Rhizobial Chemoattractants, the Taste and Preferences of Legume Symbionts

Abstract

The development of host-microbe interactions between legumes and their cognate rhizobia requires localization of the bacteria to productive sites of initiation on the plant roots. This end is achieved by the motility apparatus that propels the bacterium and the chemotaxis system that guides it. Motility and chemotaxis aid rhizobia in their competitiveness for space, resources, and nodulation opportunities. Here, we examine studies on chemotaxis of three major model rhizobia, namely Sinorhizobium meliloti, Rhizobium leguminosarum, and Bradyrhizobium japonicum, cataloging their range of attractant molecules and correlating this in the context of root and seed exudate compositions. Current research areas will be summarized, gaps in knowledge discussed, and future directions described.

Rhizobial Chemotaxis and Motility Systems at Work in the Soil

“…Firstly, more detailed knowledge of the chemotaxis and motility systems in rhizobia could determine the applicability of studies in other bacteria, such as E. coli, and enable mobility improvements to existing symbiont inoculants. For instance, improved knowledge of the many chemoreceptors with uncharacterized sensor domains could provide critical knowledge of the chemotactic signals that are critical for effective soil motility (Compton and Scharf, 2021). Additionally, assays that emulate field conditions, such as capillary and microfluidic assays, could be used to discover the decision-making logic of soil chemotaxis to allow targeted mimicry in engineered inoculants (Mao et al, 2003;Walsh et al, 2017;Lopez-Farfan et al, 2019).…”

A bacterial chemoreceptor that mediates chemotaxis to two different plant hormones

“…In this regard, a growing body of data reveals the importance of chemotaxis towards specific nutrients for an efficient plant colonization by beneficial and pathogenic phytobacteria. In this chemotaxis‐mediated host colonization, amino acids, organic acids and sugars were found to play major roles (Oku et al ., 2012, 2014; Hida et al ., ; Cerna‐Vargas et al ., 2019; Feng et al ., 2019; O'Neal et al ., 2020; Compton and Scharf, 2021). However, determining the role of chemotaxis towards alternative plant molecules (e.g.…”

“…However, the composition of plant exudates varies qualitatively and quantitatively according to physical, chemical and biological factors (Sasse et al ., 2018; Vives‐Peris et al ., 2020; Compton and Scharf, 2021). Alterations in metabolite exudation influences plant microbiome composition (Sasse et al ., 2018; Pascale et al ., 2020) and chemotactic recruitment of bacteria is dependent on variations in the composition of plant exudates (Feng et al ., 2019; Compton and Scharf, 2021). It can therefore be hypothesized that PcpI may play a role under plant‐specific physiological conditions, for example, during the induction of systemic acquired resistance when strong increases in salicylic acid levels have been measured in plant fluids (Smith‐Becker et al ., 1998).…”

“…which major constituents are sugars, amino and organic acids (Fan et al, 2012;da Silva Lima et al, 2014;Lopez-Farfan et al, 2019). However, the composition of plant exudates varies qualitatively and quantitatively according to physical, chemical and biological factors (Sasse et al 2018;Vives-Peris et al, 2020;Compton and Scharf, 2021).…”

Rhizobial Volatiles: Potential New Players in the Complex Interkingdom Signaling With Legumes

“…Compounds present in legume root exudates, such as amino acids, quaternary ammonium compounds, and organic acids function as chemoattractants for rhizobia. Upon perception by specific bacterial chemoreceptors, they promote bacterial movement toward plant roots facilitating root colonization and the possibility of finding proper sites for infection ( Scharf et al, 2016 ; Compton et al, 2020 ; Compton and Scharf, 2021 ). Root exudates can also contain various low molecular weight compounds that mimic bacterial signals and affect quorum sensing (QS) regulation in rhizobia, thereby enhancing or inhibiting the phenotypes controlled by this cell density-dependent regulatory mechanism ( Gao et al, 2003 ; Mathesius et al, 2003 ; Bauer and Mathesius, 2004 ).…”