Function of plant defense secondary metabolite in cytokinin degradation.

The flavoenzyme cytokinin dehydrogenase (CKX) catalyzes an irreversible deactivation of plant hormones cytokinins through oxidative cleavage of the cytokinin side chain to yield adenine or adenosine and an aldehyde. In the catalytic cycle of CKX, the cytokinin-reduced flavin cofactor is reoxidized by a suitable electron acceptor. We have recently demonstrated that the oxidation products of natural hydroxamic acid 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) function as effective electron acceptors of apoplastic CKX from maize. The stable oxidation product of DIMBOA reacting with peroxidase or laccase was identified as 4-nitrosoresorcinol 1-monomethyl ether (coniferron), which, however, is only a weak electron acceptor of CKX. Further analyses suggested formation of transient free radicals that were estimated to reoxidize the cytokinin-reduced flavin cofactor of CKX with the rates comparable to those of flavin reduction.