qsmRencoding an IclR-family transcriptional factor is a core pathogenic determinant ofBurkholderia glumaebeyond the acyl-homoserine lactone-mediated quorum-sensing system
Authors: Lelis Tiago , Bruno Jobelle , Padilla Jonas , Barphagha Inderjit , Ham Jong Hyun 
Published: December 2023

The plant pathogenic bacteriumBurkholderia glumaecauses bacterial panicle blight (BPB) in rice-growing areas worldwide. It has been widely accepted that an acyl-homoserine lactone (AHL)-type quorum sensing (QS) system encoded bytofIandtofRgenes (TofIR QS) is a key regulatory mechanism underlying the bacterial pathogenesis ofB. glumae. In addition,qsmR, which encodes an IclR-family regulatory protein, has been considered an important part of TofIR QS. However, the present study with three strains ofB. glumaerepresenting different pathogenic strains revealed that this currently accepted paradigm should be modified. We characterized the regulatory function of TofIR QS andqsmRin three different strains ofB. glumae, 336gr-1 (virulent), 411gr-6 (hypervirulent) and 257sh-1 (avirulent). In 336gr-1, both TofIR QS andqsmRwere critical for the pathogenesis, being consistent with previous studies. However, in the hypervirulent strain 411gr-6, TofIR QS only partially contributes to the virulence, whereasqsmRwas critical for pathogenesis like in 336gr-1. Furthermore, we found that a single nucleotide polymorphism causing T50K substitution in theqsmRcoding sequence was the cause of the non-pathogenic trait of the naturally avirulent strain 257sh-1. Subsequent analyses of gene expression and transcriptome revealed that TofIR QS is partially controlled byqsmRat the transcriptional level in both virulent strains. Further genetic tests of additionalB. glumaestrains showed that 11 out of 20 virulent strains retained the ability to produce toxoflavin even after removing thetofI/tofM/tofRQS gene cluster like 411gr-6. In contrast, all the virulent strains tested lost the function almost completely upon deletion of theqsmRgene. Taking these results together,qsmR, rather than TofIR QS, is a master regulator that determines the pathogenic trait ofB. glumaethus a more appropriate pathogen target for successful management of BPB.

Author summary

Bacterial cell-to-cell communication, called quorum-sensing, systems mediated by homoserine lactone (HSL)-type signaling molecules play pivotal roles in the virulence-related functions of diverse plant and animal pathogenic Gram-negative bacteria.Burkholderia glumaeis the chief causal agent for bacterial panicle blight, a devastating rice disease worldwide. In this pathogen, the HSL-type quorum-sensing system dependent on thetofIandtofRgenes had long been thought to be the core regulatory/signaling system essential for virulence. However, we discovered in this study that a highly virulent strain ofB. glumae, 411gr-6, retained its virulence even after thetofI/tofR-dependent quorum-sensing function was disabled. We also found that more than a half of the natural strains of this pathogen species tested in this study also exhibited their virulence function in the absence of thetofI/tofR-dependent quorum-sensing system. Then we characterized theqsmRgene as an essential regulatory/signaling element for the bacterial pathogenesis regardless of the genetic variations among the strains ofB. glumae. This study indicates that theqsmRgene, rather than thetofI/tofR-dependent quorum-sensing system, should be considered as an essential master regulatory/signaling factor for the virulence ofB. glumaeand, thus, a promising target for suppressing bacterial panicle blight.