of allantoic acid degradation in leaves may be important in determining the sensitivity of N2 fixation in soybean to drying soil. The allantoate amidinohydrolase pathway appears to be generally advantageous in conferring drought tolerance, especially under soil conditions where the availability of Mn was low. Plants with the allantoate amidohydrolase pathway when supplied Mn, had high degradation rates and N2 fixation with increased tolerance to water deficits. Consequently, both genetic selection of the ureide degradation pathway and improved management of soil Mn availability appear to be viable approaches to minimize the deleterious consequences of inhibited N2 fixation activity with drying soils.
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Notes |
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1 Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply approval or the exclusion of other products that may also be suitable.
2 To whom correspondence should be addressed. Fax: +1 352 392 6139. E-mail: trsincl{at}gnv.ifas.ufl.edu
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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 T. R. Sinclair, V. Vadez, and K. Chenu Ureide Accumulation in Response to Mn Nutrition by Eight Soybean Genotypes with N2 Fixation Tolerance to Soil Drying Crop Sci., March 1, 2003; 43(2): 592 - 597. [Abstract] [Full Text] [PDF]
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V. Vadez and T. R. Sinclair Sensitivity of N2 Fixation Traits in Soybean Cultivar Jackson to Manganese Crop Sci., May 1, 2002; 42(3): 791 - 796. [Abstract] [Full Text] [PDF]
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