Fig. 3.

Illustration of bacterial effectors that manipulate the plant ubiquitination pathway. The action of effectors delivered from Gram-negative bacteria into the plant cell cytoplasm through the T3SS or T4SS is depicted. The effectors exert their action by acting as E3 ubiquitin ligase with domains RING/U-box or F-box of SCF or NEL, SUMO or cysteine protease, that binds with proteasome subunit and interfere with signaling pathway. (A) The P. syringae T3E AvrPtoB has U-box/RING- E3 ubiquitin ligase activity that mimics plant E3. It has both kinase and E3 ligase activity that enable functions in the ubiquitination and degradation of PRRs (FLS2, BAK, CERK, EFR) and R proteins (Pto, Fen, Prf). This prevents the downstream MAPK cascade and inhibits PTI and degradation of R protein that leads to prevention of ETI. HopZ4 acts on the 26S proteasome subunit component Rpt6 and thereby preventing SA-mediated defense. (B) The R. solanacearum has GALA protein that has LRR and an F-box domain that mimic plant E3, thereby promoting disease. (C) A. tumefaciens VirF has F-box domain and it mimics SCF E3 ligase that targets the VIP1 and VirE2 proteolysis thereby enabling integration of the DNA to promote crown gall. (D) The Rhizobium sp. NopM is a NEL family E3 and it reduce the ROS production and NopT effector induces PCD. (E) The Xanthomonas sp. harbors many effectors that has been characterized from few species. The T3E XopP from Xoo has RING/U-box domain with E3 activity. XcvXopD has SUMO protease activity that promotes senescence in plants, XccXopD alters host components RGA and interferes with signaling to promote infection. Similarly, Xcv AvrXv4 possess SUMO protease activity, Xcv 85–10 XopI with F-box domain was identified, and XcvXopL with NEL domain effector and E3 activity has been reported.