Soybean cultivars 'Williams 82' and 'Maple Arrow' produce both urea and ammonia during ureide degradation

Abstract

The ability of two soybean (Glycine max L. [Merrill]) cultlvars, 'Williams 82' and 'Maple Arrow', which were reported to use different ureide degradation pathways, to degrade the ureides allantoln and allantoate was investigated. Protein fractions and total leaf homogenates from the fourth trifoliate leaves of both cultivars were examined for the ability to evolve either 14CO2 or [14C]urea from 14C-labelled ureides in the presence of various inhibitors. 14CO2 evolution from [2,7-14C]allantoate was catalysed by 25-50% saturated ammonium sulphate fractions of both cultivars. This activity was inhibited by acetohydroxamate (AHA), which has been used to inhibit plant ureases, but not by phenylphosphorodlamldate (PPD), a more specific urease inhibitor. Thus, in both cultivars, allantoate may be metabolized by allantoate amldohydrolase. This activity was sensitive to EDTA, consistent with previous reports demonstrating that allantoate amldohydrolase requires manganese for full activity. Total leaf homogenates of both cultivars evolved both 14CO2 and [14C]urea from [2,7-14C] (ureido carbon labelled) allantoin, not previously reported in either 'Williams 82' or in 'Maple Arrow'. In situ leaf degradation of 14C-labelled allantoin confirmed that both urea and CO2/NH3 are direct products of ureide degradation. Growth of plants in the presence of PPD under fixing and non-fixing conditions caused urea accumulation in both cultivars, but did not have a significant impact on total seed nitrogen. Urea levels were higher in N-fixing plants of both cultivars. Contrary to previous reports, no significant biochemical difference was found in the ability of these two cultivars to degrade ureides under the conditions used.