Plant Physiol, June 2000, Vol. 123, pp. 725-732
Inhibition of Plant Asparagine Synthetase by Monoterpene
Cineoles1
Joanne G.
Romagni,
Stephen O.
Duke, and
Franck E.
Dayan*
United States Department of Agriculture, Agricultural Research
Service, Natural Products Utilization Research Unit, P.O. Box 8048, University, Mississippi 38677
Asparagine (Asn) synthetase (AS) is the key enzyme in Asn
biosynthesis and plays an important role in nitrogen mobilization. Despite its important physiological function, little research has been
done documenting inhibitors of plant AS. Plant growth inhibition caused
by the natural monoterpene 1,4-cineole and its structurally related
herbicide cinmethylin was reversed 65% and 55%, respectively, by
providing 100 µM Asn exogenously. Reversion of the
phytotoxic effect was dependent on the concentration of Asn. The
presence of either 1,4-cineole or cinmethylin stimulated root uptake of
[14C]Asn by lettuce (Lactuca sativa)
seedlings. Although the physiological responses suggested that both
compounds affected Asn biosynthesis, biochemical analysis of AS
activity showed that the natural monoterpene was a potent inhibitor
(I50 = approximately 0.5 µM) of the
enzyme, whereas the commercial product was not inhibitory up to levels of 10 mM. Analysis of the putative metabolite,
2-hydroxy-1,4-cineole, showed that the cis-enantiomer was much more
active than the trans-enantiomer, suggesting that the hydroxyl group
was involved in the specific ligand/active site interaction. This is
the first report that AS is a suitable herbicide target site, and that
cinmethylin is apparently a proherbicide that requires metabolic
bioactivation via cleavage of the benzyl-ether side chain.
1
This work was supported in part by the
Biotechnology Research and Development Corporation.
*
Corresponding author; e-mail fdayan{at}ag.gov; fax 662-915-1035.
© 2000 American Society of Plant Physiologists
