SUMMARY HMG-I/Y proteins are characterized by the presence of AT-hook motifs, DNA binding domains that recognize AT-rich tracts of DNA. By facilitating protein:protein and protein:DNA interactions in the vicinity of these AT-rich binding sites, HMG-I/Y positively or negatively regulates gene expression. Several pea defence gene promoters have AT-rich tracts of DNA that are potential targets for modulation via HMG-I/Y. In this study, a comparison of the expression of a pea defence gene (DRR206) mRNA relative to the expression of HMG-I/Y mRNA was monitored by Northern analysis following the inoculation of a fungal pathogen, Fusarium solani or treatment with chitosan and a F. solani DNase (Fsph DNase). In pea pod endocarp tissue, HMG-I/Y expression was observed at high levels in untreated tissue and at lower levels 6 h following inoculation or wounding of the tissue. Western blots with an antipea HMG-I/Y polyclonal antibody also revealed that pea HMG-I/Y is expressed at decreased levels 6 h following inoculation or elicitor treatment. HMG-I/Y extracted from pea caused alterations in the gel migration of radio-labelled AT-rich sequences from the pea DRR206 promoter, suggesting that similar interactions could exist in vivo. Agroinfiltration was utilized to express the pea HMG-I/Y gene in tobacco containing a chimeric gene fusion of a promoter from the PR gene, DRR206, and the beta-glucuronidase (GUS) reporter gene. Transient expression of pea HMG-I/Y led to a decrease in GUS reporter gene activity in the heterologous tobacco system. These data implicate pea HMG-I/Y abundance in the down-regulation of DRR206 gene expression, and possibly HMG-I/Y depletion in the expression of defence genes in pea.