Secretome analysis of the rice bacterium Xanthomonas oryzae (Xoo) using in vitro and in planta systems

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight disease in rice, and that severely affects yield loss (upto 50%) of total rice production. Here, we report a proteomics investigation of Xoo (compatible race K3)-secreted proteins, isolated from its in vitro culture and in planta infected rice leaves. 2DE coupled with MALDI-TOF-MS and/or nLC-ESI-MS/MS approaches identified 139 protein spots (out of 153 differential spots), encoding 109 unique proteins. Identified proteins belonged to multiple biological and molecular functions. Metabolic and nutrient uptake proteins were common up to both in vitro and in planta secretomes. However, pathogenicity, protease/peptidase, and host defense-related proteins were highly or specifically expressed during in planta infection. A good correlation was observed between protein and transcript abundances for nine proteins secreted in planta as per semiquantitative RT-PCR analysis. Transgenic rice leaf sheath (carrying PBZ1 promoter::GFP cell death reporter), when used to express a few of the identified secretory proteins, showed a direct activation of cell death signaling, suggesting their involvement in pathogenicity related with secretion effectors. This work furthers our understanding of rice bacterial blight disease, and serves as a resource for possible translation in generating disease resistant rice plants for improved seed yield.