Hydrogen peroxide regulated salicylic acid– and jasmonic acid–dependent systemic defenses in tomato seedlings

Abstract

To investigate the regulation of hydrogen peroxide (H₂O₂) in SA- and JA-dependent systemic defense pathways in tomato seedlings damaged by cotton bollworm. The lower two leaves of four-leaf tomato plant seedlings were treated with cotton bollworms. The upper leaves were then removed at various time points and assessed for systemic changes in H₂O₂ levels, enzyme activity and defense-related genes expression. Wild-type seedlings, def-5 mutant seedlings (deficient in JA accumulation) and transgenic nahG seedlings(deficient in SA content) were constructed at four-leaf stage. Wild-type plants were further treated with dimethylthiourea (DMTU) and 2,5-dihydroxycinnamic acid methyl ester (DHC). Bollworm feeding in the lower leaves induced rapid systemic accumulation of H₂O₂ in the upper leaves. H₂O₂ accumulation further increased activities for SA- and JA-dependent enzymes including proteinase inhibitors (PIs), pathogenesis-related gene 1 (PR-1), cathepsin D inhibitor (CDI), β-1,3-glucanse (BGL) and polyphenol oxidase (PPO). Furthermore, H₂O₂ accumulation enhanced mRNA expression of BGL-2, PR-1, PI, CDI and PPO genes. Suppression of H₂O₂ accumulation using H₂O₂ scavengers substantially diminished these effects. mRNA expression of PR-1 was not induced in transgenic nahG plants. SA- and JA-dependent signaling pathways are involved in the tomato systemic defense responses to herbivores, and that H₂O₂ generation is required for both systemic pathways.

Keywords:
Heliothis armigera; hydrogen peroxide; jasmonic acid; salicylic acid; tomato seedling; defense response