Methylobacterium-Mediated Phytohormone Regulation and Metabolic Priming in Plant Drought Resilience

Droughts are considered one of the major abiotic limitations constraining global plant productivity. Recent findings suggest that water-deficit responses in plants are largely mediated by associated microbial communities, instead of being purely genetically based in plants. Of these beneficial microbes, pink-pigmented, facultative, methylotrophic bacteria in the genus Methylobacterium have been recognized for their immense potential as plant-growth-promoting agents. These microbes have the ability to generate phytohormones, especially cytokinins and auxins, as well as manipulate host metabolic pathways. This review aims to compile available knowledge on hormonal and metabolic interactions in the plant holobiont mediated by Methylobacterium species, especially in relation to drought stress. Firstly, the review discusses the microbial production of phytohormones, specifically cytokinins (such as trans-zeatin) and auxins (like indole-3-acetic acid, or IAA), and their effects on plant roots and shoots. Next, the review aims to discuss metabolic priming approaches induced by Methylobacterium in plants exposed to drought, which include priming for osmolyte biosynthesis (proline, glycine betaine, trehalose, etc.) and activating antioxidant defenses. Furthermore, the review aims to explain how these interactions and responses collectively contribute to developing plant drought stress resilience via improved plant–water relations, postponing senescence, maintaining photosystem efficiency and elucidating mechanisms using omics approaches.