Physiological and molecular mechanisms of the plant-microbe interactions in the mycorrhizal symbiosis of Terfezia claveryi Chatin

Abstract

The desert truffle Terfezia claveryi is an edible ectendomycorrhizal hypogeous fungus with high culinary value and beneficial health properties. Although cultivation has advanced, production remains variable and erratic, and it is necessary to study the parameters in the fungus-bacteria-plant interaction to achieve stable production and address environmental stresses, such as water stress, and climate change. This Doctoral Thesis aims to deepen the knowledge of desert truffle cultivation using the symbiosis between Helianthemum spp. and T. claveryi as a model system from different perspectives, by assessing it from a physiological and molecular point of view, taking into account how environmental factors can affect both processes. Firstly, the impact of atmospheric CO2 on mycorrhizal symbiosis with an annual host such as Helianthemum salicifolium was studied. H. salicifolium plants were grown in culture chambers with CO2 at double the current concentration, and different physiological and morphometric variables, water relations, and mycorrhization were measured. The results showed that photosynthesis, biomass, and mycorrhization increased without altering the plant's nutritional balance. Therefore, the use of annual plants as an alternative to perennials for desert truffle cultivation in a climate change scenario is suggested. Secondly, a study was conducted to determine if mycorrhization between Helianthemum almeriense and T. claveryi could be increased by applying the "mycorrhizal helper bacteria" MHB Pseudomonas mandelii #29 under water stress, and whether this could improve the plant's water relations. The effect of inoculation with MHB under two different water regimes (well-watered and drought conditions) on mycorrhizal development, root hydraulic properties, aquaporin gene expression, and plant hormones was evaluated. The results showed that the presence of P. mandelii #29 under drought conditions had a synergistic effect, as it enhanced fungal colonization. Additionally, there was better regulation of water relations, as aquaporins were better regulated. The study encourages understanding the complex interactions between MHB bacteria and mycorrhizal fungi in the context of drought to improve plantation management. Thirdly, gene expression was studied in the roots of H. almeriense plants mycorrhized with T. claveryi during mycorrhizal development. Real-time PCR was used to analyse genes from both the plant and the fungus, and it was found that changes in morphology and mycorrhizal intensity were associated with changes in gene expression. The association between H. almeriense and T. claveryi was divided into three stages: the pre-symbiotic stage, the early symbiotic stage, and the late symbiotic stage. Certain genes were identified that play a key role in the fungus entering the root and establishing symbiosis. TcAQP1 from T. claveryi appears to play an important role in symbiosis establishment. Finally, the seasonal dynamics of the microbial community associated with a desert truffle plantation were analysed using next-generation sequencing technologies (metagenomics). The results indicate that both the bacterial and fungal communities are influenced by the seasons, although the effect was greater on the bacterial community. In addition, the physicochemical parameters of the soil also affected the microbial community. The abundance of T. claveryi in the soil was highly marked by seasonality, as it significantly increased in winter and then decreased in spring, with this extraradical mycelium possibly could be used for truffle production.