Type III secretion systems (T3SSs) are bacterial membrane-embedded nanomachines translocating effector proteins into the cytoplasm of eukaryotic cells. They have been intensively studied for their important roles in animal and plant bacterial diseases. Over the past two decades, genome sequencing has unveiled their ubiquitous distribution in many taxa of Gram-negative bacteria, including plant-beneficial ones. Here, we discuss the distribution and functions of the T3SS in two agronomically important bacterial groups: the symbiotic nodule-forming nitrogen-fixing rhizobia and the free-living plant-beneficial Pseudomonas spp. In legume-rhizobia symbiosis, T3SSs and their cognate effectors play important roles, including the modulation of the plant immune response and the initiation of the nodulation process in some cases. In plant-beneficial Pseudomonas spp., the roles of T3SSs are not fully understood, but pertain to plant immunity suppression, biocontrol against eukaryotic plant pathogens, mycorrhization facilitation, and possibly resistance against protist predation. The diversity of T3SSs in plant-beneficial bacteria points to their important roles in multifarious interkingdom interactions in the rhizosphere. We argue that the gap in research on T3SSs in plant-beneficial bacteria must be bridged to better understand bacteria/eukaryotes rhizosphere interactions and to support the development of efficient plant-growth promoting microbial inoculants.
Bacterial rhizosphere colonization is critical for phytobeneficial rhizobacteria such as phenazine-producing Pseudomonas spp. To better understand this colonization process, potential metabolic and genomic determinants required for rhizosphere colonization were identified using a collection of 60 phenazine-producing Pseudomonas strains isolated from multiple plant species and representative of the worldwide diversity. Arabidopsis thaliana and Solanum tuberosum (potato) were used as host plants. Bacterial rhizosphere colonization was measured by quantitative PCR using a newly designed primer pair and TaqMan probe targeting a conserved region of the phenazine biosynthetic operon. The metabolic abilities of the strains were assessed on 758 substrates using Biolog phenotype microarray technology. These data, along with available genomic sequences for all strains, were analyzed in light of rhizosphere colonization. Strains belonging to the P. chlororaphis subgroup colonized the rhizospheres of both plants more efficiently than strains belonging to the P. fluorescens subgroup. Metabolic results indicated that the ability to use amines and amino acids was associated with an increase in rhizosphere colonization capability in A. thaliana and/or in S. tuberosum. The presence of multiple genetic determinants in the genomes of the different strains involved in catabolic pathways and plant-microbe and microbe-microbe interactions correlated with increased or decreased rhizosphere colonization capabilities in both plants. These results suggest that the metabolic and genomic traits found in different phenazine-producing Pseudomonas strains reflect their rhizosphere competence in A. thaliana and S. tuberosum. Interestingly, most of these traits are associated with similar rhizosphere colonizing capabilities in both plant species.
IMPORTANCE Rhizosphere colonization is crucial for plant growth promotion and biocontrol by antibiotic-producing Pseudomonas spp. This colonization process relies on different bacterial determinants which partly remain to be uncovered. In this study, we combined a metabolic and a genomic approach to decipher new rhizosphere colonization determinants which could improve our understanding of this process in Pseudomonas spp. Using 60 distinct strains of phenazine-producing Pseudomonas spp., we show that rhizosphere colonization abilities correlated with both metabolic and genomic traits when these bacteria were inoculated on two distant plants, Arabidopsis thaliana and Solanum tuberosum. Key metabolic and genomic determinants presumably required for efficient colonization of both plant species were identified. Upon further validation, these targets could lead to the development of simple screening tests to rapidly identify efficient rhizosphere colonizers.
Plant-beneficial phenazine-producing
Pseudomonas
spp. are effective biocontrol agents, thanks to the broad-spectrum antibiotic activity of the phenazine antibiotics they produce. These bacteria have received considerable attention over the last 20 years, but most studies have focused only on the ability of a few genotypes to inhibit the growth of a limited number of plant pathogens.
High-density rearing conditions for the mass-production of biological control agents are known to affect individual quality and performance. However, complex phenotypic traits like dispersal behaviour and their response to rearing conditions are rarely investigated, although they are likely to affect directly biocontrol efficiency in the field. In this study, we develop an original experimental design to evaluate two complementary components of dispersal behaviour in Trichogramma. Then, we investigate how these components respond to variations in rearing density, and their correlation with traits related to parasitoid fitness. We find that under high-density conditions, a large proportion of individuals display reduced mobility and fecundity, indicative of a lower-quality phenotype. These interactive effects between dispersal performance and individual fitness highlight the need to develop integrative experimental designs to easily quantify complex phenotypic traits related to the field performance of biological control agent
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.