Abstract
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.
Highlights
Type III secretion systems (T3SSs) are syringe-like membrane-embedded nanomachines that enable the translocation of effector proteins directly into the cytoplasm of eukaryotic cells
Several studies have shown that the inoculation of rhizobia in the vicinity of leguminous root cells induces a transient increase in the expression of numerous plant defenserelated genes [26–28], corresponding to the microbe-associated molecular pattern (MAMP)triggered immunity
Our current knowledge on T3SSs of plant-beneficial bacteria points to the versatile functions they play in interkingdom interactions (Figure 2)
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The T3SS has historically been known as a major virulence determinant in many important human pathogens, especially those belonging to the Yersinia, Salmonella and Chlamydia genera [4–6] It is a major plant pathogenicity factor in several bacterial taxa [7]. Performed a comprehensive survey of about 20,000 available bacterial genomes to search for T3SS gene clusters [3] These were found in 109 genera, including many environmental strains with no known association with eukaryotic hosts, and plant-beneficial bacteria, such as rhizobia and Pseudomonas isolates. Rhizobia are a paraphyletic group that includes the Rhizobium, Bradyrhizobium, Sinorhizobium (Ensifer), and Mesorhizobium genera They are known for their symbiotic relationships with numerous legumes, mediated by nitrogen-fixing nodules sheltering the Microorganisms 2022, 10, 187. We focus on two agronomically important bacterial groups, namely the rhizobia and the plant-beneficial Pseudomonas spp., to highlight the gap in T3SS research on plant-beneficial bacteria and the potential benefits of bridging this gap
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