Summary Arbuscular mycorrhizal (AM) fungi can form beneficial associations with the most terrestrial vascular plant species. AM fungi not only facilitate plant nutrient acquisition but also enhance plant tolerance to various environmental stresses such as drought stress. However, the molecular mechanisms by which AM fungal mitogen‐activated protein kinase (MAPK) cascades mediate the host adaptation to drought stimulus remains to be investigated. Recently, many studies have shown that virus‐induced gene silencing (VIGS) and host‐induced gene silencing (HIGS) strategies are used for functional studies of AM fungi. Here, we identify the three HOG1 (High Osmolarity Glycerol 1)‐MAPK cascade genes RiSte11, RiPbs2 and RiHog1 from Rhizophagus irregularis. The expression levels of the three HOG1‐MAPK genes are significantly increased in mycorrhizal roots of the plant Astragalus sinicus under severe drought stress. RiHog1 protein was predominantly localized in the nucleus of yeast in response to 1 M sorbitol treatment, and RiPbs2 interacts with RiSte11 or RiHog1 directly by pull‐down assay. Importantly, VIGS or HIGS of RiSte11, RiPbs2 or RiHog1 hampers arbuscule development and decreases relative water content in plants during AM symbiosis. Moreover, silencing of HOG1‐MAPK cascade genes led to the decreased expression of drought‐resistant genes (RiAQPs, RiTPSs, RiNTH1 and Ri14‐3‐3) in the AM fungal symbiont in response to drought stress. Taken together, this study demonstrates that VIGS or HIGS of AM fungal HOG1‐MAPK cascade inhibits arbuscule development and expression of AM fungal drought‐resistant genes under drought stress.

Arbuscular mycorrhizal (AM) fungi play an important role in improving plant growth and development under drought stress. The MAPK cascade may regulate many physiological and biochemical processes in plants in response to drought stress.