Evidence for a relationship between malate metabolism and activity of 1-sinapoylglucose: L-malate sinapoyltransferase in radish (Raphanus sativus L.) cotyledons.

The control of malate metabolism and stimulation of 1-sinapolyglucose: L-malate sinapoyltransferase (SMT) activity in radish (Raphanus sativus L. var. sativus) cotyledons has been studied. The light-induced and nitrate-dependent activity of SMT catalyzes the formation of O-sinapoly-L-malate via 1-O-sinapoyl-β-D-glucose. When dark-grown radish seedlings, cultivated in quartz sand with nutrient solution containing NO 3 (-) as the sole N source, were treated with light, SMT activity increased concomitantly with free malate in the cotyledons. This light effect was suppressed in seedlings grown in a culture medium which contained in addition to NO 3 (-) also NH 4 (+) . However, treatment with methionine sulfoximine neutralized this ammonium effect, resulting again in both rapid accumulation of malate and rapid increase in SMT activity. When seedlings grown on NO 3 (-) nitrogen were subsequently supplied with NH 4 (+) nitrogen, the accumulated level of L-malate rapidly dropped and the SMT increase ceased. The enzyme activity decreased later on, reaching the low activity level of plants which were grown permanently on NO 3 (-) /NH 4 (+) -nitrogen. An external supply (vacuum infiltration) of malate to excised cotyledons and intact seedings, grown on NO 3 (-) /NH 4 (+) -nitrogen medium, specifically promoted a dose-dependent increase in the activity of SMT. In summary these results provide evidence indicating that the SMT activity in cotyledons of Raphanus sativus might be related to the metabolism of malic acid.