The role of microbial interactions on rhizobial fitness

Abstract

Rhizobia are soil bacteria that can establish a nitrogen-fixing symbiosis with legume plants. As horizontally transmitted symbionts, the life cycle of rhizobia includes a free-living phase in the soil and a plant-associated symbiotic phase. Throughout this life cycle, rhizobia are exposed to a myriad of other microorganisms that interact with them, modulating their fitness and symbiotic performance. In this review, we describe the diversity of interactions between rhizobia and other microorganisms that can occur in the rhizosphere, during the initiation of nodulation, and within nodules. Some of these rhizobia-microbe interactions are indirect, and occur when the presence of some microbes modifies plant physiology in a way that feeds back on rhizobial fitness. We further describe how these interactions can impose significant selective pressures on rhizobia and modify their evolutionary trajectories. More extensive investigations on the eco-evolutionary dynamics of rhizobia in complex biotic environments will likely reveal fascinating new aspects of this well-studied symbiotic interaction and provide critical knowledge for future agronomical applications.

Sinorhizobium meliloti GR4 Produces Chromosomal- and pSymA-Encoded Type IVc Pili That Influence the Interaction with Alfalfa Plants

“…Rhizobial motility is not essential for nodulation or nitrogen fixation but is an important trait for root colonization and infection, which eventually could influence nodulation efficiency and competitiveness [6][7][8][9][10][11]. Rhizobial competitiveness (i.e., the ability of a given strain to form nodules in the presence of a second rhizobial strain) is a complex trait that plays a fundamental role in the success of inoculants used as biofertilizers for legumes [12][13][14]. Therefore, the identification and characterization of components and Plants 2024, 13, 628 2 of 20 regulatory mechanisms that govern rhizobial motility might contribute to the design of elite inoculants and the development of sustainable agriculture.…”

“…In addition, the genes required for T4aP biogenesis are numerous (more than 40) and are often scattered throughout the bacterial genome. In contrast, the synthesis of T4bP and T4cP requires a smaller number of genes (12)(13)(14) and they are usually grouped in a region, sometimes with genetic-island characteristics [32].…”

CopG1, a Novel Transcriptional Regulator Affecting Symbiosis in Bradyrhizobium sp. SUTN9-2

“…It also benefits agriculture by reducing reliance on synthetic nitrogen fertilizers, which can have negative environmental impacts [ 2 ]. However, specific symbiotic relationships between rhizobia and plant species require a complex exchange of signaling compounds, which is the key factor in successful symbiosis [ 3 ]. Two crucial mechanisms are necessary for the nodulation process of many rhizobia: (i) the classical mechanism of perception of the Nod factors (NFs), which are lipochitooligosaccharides (LCOs) produced by nitrogen-fixing rhizobia.…”