The rhizosphere is a rich and dynamic area at the soil–plant root interface that hosts various pathogenic and beneficial microorganisms. Beneficial non-symbiotic rhizobacteria, such as Bacillus spp., confer multiple advantageous properties to plants that include stress tolerance, regulation of plant development, enhanced nutrient availability and protection against soil pathogens. These bacteria need to efficiently colonize the root, and the success of this partially depends on the availability of mineral nutrients such as iron. Iron is important for plant growth and defence, but it is also used by phytopathogens to cause infection. Rhizobacteria can enhance plant iron uptake to help control infections and use this element to colonize plant roots. The key machineries involved in such plant-associated bacterial interactions are secretion systems. Bacillus spp., for example, express the type VII secretion system (T7SS), but its function in the context of plant–microorganism interactions is unknown. In this study, Liu and colleagues show that the T7SS of the rhizobacterium Bacillus velezensis SQR9 secretes the effector protein YukE to cause iron leakage in plant roots and support root colonization.
The authors next explored the T7SS-mediated mechanism by which B. velezensis colonizes plant roots. They conducted RNA sequencing and real-time quantitative PCR assays, and found that T7SS induces changes in iron homeostasis in the roots of B. velezensis SQR9-colonized plants. Specifically, T7SS and YukE decreased the iron content in the roots of A. thaliana and cucumber, as opposed to the T7SS and YukE mutants. The authors further showed that supplementation of the growth medium with exogenous YukE restored the root colonization defect of mutant strains under iron-deprived conditions. This confirms that YukE mediates iron leakage from the plant root to promote B. velezensis SQR9 colonization. Measuring the iron acquired by bacterial cells in the A. thaliana rhizosphere, the authors show that YukE-dependent iron leakage increased the iron content in B. velezensis cells.
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