Hydrogen peroxide enhances antioxidative defense in the leaves of water caltrop (Trapa bicornis) seedlings treated with lead

Hydrogen peroxide (H2O2) plays versatile roles in various biological processes and in responses to stress in plants. To investigate effects of exogenous H2O2 on the transcript levels of some genes encoding antioxidative enzymes, such as superoxide dismutase Cu/Zn (Cu/ZnSOD), catalase (CAT) and ascorbate peroxidase (APX), and also on the ultrastructural alterations caused by lead, 7-day-old seedlings of water caltrop (Trapa bicornis) were treated with lead salt (0.05 mmol/L) separately, or combined with H2O2 (2.5 mmol/L). Ultrastructural observation on the mesophyll cells demonstrated that the lead toxicity, which resulted in the membrane instability and abnormal organization within organelles, was eliminated under the condition of the co-treatment with H2O2 and lead. In this relation the combined treatment with H2O2 and lead prevented the increased amount of malondialdehyde to some extent during the whole experimental period (24, 48, and 72 h). Furthermore, quantitative measurements by reverse transcriptase-polymerase chain reaction (RT-PCR) showed that the co-treatment with both compounds, in contrast to the lead treatment alone, significantly up-regulated transcripts of CAT, APX, and a lignin biosynthesis related gene encoding ferulate-5-hydroxylase (F5H). The H2O2-induced F5H expression is thought to modify cell wall and thus lower Pb entry into water caltrop seedlings, thereby enhancing tolerance to Pb stress. Morphological and molecular evidences in the present study perhaps reflect the fact that some antioxidative enzymes, especially CAT and APX, combined with the action of F5H on cell wall modification, contribute to the enhanced antioxidative defense in the examined plants treated with lead.