Hydrogen peroxide-induced proline and metabolic pathway of its accumulation in maize seedlings

Exogenous H2O2 treatment led to a significant accumulation of proline in coleoptiles and radicles of maize seedlings. It also induced an almost immediate and rapid increase of activity of the key enzymes Δ1-pyrroline-5-carboxylate synthetase and glutamate dehydrogenase of the glutamate pathway of proline biosynthesis and an up-regulation of Δ1-pyrroline-5-carboxylate synthetase gene expression. Activities of the key enzymes arginase and ornithine aminotransferase of the ornithine pathway of proline biosynthesis increased only after 12 h of H2O2 treatment. Furthermore, the H2O2 treatment caused an early decrease of the activity of proline dehydrogenase, a key enzyme of proline degradation. These results indicate that H2O2 might be involved in signal transduction events, leading to proline accumulation in maize seedlings, and that the H2O2-induced proline accumulation is a combined result of the sequential activation of the glutamate and ornithine pathways of proline biosynthesis and the simultaneous inhibition of proline degradation by H2O2.