PcsR2 Is a LuxR-Type Regulator That Is Upregulated on Wheat Roots and Is Unique to Pseudomonas chlororaphis

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, we validated a recently published cumate-inducible hopanoid mutant of Bradyrhizobium diazoefficiens USDA110, Pcu-shc::∆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::∆shc is an inefficient soybean symbiont with significantly lower rates of nitrogen fixation and low survival within the host tissue. RNA-seq revealed that hopanoid loss reduces the expression of flagellar motility and chemotaxis-related genes, further confirmed by swim plate assays, and enhances the 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.

A major problem for global sustainability is feeding our exponentially growing human population while available arable land decreases. Harnessing the power of plant-beneficial microbes is a potential solution, including 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 significant 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.

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 ::Δ 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 ::Δ 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.

IMPORTANCE

A 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.

Structure and Signal Regulation Mechanism of Interspecies and Interkingdom Quorum Sensing System Receptors

Quorum sensing (QS) is a signaling mechanism for cell-to-cell communication between bacteria, fungi, and even eukaryotic hosts such as plant and animal cells. Bacteria in real life do not exist as isolated organisms but are found in complex, dynamic, and microecological environments. The study of interspecies QS and interkingdom QS is a valuable approach for exploring bacteria-bacteria interactions and bacteria-host interaction mechanisms and has received considerable attention from researchers. The correct combination of QS signals and receptors is key to initiating the QS process. Compared with intraspecies QS, the signal regulation mechanism of interspecies QS and interkingdom QS is often more complicated, and the distribution of receptors is relatively wide. The present review focuses on the latest progress with respect to the distribution, structure, and signal transduction of interspecies and interkingdom QS receptors and provides a guide for the investigation of new QS receptors in the future.

Transcriptomic and Functional Analyses Reveal Roles of AclR, a luxR-type Global Regular, in Regulating Motility and Virulence of Acidovorax citrulli

LuxR-type transcriptional regulators are essential for many physiological processes in bacteria, including pathogenesis. Acidovorax citrulli is a seedborne bacterial pathogen responsible for bacterial fruit blotch, which causes great losses in melon and watermelon worldwide. However, the LuxR-type transcriptional factors in A. citrulli have not been well studied, except for the previously reported LuxR-type regulatory protein, AcrR, involved in regulating virulence and motility. Here, we characterized a second LuxR-type regulator, AclR, in the group II strain Aac-5 of A. citrulli by mutagenesis, virulence and motility assays, and transcriptomic analysis. Deletion of aclR resulted in impaired twitching and swimming motility and flagellar formation and diminished virulence but increased biofilm formation. Transcriptomic analysis revealed that 1,379 genes were differentially expressed in the aclR mutant strain, including 29 genes involved in flagellar assembly and 3 involved in pili formation, suggesting a regulatory role for AclR in multiple important biological functions of A. citrulli. Together, our results not only indicate that AclR plays a global role in transcriptional regulation in A. citrulli influencing motility, biofilm formation, and virulence but also provide perspective regarding the regulatory network of biological functions in A. citrulli.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pseudomonas chlororaphis metabolites as biocontrol promoters of plant health and improved crop yield

The Pseudomonas fluorescens complex contains at least eight phylogenetic groups and each of these includes several bacterial species sharing ecological and physiological traits. Pseudomonas chlororaphis classified in a separate group is represented by three different subspecies that show distinctive traits exploitable for phytostimulation and biocontrol of phytopathogens. The high level of microbial competitiveness in soil as well as the effectiveness in controlling several plant pathogens and pests can be related to the P. chlororaphis ability to implement different stimulating and toxic mechanisms in its interaction with plants and the other micro- and macroorganisms. Pseudomonas chlororaphis strains produce antibiotics, such as phenazines, pyrrolnitrine, 2-hexyl, 5-propyl resorcinol and hydrogen cyanide, siderophores such as pyoverdine and achromobactine and a complex blend of volatile organic compounds (VOCs) that effectively contribute to the control of several plant pathogens, nematodes and insects. Phenazines and some VOCs are also involved in the induction of systemic resistance in plants. This complex set of beneficial strategies explains the high increasing interest in P. chlororaphis for commercial and biotechnological applications. The aim of this review is to highlight the role of the different mechanisms involved in the biocontrol activity of P. chlororaphis strains.