Inhibition of plant asparagine synthetase-by monoterpene cineoles

Asparagine (Asn) synthetase (AS) is the key enzyme in Asn biosynthesis and plays an important role in nitrogen mobilization. Despite its important phy siological function, little research has been done documenting inhibitors o f plant AS. Plant growth inhibition caused by the natural monoterpene 1,4-c ineole and its structurally related herbicide cinmethylin was reversed 65% and 55%, respectively, by providing 100 mu M Asn exogenously. Reversion of the phytotoxic effect was dependent on the concentration of Asn. The presen ce of either 1,4-cineole or cinmethylin stimulated root uptake of [C-14]Asn by lettuce (Lactuca sativa) seedlings. Although the physiological response s suggested that both compounds affected Asn biosynthesis, biochemical anal ysis of AS activity showed that the natural monoterpene was a potent inhibi tor (I-50 = approximately 0.5 mu M) of the enzyme, whereas the commercial p roduct was not inhibitory up to levels of 10 mM. Analysis of the putative m etabolite, 2-hydroxy-1,4-cineole, showed that the cis-enantiomer was much m ore active than the trans-enantiomer, suggesting that the hydroxyl group wa s involved in the specific ligand/active site interaction. This is the firs t report that AS is a suitable herbicide target site, and that cinmethylin is apparently a proherbicide that requires metabolic bioactivation via clea vage of the benzyl-ether side chain.