Biology subjects

Belin, B. J.

Publications and source records attributed to Belin, B. J..

5 recordsLinked to original sources

bioRxiv2025

Aeschynomene americana induces terminal bacteroid differentiation in Bradyrhizobium sp. USDA3516, a novel model for Dalbergioid-rhizobia symbiosis

The paradigms of legume-rhizobia symbiosis are derived primarily from conserved features of Inverted- Repeat Lacking Clade (IRLC) legumes and closely related species. The Dalbergioids diverged from the IRLC early in legume evolution and possess unique symbiotic features but few genetically tractable models. The small, diploid Dalbergioid Aeschynomene americana (American jointvetch) has promise as a genetic model for Dalbergioid-rhizobia symbiosis, yet only a few studies have examined its symbiotic properties. We examined the symbiont range of A. americana from central Florida and characterized a native A. americana nodule isolate, Bradyrhizobium sp. USDA3516. We find that A. americana forms effective symbioses with B. sp. USDA3516, which is closely related to Thai A. americana symbiont B. sp DOA9, and with symbionts from the Dalbergioids stylo and peanut. Interestingly, several strains that effectively nodulated A. americana exhibited branched bacteroid morphologies, but we found that branching was neither necessary nor sufficient for effective symbiosis. Our study contradicts the prevailing view that bacteroid shape is a major determinant of symbiotic efficiency and presents the A. americana-B. sp. USDA3516 interaction as an optimal model of A. americana symbiosis.

plant biology↗

bioRxiv2025

Characterization of chemotaxis in soybean symbiont Bradyrhizobium diazoefficiens

ABSTRACTSymbiotic relationships between nitrogen-fixing soil bacteria and legumes provide nearly half of all biologically fixed nitrogen on Earth, playing a crucial role in sustainable agriculture. These relationships rely on bacterial navigation of complex, dynamic soil environments to reach their plant hosts. Central to this behavior are bacterial motility and chemotaxis, the ability to sense and move toward host-derived signals in the rhizosphere. In the soybean symbiont Bradyrhizobium diazoefficiens USDA110, motility is controlled by dual flagellar systems, and this strain contains three putative but uncharacterized chemotaxis operons (che1, che2, and che3). Using targeted deletions of all three predicted cheA genes, we show that cheA2 is the primary driver of chemotaxis toward soybean seed exudate in soft agar assays, and that the greater contribution of cheA2 vs. cheA1 in soft agar chemotaxis is due to its genomic context. Interestingly, we also found that B. diazoefficiens mutants that are incapable of chemotaxis in semisolid media retain wild type-like swimming speeds in aqueous media. These findings provide insight into how the agricultural inoculant B. diazoefficiens coordinates its chemosensory systems to respond to its host plant. IMPORTANCEChemotaxis is crucial for the establishment of beneficial plant-microbe associations, yet mechanistic studies of chemotaxis have been limited to a handful of soil bacterial models, namely Azospirillum brasilense, Sinorhizobium meliloti, and Rhizobium leguminosarum. These three models represent only a fraction of the diversity found among plant- beneficial bacteria and agricultural inoculants. The soybean symbiont Bradyrhizobium diazoefficiens USDA110 is a commonly used soybean inoculant with exceptional nitrogen fixation efficiency, but the genetic control of chemotaxis in B. diazoefficiens has not been examined. Establishing B. diazoefficiens as a model of chemotaxis provides an opportunity to understand how multiple chemotaxis systems coordinate root colonization in this major agricultural symbiont and can enable comparative analyses of plant-microbe recognition strategies across agricultural bacteria.

microbiology↗

bioRxiv2025

Origin and novel transport pathways of bacterial hopanoids

Hopanoids are a class of steroid-like lipids that fortify bacterial membranes. Derivatives of hopanoids known as "geo-hopanes" are abundant in sediments and are signatures of ancient bacteria, yet there are conflicting views on their use as markers of specific taxa or environments. Here we analyze conservation of hopanoid biosynthesis across bacterial genomes using modern taxonomic tools. We find that hopanoids most likely originated in an ancestor of marine alphaproteobacteria, and that tolerance of high osmolarity is the most common feature of hopanoid-producing strains. Additionally, microsynteny with hopanoid-related loci revealed new hopanoid-associated gene families, which we term hpnT and hpnS. Structural predictions and the restriction of these gene families to diderms suggests they participate in hopanoid trafficking, potentially forming a pathway analogous to Mla proteins in E. coli.

microbiology↗

bioRxiv2023

Hopanoid lipids promote soybean-Bradyrhizobium symbiosis

The symbioses between leguminous plants and nitrogen-fixing bacteria known as rhizobia are well known for promoting plant growth and sustainably increasing soil nitrogen. Recent evidence indicates that hopanoids, a family of steroid-like lipids, promote Bradyrhizobium symbioses with tropical legumes. To characterize hopanoids in Bradyrhizobium symbiosis with soybean, the most economically significant Bradyrhizobium host, we validated a recently published cumate-inducible hopanoid mutant of Bradyrhizobium diazoefficiens USDA110, Pcu-shc::{Delta}shc. GC-MS analysis showed that this strain does not produce hopanoids without cumate induction, and under this condition, is impaired in growth in rich medium and under osmotic, temperature, and pH stress. In planta, Pcu-shc::{Delta}shc is an inefficient soybean symbiont with significantly lower rates of nitrogen fixation and low survival within host tissue. RNA-seq revealed that hopanoid loss reduces expression of flagellar motility and chemotaxis-related genes, further confirmed by swim plate assays, and enhances expression of genes related to nitrogen metabolism and protein secretion. These results suggest that hopanoids provide a significant fitness advantage to B. diazoefficiens in legume hosts and provide a foundation for future mechanistic studies of hopanoid function in protein secretion and motility. IMPORTANCEA major problem for global sustainability is feeding our exponentially growing human population while available arable land is decreasing, especially in areas with the greatest population growth. Harnessing the power of plant-beneficial microbes has gained attention as a potential solution, including the increasing our reliance on the symbioses of leguminous plants and nitrogen-fixing rhizobia. This study examines the role of hopanoid lipids in the symbiosis between Bradyrhizobium diazoefficiens USDA110, an important commercial inoculant strain, and its economically important host soybean. Our research extends our knowledge of the functions of bacterial lipids in symbiosis to an agricultural context, which may one day help improve the practical applications of plant-beneficial microbes in agriculture.

microbiology↗

bioRxiv2018

Extended hopanoid lipids promote bacterial motility, surface attachment, and root nodule development in the Bradyrhizobium diazoefficiens-Aeschynomene afraspera symbiosis

Hopanoids are steroid-like bacterial lipids that enhance membrane rigidity and promote bacterial growth under diverse stresses. Hopanoid biosynthesis genes are conserved in nitrogen-fixing plant symbionts, and we previously found that the extended (C35) class of hopanoids in Bradyrhizobium diazoefficiens are required for efficient symbiotic nitrogen fixation in the tropical legume host Aeschynomene afraspera. Here we demonstrate that the nitrogen fixation defect conferred by extended loss can fully be explained by a reduction in root nodule sizes rather than per-bacteroid nitrogen fixation levels. Using a single-nodule tracking approach to track A. afraspera nodule development, we provide a quantitative model of root nodule development in this host, uncovering both the baseline growth parameters for wild-type nodules and a surprising heterogeneity of extended hopanoid mutant developmental phenotypes. These phenotypes include a delay in root nodule initiation and presence of a subpopulation of nodules with slow growth rates and low final volumes, which are correlated with reduced motility and surface attachment in vitro and lower bacteroid densities in planta, respectively. This work provides a quantitative reference point for understanding the phenotypic diversity of ineffective symbionts in A. afraspera and identifies specific developmental stages affected by extended hopanoid loss for future mechanistic work.

microbiology↗