Rhizobial Chemotaxis and Motility Systems at Work in the Soil

Abstract

Bacteria navigate their way often as individual cells through their chemical and biological environment in aqueous medium or across solid surfaces. They swim when starved or in response to physical and chemical stimuli. Flagella-driven chemotaxis in bacteria has emerged as a paradigm for both signal transduction and cellular decision-making. By altering motility, bacteria swim toward nutrient-rich environments, movement modulated by their chemotaxis systems with the addition of pili for surface movement. The numbers and types of chemoreceptors reflect the bacterial niche and lifestyle, with those adapted to complex environments having diverse metabolic capabilities, encoding far more chemoreceptors in their genomes. The Alpha-proteobacteria typify the latter case, with soil bacteria such as rhizobia, endosymbionts of legume plants, where motility and chemotaxis are essential for competitive symbiosis initiation, among other processes. This review describes the current knowledge of motility and chemotaxis in six model soil bacteria: Sinorhizobium meliloti, Agrobacterium fabacearum, Rhizobium leguminosarum, Azorhizobium caulinodans, Azospirillum brasilense, and Bradyrhizobium diazoefficiens. Although motility and chemotaxis systems have a conserved core, rhizobia possess several modifications that optimize their movements in soil and root surface environments. The soil provides a unique challenge for microbial mobility, since water pathways through particles are not always continuous, especially in drier conditions. The effectiveness of symbiont inoculants in a field context relies on their mobility and dispersal through the soil, often assisted by water percolation or macroorganism movement or networks. Thus, this review summarizes the factors that make it essential to consider and test rhizobial motility and chemotaxis for any potential inoculant.

Symbiotic Algae of Hydra viridissima Play a Key Role in Maintaining Homeostatic Bacterial Colonization

“…It is known for the renowned squid-vibrio symbiotic system that the Vibrio bacteria show chemotaxis in response to the host-derived chitin oligosaccharides ( Mandel et al, 2012 ). Similar chemotactic behavior has been observed for several species of Rhizobium bacteria toward the plant roots ( Aroney et al, 2021 ). Similarly, we hypothesize that algal metabolites in Hydra might be actively attracting the invasive species such as Legionella and altering the colonization niche to stop its further spread like “lure-and-kill” mechanism.…”

Surface Motility Regulation of Sinorhizobium fredii HH103 by Plant Flavonoids and the NodD1, TtsI, NolR, and MucR1 Symbiotic Bacterial Regulators

“…Bacterial motility is not crucial for the establishment of rhizobia-legume symbiosis, but it affects the ability of rhizobial strains to quickly colonise and infect host legume roots (revised by [28]), which might be an important feature in natural conditions where different rhizobial strains can compete for nodulating the same plant. Unfortunately, all the HH103 mutants tested in this study and shown to be impaired in surface motility are affected in genes with relevant symbiotic functions [16,17,19,20,53,61], which prevents gaining any knowledge about the putative relevance of genistein-induced surface motility for HH103 symbiotic capabilities.…”

“…Biofilm formation and surface motility are opposite behaviours exhibited by bacteria living on surfaces [ 22 , 25 ]. Rhizobial motility is not essential for nodulation and nitrogen fixation but can influence nodulation efficiency and competitiveness by approaching bacteria to potential infection sites and/or facilitating their progression inside infection threads [ 26 , 27 , 28 ]. Several studies have shown that rhizobial mutants affected in flagella synthesis exhibit partial defects in symbiotic performance such as reduced ability for root colonisation [ 29 ], delayed nodulation and/or reduced competitiveness for nodulating host legumes [ 25 , 30 , 31 , 32 , 33 ].…”

Ecological and Evolutionary Implications of Microbial Dispersal

“…Worth mentioning, such a type of motility is also important in terrestrial systems. For instance, assisting the movement of species within diffusible hotspots in the soil [e.g., plant-root surfaces ( Aroney et al, 2021 ), fungal hyphae ( Pion et al, 2013 ), see below].…”