The Protective Roles of S-adenosylmethionine Decarboxylase (SAMDC) Gene in Melon Resistance to Powdery Mildew Infection

Powdery mildew caused by Podosphaera xanthii (P. xanthii) is one of important diseases in melon. We have previously investigated the differential gene expression in the incompatible P. xanthii-melon interactions and identified one EST containing homologous sequences to S-adenosylmethionine decarboxylase (SAMDC) cDNA. Given this, SAMDC gene of Cucumis melo was cloned and designated as CmSAMDC in this study. It was 1,095 bp long and encoded a 364-amino acid peptide with a molecular mass of 40 kD. By sequence analyzing, the deduced CmSAMDC protein was shown to have two conserved regions of a putative proenzyme cleavage site and a PEST domain. In addition, the expressions of CmSAMDC in the resistant melon materials increased more sharply than in the susceptible the melon materials, and the higher polyamines (PAs) and hydrogen peroxide (H2O2) contents in resistant melon materials were found as well, which were accompanied by up-regulation of the stress-responsive defense enzyme activities. Over-expression of CmSAMDC in Arabidopsis resulted in greatly reduced pathogen infection in the inoculated leaves of transgenic lines, enhanced resistance to powdery mildew, and the enhanced resistance appeared to be associated with pathogen-induced cell death. Taken together, our results suggested that CmSAMDC and perhaps its orthologous genes might be involved in responses of plants to biotrophic pathogens.