A possible mechanism for the apocynin-induced nitric oxide accumulation in plants.

Nitric oxide (NO) is a small, ubiquitous bioactive molecule, postulated as a broad spectrum anti-stress compound. The NADPH oxidase inhibitor apocynin induces the accumulation of endogenous NO in leaves of maize seedlings through a nitric oxide synthase (NOS)-like activity, and confers an augmented tolerance to UV-B-induced oxidative damage. Here we propose a mechanism for the apocynin-induced NO increase in plants. NOS catalyzes the oxidation of arginine to citrulline and NO. It is suggested that apocynin inhibits arginase, the enzyme that hydrolyzes L-arginine to urea and L-ornithine, increasing the arginine availability for arginine-dependent NO synthesis. Superoxide (O(2)(-)) is a strong NO scavenger due to its high reactivity with NO to give peroxynitrite (ONOO(-)). Superoxide is mainly produced by plant NADPH oxidase (pNOX). Inhibition of pNOX by apocynin at relatively high NO concentration, could reduces the formation of O(2)(-) and ONOO(-), increasing the availability of a huge amount of NO. We consider apocynin as a very attractive compound for studying NO-regulated processes in plants since it can replace the use of NO donors and overcome the subsequent technical problems.