Isolation and characterization of effector genes during symbiotic interaction between Chinese cabbage and Piriformospora indica

Piriformospora indica is a mutualistic endophyte with a broad host range and great application potential in agriculture. Recent studies indicate that, like pathogenic microbes, mutualistic microbes use effector proteins to adjust plant physiology resulting a promotion of symbiosis with plant. The study of P.indica effector proteins and their mechanism can be a great model for endophyte symbiosis. Consequently, the objective of our study is to establish a examine system for identification and functional characterization of P.indica effector proteins. Based on previous studies, double subtractive cDNA library and bioinformatics methods, we picked nine genes to exam their expression pattern during symbiosis stage and the subcellular localization in plant cell. PIIN_09643 and PIIN_11103 are both induced during symbiosis stage, and their encoded proteins have the subcellular localization in nucleus. For functional study of these putative effector genes, we established transgenic hairy-root system of Chinese cabbage and recombinant protein system of Escherichia coli to investigate the role of these genes during symbiosis. In hairyroot system, we found PIIN_09643 and PIIN_11103 significantly suppress the expression level of SA pathway marker gene, PR1; and cause a promotion of symbiosis colonization. However, for JA pathway marker gene, PIIN_09643 and PIIN_11103 do not directly regulate the expression level of PDF1.2. As the supporting data, the treatment of PIIN_09643 and PIIN_11103 recombinant protein to Chinese cabbage leaves show a similar effect on expression level of PR1 and PDF1.2. It suggests that P. indica apply PIIN_09643 and PIIN_11103 to suppress SA response, and further facilitate the symbiosis process in planta. Moreover, we found PIIN_09643 and PIIN_11103 can enhance the expression level of redox status relative genes, DHAR5 and GSTU. It might be the indirect result of the downregulation of SA pathway.