Molecular and functional profiling of Arabidopsis pathogenesis-related genes: insights into their roles in salt response of seed germination.

Pathogenesis-related (PR) proteins are a group of heterogeneous proteins encoded by genes that are rapidly induced by pathogenic infections and by salicylic acid (SA), jasmonic acid (JA) and ethylene (ET). They are widely used as molecular markers for resistance response to pathogens and systemic acquired response (SAR). However, recent studies have shown that the PR genes are also regulated by environmental factors, including light and abiotic stresses, and by developmental cues, suggesting that they also play a role in certain stress responses and developmental processes. In this work, we systematically examined the expression patterns of Arabidopsis PR genes. We also investigated the effects of environmental stresses and growth hormones on the expression of PR genes. We found that individual PR genes are temporally and spatially regulated in distinct patterns. In addition, they are differentially regulated by plant growth hormones, including SA, ABA, JA, ET and brassinosteroid (BR), and by diverse abiotic stresses, supporting the contention that the PR proteins play a role in plant developmental processes other than disease resistance response. Interestingly, PR-3 was induced significantly by high salt in an ABA-dependent manner. Consistent with this, a T-DNA insertional knockout plant with disruption of the PR-3 gene showed a significantly reduced rate of seed germination in the presence of high salt. It is thus proposed that PR-3 mediates ABA-dependent salt stress signals that affect seed germination in Arabidopsis. PR-4 and PR-5 also contributed to salt regulation of seed germination, although their effects were not as evident as those of PR-3.