Expression of potential defense response genes in cotton

Abstract

Induction of genes needed to synthesize terpenoid compounds that have antifungal activity has previously been demonstrated to occur earlier in SBSI, a wilt resistant cotton cultivar, than in the susceptible cultivar, Rowden. In this study we have used cloned probes to test the same cultivars for induced expression of genes from other known defense response pathways. The time course of mRNA appearance was followed for up to 4 days in stele tissues of control plants and in plants inoculated with conidia of the wilt pathogen, Verticillium dahliae. Among genes that code for enzymes in the phenylpropanoid defense pathway, phenylalanine ammonia-lyase mRNA was constitutively expressed. Levels of mRNA transcripts coding for chalcone synthase, an enzyme required for flavonoid biosynthesis, and for caffeic-O-methyltransferase, an enzyme used in synthesis of lignin, were greatly elevated in inoculated plants as compared to water treated controls. Greater and/or earlier initial response was detected in the wilt resistant cultivar than in the susceptible cultivar. Among genes coding for enzymes that can degrade fungal cell walls, equivalent levels of β-1,3 glucanase mRNA were present in both fungal and water-treated plants. Chitinase mRNA synthesis was strongly induced in inoculated plants. Levels of message continued to increase in Rowden, but diminished rapidly after 48–60 h in SBSI.