Pan Huiqiao, Pierson Leland S, Pierson Elizabeth A
Molecular and Environmental Plant Sciences Program, Texas A&M University, College Station, TX, United States.
Department of Horticulture Sciences, Texas A&M University, College Station, TX, United States.
Front Microbiol. 2020 Nov 10;11:560124. doi: 10.3389/fmicb.2020.560124. eCollection 2020.
LuxR solos are common in plant-associated bacteria and increasingly recognized for playing important roles in plant-microbe interkingdom signaling. Unlike the LuxR-type transcriptional regulators of prototype LuxR/LuxI quorum sensing systems, solos do not have a LuxI-type autoinducer synthase gene associated with them. LuxR solos in plant-pathogenic bacteria are important for virulence and in plant endosymbionts contribute to symbiosis. In the present study, we characterized an atypical LuxR solo, PcsR2, in the biological control species 30-84 that is highly conserved among sequenced s strains. Unlike most LuxR solos in the plant-associated bacteria characterized to date, is not associated with a proline iminopeptidase gene and the protein has an atypical N-terminal binding domain. We created a deletion mutant and used quantitative RT-PCR to show that the expression of and genes in the operon immediately downstream was upregulated ∼10-fold when the wild type strain was grown on wheat roots compared to planktonic culture. PcsR2 was involved in upregulation. Using a GFP transcriptional reporter, we found that expression of responded specifically to root-derived substrates as compared to leaf-derived substrates but not to endogenous AHLs. Compared to the wild type, the mutant was impaired in the ability to utilize root carbon and nitrogen sources in wheat root macerate and to colonize wheat roots. Phenazine production and most biofilm traits previously shown to be correlated with phenazine production also were diminished in the mutant. Gene expression of several of the proteins in the phenazine regulatory network including PhzR, Pip (phenazine inducing protein) and RpeA/RpeB were reduced in the mutant, and overexpression of these genes restored phenazine production in the mutant to wild-type levels, indicating PcsR2 affects the activity of the these regulatory genes upstream of RpeA/RpeB via an undetermined mechanism. Our results indicate PcsR2 upregulates the expression of the adjacent operon in response to unknown wheat root-derived signals and belongs to a novel subfamily of LuxR-type transcriptional regulators found in sequenced strains.
孤独型LuxR在与植物相关的细菌中很常见，并且因其在植物 - 微生物跨界信号传导中发挥重要作用而日益受到认可。与原型LuxR/LuxI群体感应系统的LuxR型转录调节因子不同，孤独型LuxR没有与之相关的LuxI型自诱导物合成酶基因。植物致病细菌中的孤独型LuxR对毒力很重要，而在植物内共生体中则有助于共生。在本研究中，我们对生物防治菌株30 - 84中一种非典型的孤独型LuxR——PcsR2进行了表征，该菌株在已测序菌株中高度保守。与迄今为止在与植物相关细菌中表征的大多数孤独型LuxR不同，它与脯氨酸亚氨基肽酶基因不相关，并且该蛋白质具有非典型的N端结合结构域。我们创建了一个缺失突变体，并使用定量RT - PCR表明，与浮游培养相比，野生型菌株在小麦根上生长时，紧邻其下游操纵子中的 和 基因的表达上调了约10倍。PcsR2参与了这种上调。使用GFP转录报告基因，我们发现 的表达对根源底物有特异性反应，与叶源底物相比，但对内源性AHLs没有反应。与野生型相比，突变体在利用小麦根浸出液中的根碳源和氮源以及在小麦根上定殖的能力方面受损。吩嗪的产生以及先前显示与吩嗪产生相关的大多数生物膜特性在突变体中也有所降低。吩嗪调节网络中几种蛋白质的基因表达，包括PhzR、Pip（吩嗪诱导蛋白）和RpeA/RpeB在突变体中降低，并且这些基因的过表达将突变体中的吩嗪产生恢复到野生型水平，表明PcsR2通过一种未确定的机制影响RpeA/RpeB上游这些调节基因的活性。我们的结果表明，PcsR2响应未知的小麦根源信号上调相邻操纵子的表达，并且属于在已测序菌株中发现的LuxR型转录调节因子的一个新亚家族。
