A mycorrhizal helper bacterium alleviates drought stress in mycorrhizal Helianthemum almeriense plants by regulating water relations and plant hormones

Complex interactions between mycorrhizal helper bacteria (MHBs) and mycorrhizal fungi have rarely been studied in the context of drought. Separately, both MHB inoculation and moderate drought stress have previously shown to enhance desert truffle mycorrhization in Helianthemum almeriense plants, but the combined effect of both of them has not been evaluated yet. The goal of this study is to determine whether mycorrhization between Helianthemum almeriense and the desert truffle Terfezia claveryi can be triggered by applying Pseudomonas mandelii #29 under drought stress and whether this could improve plant water-relations. To investigate this, we evaluated the effect of the combination of MHB inoculation with two different water regimes (well-watered and water-deficit) over the mycorrhizal development, root hydraulic properties, gene expression of AQPs and plant hormones. The results showed that presence of P. mandelii under drought stress caused a synergistic increase of fungal colonization and a higher nutrient uptake. Although drought stress decreased root hydraulic conductivity (Lpr) regardless of whether plants were MHB-inoculated or not, this decrease was buffered in MHB-inoculated plants, concomintantly with regulation of plant hormones, such as abscisic acid, and aquaporin gene expression. Our results indicate that drought-mediated tripartite interactions (P. mandelii #29 x T. claveryi x H. almeriense) are beneficial for the plant to cope with water stress.