Department of Genetics, Institute for Biology, Martin Luther University Halle-Wittenberg, Halle, Germany.
Department of Biochemistry of Plant Interactions, Leibniz Institute for Plant Biochemistry, Halle, Germany.
PLoS Pathog. 2023 Aug 14;19(8):e1011263. doi: 10.1371/journal.ppat.1011263. eCollection 2023 Aug.
Pathogenic Xanthomonas bacteria cause disease on more than 400 plant species. These Gram-negative bacteria utilize the type III secretion system to inject type III effector proteins (T3Es) directly into the plant cell cytosol where they can manipulate plant pathways to promote virulence. The host range of a given Xanthomonas species is limited, and T3E repertoires are specialized during interactions with specific plant species. Some effectors, however, are retained across most strains, such as Xanthomonas Outer Protein L (XopL). As an 'ancestral' effector, XopL contributes to the virulence of multiple xanthomonads, infecting diverse plant species. XopL homologs harbor a combination of a leucine-rich-repeat (LRR) domain and an XL-box which has E3 ligase activity. Despite similar domain structure there is evidence to suggest that XopL function has diverged, exemplified by the finding that XopLs expressed in plants often display bacterial species-dependent differences in their sub-cellular localization and plant cell death reactions. We found that XopL from X. euvesicatoria (XopLXe) directly associates with plant microtubules (MTs) and causes strong cell death in agroinfection assays in N. benthamiana. Localization of XopLXe homologs from three additional Xanthomonas species, of diverse infection strategy and plant host, revealed that the distantly related X. campestris pv. campestris harbors a XopL (XopLXcc) that fails to localize to MTs and to cause plant cell death. Comparative sequence analyses of MT-binding XopLs and XopLXcc identified a proline-rich-region (PRR)/α-helical region important for MT localization. Functional analyses of XopLXe truncations and amino acid exchanges within the PRR suggest that MT-localized XopL activity is required for plant cell death reactions. This study exemplifies how the study of a T3E within the context of a genus rather than a single species can shed light on how effector localization is linked to biochemical activity.
病原黄单胞菌可引起 400 多种植物物种的疾病。这些革兰氏阴性细菌利用 III 型分泌系统将 III 型效应蛋白（T3E）直接注射到植物细胞质中，在那里它们可以操纵植物途径以促进毒力。给定黄单胞菌物种的宿主范围是有限的，并且在与特定植物物种相互作用时，T3E 库是专门化的。然而，一些效应子在大多数菌株中都被保留下来，例如黄单胞菌外蛋白 L（XopL）。作为一种“祖先”效应子，XopL 有助于多种黄单胞菌的毒力，感染多种植物物种。XopL 同源物具有富含亮氨酸重复（LRR）结构域和具有 E3 连接酶活性的 XL 盒的组合。尽管结构域相似，但有证据表明 XopL 的功能已经分化，例如在植物中表达的 XopL 通常在其亚细胞定位和植物细胞死亡反应中表现出细菌种依赖性差异的发现中得到证实。我们发现，来自 X. euvesicatoria（XopLXe）的 XopL 直接与植物微管（MTs）结合，并在 agroinfection 试验中在 N. benthamiana 中引起强烈的细胞死亡。来自三个不同感染策略和植物宿主的另外三个黄单胞菌物种的 XopLXe 同源物的定位揭示了远缘 X. campestris pv。campestris 携带的 XopL（XopLXcc）不能定位到 MTs 并引起植物细胞死亡。对 MT 结合 XopLs 和 XopLXcc 的比较序列分析确定了一个脯氨酸丰富区域（PRR）/α-螺旋区域对于 MT 定位很重要。XopLXe 截断和 PRR 内氨基酸交换的功能分析表明，MT 定位的 XopL 活性是植物细胞死亡反应所必需的。这项研究说明了在属的范围内而不是在单个物种的范围内研究 T3E 如何揭示效应子定位与生化活性之间的联系。
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