Bacterial LuxR solos have evolved to respond to different molecules including signals from plants

Abstract

A future challenge will be understanding the extensive communication that most likely takes place in bacterial interspecies and interkingdom signaling between plants and bacteria. A major bacterial inter-cellular signaling system in Gram-negative bacteria is LuxI/R quorum sensing (QS) based on the production (via the LuxI-family proteins) and detection (via the LuxR-family proteins) of N-acyl homoserine lactones (AHLs) signaling molecules. LuxR proteins which have the same modular structure of QS LuxRs but are devoid of a cognate LuxI AHL synthase are called solos. LuxR solos have been shown to be responsible to respond to exogenous AHLs produced by neighboring cells as well endogenously produced AHLs. It is now also evident that some LuxR proteins have evolved from the ability to binding AHLs and respond to other molecules/signals. For example, recent research has shown that a sub-family of LuxR solos responds to small molecules produced by plants. This indicates the presence of a uni-directional interkingdom signaling system occurring from plants to bacteria. In addition LuxR solos have now been also implicated to respond to endogenously produced signals which are not AHLs. In this Mini Review article we will discuss current trends and implications of the role of LuxR solos in bacterial responses to other signals using proteins related to AHL QS systems.

Azospirillum brasilenseAz39, a model rhizobacterium withAHLquorum‐quenchingcapacity

“…The conservation of amino acid residues present in the LuxR of Az39 is a fact that could be associated with LuxRs that respond to exogenous AHLs (by “eavesdropping”) from bacteria with which they share niche and/or other molecules chemically similar from their host plants (Patel et al . ; Venturi et al . ).…”

“…An analysis of multiple sequence alignment of this LuxR compared with LuxR cognates and LuxR solos already described in the literature allowed to show that some amino acid residues characteristic of the N-terminal domain of binding to the autoinductor remain conserved, which classified them outside the family of typical LuxR regulators (data not shown). The conservation of amino acid residues present in the LuxR of Az39 is a fact that could be associated with LuxRs that respond to exogenous AHLs (by "eavesdropping") from bacteria with which they share niche and/or other molecules chemically similar from their host plants (Patel et al 2013;Venturi et al 2018).…”

“…The appearance of LuxR solos regulators indicates that these protein families could be involved in intrakingdom or interkingdom signalling systems through the detection of different compounds produced by other prokaryote or eukaryote organisms (Patankar and Gonzalez ; Patel et al . ).…”

“…These systems are called LuxR orphans or LuxR solos (Patankar and Gonzalez 2009), and in some cases they act in concert with the LuxR/LuxI canonical system. The appearance of LuxR solos regulators indicates that these protein families could be involved in intrakingdom or interkingdom signalling systems through the detection of different compounds produced by other prokaryote or eukaryote organisms (Patankar and Gonzalez 2009;Patel et al 2013).…”

Application of Next Generation Sequencing Technologies in Revealing Plant-Microbe Interactions

“…Moreover, chemical interaction through bacterial QS (AHL) signaling molecule is recently highlighted due to their assessment in plant growth, development, and performance (Hartmann et al 2014). LuxR solo receptors have been identified in both associated bacterial communities and plant host [16,17]. Bacterial LuxR solos are generally studied in pathogenic bacteria and rhizobia.…”