Ureide Accumulation in Response to Mn Nutrition by Eight Soybean Genotypes with N2 Fixation Tolerance to Soil Drying

was leaf gas exchange. The basis of the N2 fixation tolerance to water deficit in the eight selected PI lines is Nitrogen fixation in soybean (Glycine max Merr.) is especially not known. sensitive to soil drying. The basis of this sensitivity appears to be related to the fact that ureides are transported from the nodules, Recent studies have focused on ureide accumulation and the ureide concentrations increase with water deficits in leaves and feedback on nodule activity as being crucial in influresulting in an apparent feedback to nodules involving ureides that encing soybean N2 fixation activity. Dramatic accumulainhibit activity. Therefore, sustained ureide catabolism in the leaves tion of ureides in response to water deficits has been under water deficit appears to be critical for N2 fixation tolerance. observed in shoots of soybean grown in controlled enviRecently, eight plant introduction lines were identified that expressed ronments (deSilva et al., 1996; Serraj and Sinclair, substantial N2 fixation tolerance of water deficits. The focus of this 1996a) and in the field (Serraj et al., 1997; Purcell et al., study was to explore the basis for the tolerance previously observed 1998). Experiments in which ureide was fed to soybean in the eight lines. Specifically, the objective of this study was to plants showed that N2 fixation activity was readily inhibevaluate in these genotypes the dependence for ureide catabolism on ited as a result of increased ureide concentrations in the allantoate amidinohydrolase, which in other cultivars appears to be related to N2 fixation tolerance of water deficits. Since allantoate plant (Serraj et al., 1999b; Vadez et al., 2000). Conseamidinohydrolase does not require Mn as a co-factor in contrast to quently, ureide accumulation as a result of a failure in the alternate enzyme for allantoic acid catabolism, ureide accumulaureide catabolism in the shoot was hypothesized as an tion was measured in leaves of these genotypes after the plants were explanation of N2 fixation sensitivity in soybean to soil fed allantoic acid following growth on low Mn hydroponic solutions. drying (Serraj et al., 1999a). This treatment confirmed that ureide accumulation was independent Two enzymes have been identified for catalyzing alof Mn nutrition level in six of the eight tolerant lines. Ureide accumulalantoic acid breakdown in soybean. Shelp and Ireland tion in PI 429328 was consistently the most insensitive to Mn nutrition (1985) identified the catabolic enzyme in the cultivar level. Overall, these results indicated that ureide catabolism indepenMaple Arrow as allantoate amidinohydrolase (EC dent of Mn is active in six of the eight plant introduction lines identified 3.5.3.4). Winkler et al. (1987) could not confirm this to express N2 fixation tolerance to soil drying. observation in the cultivar Williams and found instead that allantoate amidohydrolase (EC 3.5.3.9) catalyzed allantoic acid degradation. This second enzyme was A in symbiotic N2 fixation of soybean early found to require Mn as a cofactor (Winkler et al., 1987; in soil drying has been known for some time (Kuo Lukaszewski et al., 1992). On the basis of responses to and Boersma, 1971; Sprent, 1971). A number of field Mn, Vadez and Sinclair (2000) subsequently concluded studies subsequently confirmed that N2 fixation in soythat these two cultivars, in fact, employed differing catabean was more sensitive to soil drying than was mass bolic enzymes as originally reported, and that there was accumulation, and that this sensitivity had a deleterious genetic variation in the Mn requirement involved in affect on yield potential (Serraj et al., 1999a). This sensiureide degradation. Further, Vadez and Sinclair (2001a) tivity is not universal among grain legumes and appears reported that Maple Arrow, which had the enzyme to be a trait of those species that transport ureides (allanseemingly not requiring Mn, expressed tolerance of N2 toin and allantoic acid) from the nodules to the shoot fixation to water deficit and that Williams, which re(Sinclair and Serraj, 1995). quired Mn, had N2 fixation sensitive to water deficits. Soybean genotypes have been identified, however, The possibility of differing catabolic enzymes for althat express substantial tolerance of N2 fixation to water lantoic acid leading to differences in N2 fixation sensitivdeficit. Sall and Sinclair (1991) identified the cultivar ity to water deficits opens the possibility of genotypic Jackson as having N2 fixation sensitivity to water deficit segregation based on ureide degradation characteristics. that was no worse than that of mass accumulation. This Ureide accumulation (Purcell et al., 2000) and degradaencouraged a screen of a large collection of soybean tion (Vadez and Sinclair, 2002) in the tolerant cultivar plant introduction lines ( 3000 lines) in an effort to Jackson is insensitive to Mn concentration in the leaves, identify lines that exhibited N2 fixation drought tolerindicating the presence of allantoate amidinohydrolase. ance (Sinclair et al., 2000). That study resulted in the Further, Vadez and Sinclair (2001a) compared ureide identification of eight plant introduction lines that had accumulation in leaves of nine soybean cultivars with N2 fixation that was more tolerant of soil drying than varying sensitivity of N2 fixation to water deficit, including five of the genotypes identified as being very tolerant T.R. Sinclair, USDA-ARS, and Agronomy Physiology and Genetics of soil drying, after growing them on nutrient solutions Laboratory; V. Vadez and K. Chenu, Agronomy Dep., Agronomy Physiology and Genetics Laboratory, Univ. of Florida, P.O. Box containing either 0or 6.6M Mn. Ureide accumulation 110965, Gainesville, FL 32611-0965. Received 16 July 2001. *Correin leaves of four of the tolerant lines was insensitive to sponding author ([email protected]). Mn in the nutrient solution and there was relatively low ureide accumulation even under a zero-Mn treatment. Published in Crop Sci. 43:592–597 (2003).