1 Agriculture and Agri-Food Canada, Saskatoon Research Centre, 107 Science Place, Saskatoon, SK., S7N 0X2, Canada.
2 Boyce Thompson Institute, Cornell University, 533 Tower Road, Ithaca, NY 14853, U.S.A.; and.
Mol Plant Microbe Interact. 2019 Mar;32(3):296-305. doi: 10.1094/MPMI-07-18-0192-R. Epub 2019 Feb 4.
The plant hormone salicylic acid (SA) plays a critical role in defense against biotrophic pathogens such as Plasmodiophora brassicae, which is an obligate pathogen of crucifer species and the causal agent of clubroot disease of canola (Brassica napus). P. brassicae encodes a protein, predicted to be secreted, with very limited homology to benzoic acid (BA)/SA-methyltransferase, designated PbBSMT. PbBSMT has a SA- and an indole-3-acetic acid-binding domain, which are also present in Arabidopsis thaliana BSMT1 (AtBSMT1) and, like AtBSMT1, has been shown to methylate BA and SA. In support of the hypothesis that P. brassicae uses PbBSMT to overcome SA-mediated defenses by converting SA into inactive methyl salicylate (MeSA), here, we show that PbBSMT suppresses local defense and provide evidence that PbBSMT is much more effective than AtBSMT1 at suppressing the levels of SA and its associated effects. Basal SA levels in Arabidopsis plants that constitutively overexpress PbBSMT compared with those in Arabidopsis wild-type Col-0 (WT) were reduced approximately 80% versus only a 50% reduction in plants overexpressing AtBSMT1. PbBSMT-overexpressing plants were more susceptible to P. brassicae than WT plants; they also were partially compromised in nonhost resistance to Albugo candida. In contrast, AtBSMT1-overexpressing plants were not more susceptible than WT to either P. brassicae or A. candida. Furthermore, transgenic Arabidopsis and tobacco plants overexpressing PbBSMT exhibited increased susceptibility to virulent Pseudomonas syringae pv. tomato DC3000 (DC3000) and virulent Pseudomonas syringae pv. tabaci, respectively. Gene-mediated resistance to DC3000/AvrRpt2 and tobacco mosaic virus (TMV) was also compromised in Arabidopsis and Nicotiana tabacum 'Xanthi-nc' plants overexpressing PbBSMT, respectively. Transient expression of PbBSMT or AtBSMT1 in lower leaves of N. tabacum Xanthi-nc resulted in systemic acquired resistance (SAR)-like enhanced resistance to TMV in the distal systemic leaves. Chimeric grafting experiments revealed that, similar to SAR, the development of a PbBSMT-mediated SAR-like phenotype was also dependent on the MeSA esterase activity of NtSABP2 in the systemic leaves. Collectively, these results strongly suggest that PbBSMT is a novel effector, which is secreted by P. brassicae into its host plant to deplete pathogen-induced SA accumulation.
植物激素水杨酸（SA）在防御生物营养型病原体（如芸薹根肿菌）中起着关键作用，该菌是十字花科物种的专性病原体，也是油菜（甘蓝型油菜）根肿病的病原体。芸薹根肿菌编码一种预测为分泌的蛋白，与苯甲酸（BA）/SA-甲基转移酶具有非常有限的同源性，称为 PbBSMT。PbBSMT 具有 SA 和吲哚-3-乙酸结合域，该域也存在于拟南芥 BSMT1（AtBSMT1）中，并且与 AtBSMT1 一样，已被证明可甲基化 BA 和 SA。为了支持芸薹根肿菌利用 PbBSMT 将 SA 转化为无活性的甲基水杨酸（MeSA）来克服 SA 介导的防御的假设，在这里，我们表明 PbBSMT 抑制了局部防御，并提供了证据表明 PbBSMT 比 AtBSMT1 更有效地抑制 SA 及其相关效应的水平。与拟南芥野生型 Col-0（WT）相比，组成型过表达 PbBSMT 的拟南芥植物中的基础 SA 水平降低了约 80%，而过表达 AtBSMT1 的植物仅降低了 50%。过表达 PbBSMT 的植物比 WT 植物更容易感染芸薹根肿菌；它们在非寄主对 Albugo candida 的抗性方面也受到部分损害。相比之下，过表达 AtBSMT1 的植物对 P. brassicae 或 A. candida 的敏感性并不高于 WT。此外，过表达 PbBSMT 的转基因拟南芥和烟草植物对强毒力丁香假单胞菌 pv。番茄 DC3000（DC3000）和强毒力丁香假单胞菌 pv。烟斑病毒分别表现出增加的易感性。对 DC3000/AvrRpt2 和烟草花叶病毒（TMV）的基因介导抗性也在过表达 PbBSMT 的拟南芥和烟草 N. tabacum 'Xanthi-nc'植物中受到损害，分别。在 N. tabacum Xanthi-nc 的下叶中瞬时表达 PbBSMT 或 AtBSMT1 导致系统获得性抗性（SAR）样增强对系统远端叶中 TMV 的抗性。嵌合嫁接实验表明，与 SAR 类似，PbBSMT 介导的 SAR 样表型的发育也依赖于系统叶中 NtSABP2 的 MeSA 酯酶活性。总之，这些结果强烈表明 PbBSMT 是一种新型效应物，它由芸薹根肿菌分泌到宿主植物中，以耗尽病原体诱导的 SA 积累。
