Figure 8. Working model of SSB_X function.

SSB_Xoc, like Hpa1, which are regulated at the transcriptional level by HrpG and HrpX, are potentially secreted into the plant apoplast via the T3SS. The grey circle and black triangle indicate SSB_X and Hpa1 proteins that may be secreted through the Hrp pilus (encoded by hrpE), but not translocated through the translocon as other T3SEs (different white shapes), and then are possibly recognized by an unidentified receptor (question mark) which associates with BAK1. This interaction may result in phosphorylation of BIK1 and subsequent phosphotransfer to the MAPK cascade to activate the expression of genes involving in SA-, JA- and Eth-signaling pathways that lead to induced resistance (SAR and/or ISR) accompanied by callose deposition on cell walls and enhanced plant growth. MAPK signaling regulates NADPH oxidase-dependent oxidative burst in the early stages of plant defense.