Chitosan enhances resistance in rubber tree (Hevea brasiliensis), through the induction of abscisic acid (ABA)

Abstract

Chitosan is considered as a natural biodegradable compound that has potential to control plant diseases. In this work, we investigated the effect of chitosan in stimulating defense responses in rubber tree against Phytophthora palmivora. Foliar spraying of either 0.2% (w/v) chitosan or 50 μM abscisic acid (ABA) were efficient in the reduction of disease severity in P. palmivora infected rubber tree. In addition, the increases of enzyme activities, i.e. catalase, peroxidase, polyphenol oxidase and phenylalanine ammonia lyase, the expression of HbPR1, HbGLU, HbASI and HbCAT genes and the deposition of callose and lignin were observed in chitosan- or ABA-treated rubber tree leaves. Besides, a partial cDNA sequence for 9-cis-epoxycarotenoid dioxygenase (NCED), a key enzyme involved in ABA biosynthesis was firstly isolated from rubber tree using RT-PCR (designated HbNCED). The HbNCED gene fragment comprised of 753 bp and the deduced HbNCED protein showed 94% similarity to the NCED5 from Ricinus communis. Moreover, the up-regulation of HbNCED gene by exogenous chitosan was correlated with the induction of endogenous ABA measured by HPLC. Our results suggested that the exogenous chitosan could effectively reduce disease severity through the up-regulation of defense-related genes and ABA-biosynthesis gene, which led to the activation of defense-related proteins and ABA level in rubber tree.