Ectopic Expression of a Proteinase Inhibitor I4 (MtPiI4) Gene from Medicago truncatula Confers Plant Resistance to Pseudomonas syringae pv. Tomato DC3000

Proteinase inhibitors (PIs) play an important role in plant responses to biotic and environmental stimuli, but little is known about the role of PIs in mediating plant immune responses to microbial infection. In this study, a gene named proteinase inhibitor I4 (MtPiI4) was isolated from Medicago truncatula and characterized as a serpin family gene with a typically conserved DUF716 domain. MtPiI4 was differentially expressed in seed, root, leaf, stem and flower tissues. Expression of MtPiI4 was induced by inoculation with a typical bacterial pathogen Pseudomonas syringae pv. tomato DC3000 strain (Pst DC3000). It was also up-regulated by methyl jasmonate (MeJA) treatment. To identify its function in regulating plant immunity against Pst DC3000, we constructed transgenic Arabidopsis plants over-expressing MtPiI4. Compared to wild type, 35S::MtPiI4 plants showed enhanced resistance to Pst DC3000. Expression of JA biosynthetic and responsive genes such as LOX2, PDF1.2, and VSP1 was depressed in 35S::MtPiI4 plants as compared to wild type, suggesting that the JA signaling response was attenuated in 35S::MtPiI4 plants upon Pst DC3000 exposure. Furthermore, over-expression of MtPiI4 led to up-regulation of NPR1 (nonexpressor of pathogenesis-related gene 1—a negative regulator of JA signaling) and down-regulation of MAPK4 (mitogen-activated protein kinase4—a positive regulator of JA signaling). These results indicate that MtPiI4 regulation of plant resistance to Pst DC3000 is involved in the JA signaling transduction pathway.