Bacterial effectors mimicking ubiquitin-proteasome pathway tweak plant immunity

Abstract

Plant pathogenic Gram-negative bacteria evade the host plant immune system by secreting Type III (T3E) and Type IV effector (T4E) proteins into the plant cytoplasm. Mostly T3Es are secreted into the plant cells to establish pathogenicity by affecting the vital plant process viz. metabolic pathways, signal transduction and hormonal regulation. Ubiquitin-26S proteasome system (UPS) exists as one of the important pathways in plants to control plant immunity and various cellular processes by employing several enzymes and enzyme components. Pathogenic and non-pathogenic bacteria are found to secrete effectors into plants with structural and/or functional similarity to UPS pathway components like ubiquitin E3 ligases, F-box domains, cysteine proteases, inhibitor of host UPS or its components, etc. The bacterial effectors mimic UPS components and target plant resistance proteins for degradation by proteasomes, thereby taking control over the host cellular activities as a strategy to exert virulence. Thus, the bacterial effectors circumvent plant cellular pathways leading to infection and disease development. This review highlights known bacterial T3E and T4E proteins that function and interfere with the ubiquitination pathway to regulate the immune system of plants.

Keywords: E3 ubiquitin ligases; Effectors; Pathogen-associated molecular patterns; Plant immunity; T3E; Ubiquitin-proteasome system.

Fig. 1.

Activation of plant immune system. A two-tiered immune system comprising pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) occur in plants. The instant immune response is mediated by PRRs that triggers PTI whereas NLR activate ETI upon translocation of T3E into the cytoplasm via. T3SS. Signalling events by PRRs and NLRs leads to overlapping downstream cellular responses, including defense-gene expression, production of ROS and callose deposition. The primary plant immune receptors viz. PRRs and NLRs function synergistically to ensure a robust level of key immune components during ETI. SAR is induced when the ETI takes over the pathogenic effector and plants are able to resist secondary infection in uninfected parts of plants.

Fig. 2.

(A). Ubiquitination-26S proteasome pathway. Ubiquitin is first activated by ubiquitin-activating enzyme (E1), at the expenditure of ATP. Then, the ubiquitin molecule is passed on to the second enzyme, ubiquitin-conjugating enzyme (E2). The final enzyme, ubiquitin ligase (E3), recognizes target substrate and binds and labels it with the ubiquitin. The poly-ubiquitination is facilitated by E4, which transfers additional ubiquitin moieties. Proteins modified by sequential linkage of multiple ubiquitin residues of at least four via ubiquitin degradation are targeted by the 26S proteasome. In the non-proteasomal pathway, deubiquitinase (DUBs) catalyze the disassembly and editing of the Ub moieties attached to protein substrates. The various plant effectors that interfere with the UPS are indicated along the pathway. (B). Plant E3 ubiquitin ligases. E3 can be divided into HECT and RING/U-box domain-containing E3s based on their mode of transfer of Ub. RING E3s catalyze the transfer of Ub, whereas in the HECT, the E3 forms an intermediate and transfers to the target. The RING/U-box E3s are the multi-subunit complex of E3s viz., SCF, CUL3-BTB, CUL4- DDB1 and APC/C.

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.