The importance of the Phosphate Starvation Response during the arbuscular mycorrizal symbiosis

Plant growth depends on the availability of phosphate (Pi). Since most soils are Pi deficient, plants evolved several strategies to allow more efficient uptake of the macronutrient. One strategy is based on the formation of a symbiotic relationship with arbuscular mycorrhizal (AM) fungi, which can be observed in more than 80% of land plants. AM fungi are very efficient in the acquisition of Pi from soil and in the transfer to the host plants at the level of specialized fungal structures called arbuscules formed within cortical cells. In exchange, the plants provide the fungus with sugars and lipids, required by these organisms to complete their life cycle. Recent studies, presented in this thesis, revealed the presence of a complex transcriptional regulator network controlling the establishment of the AM symbiosis in connection with the Pi starvation response. In rice, PHR transcription factors have been identified as master regulators of this network, connecting the direct Pi uptake pathway, in which they induce Pi starvation-induced (PSI) genes, and the mycorrhizal (or indirect) pathway. It has been found that many AM relevant genes present P1BS cis-elements in their promoters, which make them targets for PHRs. To prevent overaccumulation of Pi, since it has toxic effects in plants, the PHR-regulated network is controlled by SPX domain-containing proteins, working as Pi sensors and inhibiting the binding between PHRs to P1BS elements in a Pi-dependent manner. In Medicago, however, SPX proteins were shown to have a positive effect during AM colonization under low Pi, since they facilitate the root exudation of strigolactones which are known to boost AM fungal branching during pre-contact stages and promote mycorrhization. Furthermore, a novel negative feedback loop mechanism has been demonstrated based on the interactions between the AM-relevant transcription factors WRINKLED 1, ERM1 (both involved in the control of lipid and Pi exchange) and the repressor ERF12. This regulatory mechanism allows plant to coordinate arbuscule development and maintain homeostasis of nutrient transport during symbiosis. The increasing knowledge on the molecular mechanisms that allow the formation of the stable interaction between plants and AM fungi could lead to potential applications for sustainable agriculture. Harnessing these organisms, it’s not only beneficial for plants but it would allow to a decreased use of chemical fertilizers, which adversely affect the environment.