Activity of ATP sulfurylase in reproductive soybean

A low concentration of the S-containing amino acids, cysteine and methionin e, often limits the nutritive value of soybean [Glycine max (L.) Merr.] see d protein. On a whole-plant level, the localization and duration of cystein e and methionine synthesis during seed filling in soybean have not been rep orted previously. The objective of this study was to characterize the local ization and duration of ATP sulfurylase activity in reproductive soybean. A TP sulfurylase is the first enzyme in the S-reduction pathway. Samples of s oybean (cv. Kenwood 94) root, leaf, stem, pod wall, and seed tissues on a w hole-plant basis were taken at beginning seed filling (approximately R5.5) from two field sites in Iowa in 1997. In addition, samples of leaf, pod, an d seed tissue from the same nodal position (Node 11) were collected weekly from R5 to R8 (full maturity) at one of the sites. At R5.5 almost half of w hole-shoot ATP sulfurylase activity was Located in seed tissue. Leaf tissue contributed approximately 30 to 40% of the shoot ATP sulfurylase activity, and stems and pod wall contributed approximately 10% each. The profile of specific activity (fresh weight basis) for leaf, pod, and seed tissue at no de 11 over time showed that activity of leaf tissue steadily declined durin g seed filling, whereas that of seed tissue increased initially with seed e xpansion and remained high until physiological maturity. Thus, in terms of both specific activity, and sustained duration of activity, seed tissue was the dominant site of ATP sulfurylase activity during seed filling. It appe ars, then, that most of the S-containing amino acids synthesized during rep roductive growth may be synthesized in the seed itself, rather than being i mported from other tissues.