Ureide accumulation in response to Mn nutrition by eight soybean genotypes with N2 fixation tolerance to soil drying

Nitrogen fixation in soybean (Glycine mar Merr.) is especially sensitive to soil drying. The basis of this sensitivity appears to be related to the fact that ureides are transported from the nodules, and the ureide concentrations increase with water deficits in leaves resulting in an apparent feedback to nodules involving ureides that inhibit activity. Therefore, sustained ureide catabolism in the leaves under water deficit appears to be critical for N-2 fixation tolerance. Recently, eight plant introduction lines were identified that expressed substantial N-2 fixation tolerance of water deficits. The focus of this study was to explore the basis for the tolerance previously observed in the eight lines. Specifically, the objective of this study was to evaluate in these genotypes the dependence for ureide catabolism on allantoate amidinohydrolase, which in other cultivars appears to be related to N-2 fixation tolerance of water deficits. Since allantoate amidinohydrolase does not require Mn as a co-factor in contrast to the alternate enzyme for allantoic acid catabolism, ureide accumulation was measured in leaves of these genotypes after the plants were fed allantoic acid following growth on low Mn hydroponic solutions. This treatment confirmed that ureide accumulation was independent of Mn nutrition level in six of the eight tolerant lines. Ureide accumulation in PI 429328 was consistently the most insensitive to Mn nutrition level. Overall, these results indicated that ureide catabolism independent of Mn is active in six of the eight plant introduction lines identified to express N-2 fixation tolerance to soil drying.