SPERMIDINE BIOSYNTHESIS AS AFFECTED BY OSMOTIC-STRESS IN OAT LEAVES

A new assay for the evaluation of spermidine (Spd) synthase activity w as developed. It involves a coupled reaction and avoids the use of dec arboxylated S-adenosylmethionine, which is unstable and not easily ava ilable. This assay was applied to assess changes in enzyme activity in oat leaves subjected to osmotic stress in the dark. The results indic ate that osmotically-induced putrescine (Put) accumulation in cereals results not only from the activation of the arginine decarboxylase pat hway, but also from the inhibition of the activity of Spd synthase, th e enzyme which catalyzes the transformation of Put to Spd. Other possi bilities which could contribute to the decline of Spd and spermine lev els under osmotic stress are also discussed.