Evaluation and management of soil microbial potentialities for improving the carob tree tolerance to water constraints

The carob tree (Ceratonia siliqua L.), an ecological, cultural and industrial component of the Mediterranean basin, has been experiencing a decline of its abundance in recent decades. Three contributing factors have been identified, an agricultural policy in favour of other plant species, an increasing urbanisation of the Mediterranean coastline, which leads to habitat fragmentation, and negative climatic conditions, notably longer periods of drought. The dependency of the carob tree on water resources is an underestimated trait, and yet a crucial one for its survival. The optimisation of the symbiotic association between carob and mycorrhizal fungi has been proposed as a promising strategy to improve its resistance to water stress. However, a more holistic view of plant-microorganism interactions within the environment seems essential to sustainably address the challenges of plant adaptation to environmental stresses. The central hypothesis of the thesis is that the management of interactions between the carob tree and the microbiota of some pioneer plants in its habitat could be the key for a better adaptation of the carob tree to drought. The objectives were (i) to assess the specificity of microbiota associated with plants living with the carob, and (ii) to quantify their impact on carob growth and resistance to water deficit. The results showed a predominance of actinobacteria, proteobacteria and ascomycetes in the studied habitats, with significant functional divergences between North and South of Morocco. Common garden experiments revealed that the type of microbiota was a major driver of the effect on carob growth and resistance, with strong variations depending on the water status. Indeed, the native microbiota of the carob tree is one of the most efficient under non-limiting water conditions but confers little resistance to drought. On the other hand, the microbiota associated with the lentisk provides the best growth-resistance trade-off. This benefit seems to be attributable to a higher abundance of DSE (dark septate endophytes) fungi and an increase in mycorrhizal colonisation in response to water stress. The integration of lentisk in technical practices as a lever for the management of a beneficial microbiota could therefore represent a future strategy for the adaptation of carob to increasing drought in the Mediterranean basin.