REGULATORY EFFECTS OF EXOGENOUS BRANCHED-CHAIN AMINO-ACIDS IN NICOTIANA-PLUMBAGINIFOLIA CELL-SUSPENSION CULTURES

Nicotiana plumbaginifolia suspension cultured cells were grown on medi um supplemented with valine, leucine and isoleucine, singly or in comb ination. The effects of the three branched-chain amino acids on cell g rowth rate and on the activity of acetohydroxyacid synthase (AHAS), th e first enzyme (and the main regulative site) of their biosynthetic pa thway, were studied. Results showed that valine and leucine, at concen trations ranging from 10(-4) to 10(-3) M, inhibit growth, and at highe r doses (from 10(-2) to 10(-1) M) AHAS activity. Growth, but not AHAS activity, was affected also by isoleucine. The addition of ammonium su ccinate to the culture medium, in order to counteract a possible gener al inhibitory effect of these compounds on nitrogen metabolism, reliev ed only partially their cytotoxicity. Feeding cells with equimolar mix tures of the three amino acids resulted in a minor but reproducible de crease in AHAS level, which was proportional to the dose. A similar re sult was obtained also on N. plumbaginifolia seedlings, suggesting tha t in this species a modulation of enzyme level could play a role in co ntrolling the flow of metabolites through the pathway.