The Xanthomonas campestris pv. vesicatoria Type III Effector Protein XopJ Inhibits Protein Secretion: Evidence for Interference with Cell Wall–Associated Defense Responses

Abstract

The phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria uses the type III secretion system (T3SS) to inject effector proteins into cells of its Solanaceous host plants. It is generally assumed that these effectors manipulate host pathways to favor bacterial replication and survival. However, the molecular mechanisms by which type III effectors suppress host defense responses are far from being understood. Based on sequence similarity, Xanthomonas outer protein J (XopJ) is a member of the YopJ/AvrRxv family of SUMO peptidases and acetyltranferases, although its biochemical activity has not yet been demonstrated. Confocal laser scanning microscopy revealed that green fluorescent protein (GFP) fusions of XopJ are targeted to the plasma membrane when expressed in plant cells, which most likely involves N-myristoylation. In contrast to a XopJ(C235A) mutant disrupted in the catalytic triad sequence, the wild-type effector GFP fusion protein was also localized in vesicle-like structures colocalizing together with a Golgi marker protein, suggesting an effect of XopJ on vesicle trafficking. To explore an effect of XopJ on protein secretion, we used a GFP-based secretion assay. When a secreted (sec)GFP marker was coexpressed with XopJ in leaves of Nicotiana benthamiana, GFP fluorescence was retained in reticulate structures. In contrast, in plant cells expressing secGFP alone or along with the XopJ(C235A) mutant, no GFP fluorescence accumulated within the cells. Moreover, coexpressing secGFP together with XopJ led to a reduced accumulation of secGFP within the apoplastic fluid of N. benthamiana leaves, further showing that XopJ affects protein secretion. Transgenic expression of XopJ in Arabidopsis suppressed callose deposition elicited by a T3SS-negative mutant of Pseudomonas syringae pv. tomato DC3000. A role of XopJ in the inhibition of cell wall-based defense responses is discussed.

Effector Biology of Plant-Associated Organisms: Concepts and Perspectives

“…This suggests that these effectors associate with dynamic focal processes that take place at pathogen penetration sites (Underwood and Somerville 2008). T3SS effectors of the plant pathogenic bacteria P. syringae and Xanthomonas campestris have been reported to target secretory pathways, suggesting that interference with polarized vesicle trafficking is a common pathogenesis process (Bartetzko et al 2009;Kaffarnik et al 2009;Nomura et al 2011). One example is the P. syringae effector HopM1 that localizes to the trans-Golgi network/early endosome to destabilize the host protein AtMIN7, also known as BEN1 regulating endosomal recycling (Tanaka et al 2009;Nomura et al 2011).…”

Functional Characterization of the Type III Secretion Substrate Specificity Switch Protein HpaC from Xanthomonas campestris pv. vesicatoria

“…T3S systems are essential pathogenicity factors of many Gram-negative plant-and animal-pathogenic bacteria and mediate the translocation of bacterial proteins, also referred to as type III effector proteins, into the cytosol of eukaryotic cells (12,31,60). Type III effector proteins presumably interfere with host cellular functions such as basal immune responses, to the benefit of the pathogen, and thus promote bacterial multiplication (5,12,36,69).…”

Dialogue between Xop-T3SS Effectors of <I>Xanthomonas oryzae Pv. Iryzae</I> and Rice during Bacterial Blight Pathogenesis

“…citri, X. campestris pv. campestris, Xanthomonas perforans have been documented (Kim et al, 2009;Lee et al, 2005;Ochiai et al, 2005;Salzberg et al, 2008), only a few of them have been studied extensively for their functional role in disease induction (AkimotoTomiyama et al, 2012;Bartetzko et al, 2009;Kim et al, 2009;Kumar and Mondal, 2013;Mondal et al, 2016;Song and Yang, 2010). The Indian Xoo strain has been studied recently in details for their composition of T3SS effectors.…”

“…The effectors of Xanthomonas outer protein (Xop) family (non-TAL effectors) function as major virulence determinants (Kim et al, 2008;Kumar and Mondal, 2013;White et al, 2009) in Xoo. The role of many Xop effectors, including XopD, XopJ, XopN, XopR, XopZ in governing virulence to Xanthomonas were demonstrated and the functional motifs of these Xop effectors were identified based on similarities with the know motifs or sequences in other Xanthomonads (Akimoto-Tomiyama et al, 2012;Bartetzko et al, 2009;Kim et al, 2009;Kumar and Mondal, 2013;Song and Yang, 2010). XopN of X. campestris pv.…”

“…vesicatoria was predicted to interact with tomato a typical receptor-like kinase 1 (TARK1) and four tomato 14-3-3 isoforms (Kim et al, 2009) during the suppression of PTI in tomato. XopD of the same pathogen was reported to desumolyze plant proteins (Kim et al, 2008) while XopJ was shown to be involved in the ubiquitin/proteasome system and help in removing SUMO proteins, resulting in the inactivation of SUMOlyated proteins (Bartetzko et al, 2009). XopN of Xanthomonas axonopodis pv.…”