Solé Magali, Scheibner Felix, Hoffmeister Anne-Katrin, Hartmann Nadine, Hause Gerd, Rother Annekatrin, Jordan Michael, Lautier Martine, Arlat Matthieu, Büttner Daniela
Institute of Biology, Department of Genetics, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany.
Institute of Biology, Department of Genetics, Martin Luther University Halle-Wittenberg, Halle (Saale), Germany
J Bacteriol. 2015 Sep;197(17):2879-93. doi: 10.1128/JB.00322-15. Epub 2015 Jun 29.
Many plant-pathogenic bacteria utilize type II secretion (T2S) systems to secrete degradative enzymes into the extracellular milieu. T2S substrates presumably mediate the degradation of plant cell wall components during the host-pathogen interaction and thus promote bacterial virulence. Previously, the Xps-T2S system from Xanthomonas campestris pv. vesicatoria was shown to contribute to extracellular protease activity and the secretion of a virulence-associated xylanase. The identities and functions of additional T2S substrates from X. campestris pv. vesicatoria, however, are still unknown. In the present study, the analysis of 25 candidate proteins from X. campestris pv. vesicatoria led to the identification of two type II secreted predicted xylanases, a putative protease and a lipase which was previously identified as a virulence factor of X. campestris pv. vesicatoria. Studies with mutant strains revealed that the identified xylanases and the protease contribute to virulence and in planta growth of X. campestris pv. vesicatoria. When analyzed in the related pathogen X. campestris pv. campestris, several T2S substrates from X. campestris pv. vesicatoria were secreted independently of the T2S systems, presumably because of differences in the T2S substrate specificities of the two pathogens. Furthermore, in X. campestris pv. vesicatoria T2S mutants, secretion of T2S substrates was not completely absent, suggesting the contribution of additional transport systems to protein secretion. In line with this hypothesis, T2S substrates were detected in outer membrane vesicles, which were frequently observed for X. campestris pv. vesicatoria. We, therefore, propose that extracellular virulence-associated enzymes from X. campestris pv. vesicatoria are targeted to the Xps-T2S system and to outer membrane vesicles.
The virulence of plant-pathogenic bacteria often depends on TS2 systems, which secrete degradative enzymes into the extracellular milieu. T2S substrates are being studied in several plant-pathogenic bacteria, including Xanthomonas campestris pv. vesicatoria, which causes bacterial spot disease in tomato and pepper. Here, we show that the T2S system from X. campestris pv. vesicatoria secretes virulence-associated xylanases, a predicted protease, and a lipase. Secretion assays with the related pathogen X. campestris pv. campestris revealed important differences in the T2S substrate specificities of the two pathogens. Furthermore, electron microscopy showed that T2S substrates from X. campestris pv. vesicatoria are targeted to outer membrane vesicles (OMVs). Our results, therefore, suggest that OMVs provide an alternative transport route for type II secreted extracellular enzymes.
许多植物致病细菌利用II型分泌（T2S）系统将降解酶分泌到细胞外环境中。T2S底物可能在宿主-病原体相互作用过程中介导植物细胞壁成分的降解，从而促进细菌毒力。此前，野油菜黄单胞菌疮痂致病变种的Xps-T2S系统被证明有助于细胞外蛋白酶活性以及一种与毒力相关的木聚糖酶的分泌。然而，野油菜黄单胞菌疮痂致病变种其他T2S底物的身份和功能仍然未知。在本研究中，对野油菜黄单胞菌疮痂致病变种的25种候选蛋白进行分析，鉴定出两种II型分泌的预测木聚糖酶、一种假定的蛋白酶和一种先前被鉴定为野油菜黄单胞菌疮痂致病变种毒力因子的脂肪酶。对突变菌株的研究表明，鉴定出的木聚糖酶和蛋白酶有助于野油菜黄单胞菌疮痂致病变种的毒力和在植物体内的生长。当在相关病原体野油菜黄单胞菌野油菜致病变种中进行分析时，野油菜黄单胞菌疮痂致病变种的几种T2S底物独立于T2S系统分泌，这可能是由于两种病原体在T2S底物特异性上的差异。此外，在野油菜黄单胞菌疮痂致病变种T2S突变体中，T2S底物的分泌并未完全缺失，这表明其他转运系统对蛋白质分泌有贡献。与此假设一致，在野油菜黄单胞菌疮痂致病变种中经常观察到的外膜囊泡中检测到了T2S底物。因此，我们提出野油菜黄单胞菌疮痂致病变种的细胞外毒力相关酶靶向Xps-T2S系统和外膜囊泡。
植物致病细菌的毒力通常取决于TS2系统，该系统将降解酶分泌到细胞外环境中。目前正在对几种植物致病细菌中的T2S底物进行研究，包括在番茄和辣椒中引起细菌性斑点病的野油菜黄单胞菌疮痂致病变种。在这里，我们表明野油菜黄单胞菌疮痂致病变种的T2S系统分泌与毒力相关的木聚糖酶、一种预测的蛋白酶和一种脂肪酶。对相关病原体野油菜黄单胞菌野油菜致病变种的分泌分析揭示了两种病原体在T2S底物特异性上的重要差异。此外，电子显微镜显示野油菜黄单胞菌疮痂致病变种的T2S底物靶向外膜囊泡（OMV）。因此，我们的结果表明，OMV为II型分泌的细胞外酶提供了一条替代转运途径。
