INDUCTION OF ALTERNATIVE OXIDASE SYNTHESIS BY HERBICIDES INHIBITING BRANCHED-CHAIN AMINO-ACID SYNTHESIS

Sycamore suspension cells (Acer pseudoplatanus L.) were incubated in t he presence of sulfonylurea and imidazolinone herbicides. These inhibi tors of acetolactate synthase (ALS), a key enzyme of branched-chain am ino acid synthesis, triggered a dramatic induction of the alternative oxidase (AOX). AOX activity increased in treated cells, eventually exc eeding cytochrome (cyt) pathway activity. This induction of AOX activi ty was correlated with the accumulation of a 35 kDa AOX protein in iso lated mitochondria, detected by Western blotting with a monoclonal ant ibody against Sauromatum guttafum AOX. It was preceded by the accumula tion of putative 1.6 kb AOX mRNA, detected using an Aox cDNA probe fro m soybean. The metabolic perturbations induced by the herbicides rathe r than the herbicide molecules themselves were responsible for this in duction of AOX. However, alpha-oxobutyrate (one of the substrates of A LS) and its transamination product, alpha-aminobutyrate, which accumul ated after herbicide treatment, were not involved. The inhibition of b ranched-chain amino acid synthesis was probably somehow responsible fo r the AOX induction since: (i) a mixture of those amino acids (leucine , isoleucine, valine) prevented AOX induction by ALS inhibitors; (ii) the herbicide Hoe 704, a potent inhibitor of acetolactate reducto-isom erase (the enzyme following ALS in the branched-chain amino acid pathw ay), also triggered AOX induction.