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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 (Lp r) 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. • P. mandelii #29 is a MHB that increases the mycorrhizal colonization within the host roots especially under drought stress. • This higher mycorrrhizal colonization due to MHB is translated into higher water root hydraulic conductivity (Lp r). • This higher Lp r is associated with a modification of root hormone content and with the regulation of plant aquaporins. • The results indicate that drought-mediated tripartite interactions are beneficial for the plant to cope with water stress. [ABSTRACT FROM AUTHOR]

Desert truffles are edible hypogeous (forming fruit bodies below ground) fungi that grow in semi-arid and arid areas. They are highly valued for both their culinary and medicinal properties in the Mediterranean basin, the Middle East and the Gulf areas. Desert truffles form mycorrhizae mostly with plants belonging to the Cistaceae family, mainly with Helianthemum species. These truffles are still, usually, collected from the wild, but loss of habitats due to urbanization, desertification, intensive agriculture and global warming, along with an urgent need to develop new crops adapted to arid conditions, are currently hastening efforts towards their domestication. Here, we sum up the successful research leading to cultivation of this crop, based on plots that were established in sandy to silt soils with high pH values and low mineral contents. We report suitable methods for production of mycorrhized seedlings and preferred planting methods. We found that under natural conditions yields are affected by water availability, so irrigation regimes to ensure good yields were sought. Although good yields were indeed obtained in some years, fluctuations in yields over the years were significant; the reasons for this are not entirely clear and are currently under study. This crop is particularly well suited to relatively marginal conditions but prospects for establishment of desert truffles as a niche crop for arid and semi-arid areas depend on further improvements in yields. [ABSTRACT FROM AUTHOR]