Physiological regulations of a highly tolerant cactus to dry season modify its rhizospheric microbial communities

Abstract

Echinocactus platyacanthus is a Mexican cactus highly tolerant to stressful conditions. In recent years it has been increased evidence that plant microbiome is closely linked to plant physiology. We hypothesize that E. platyacanthus responds to environmental changes by altering its physiology, which modifies the bacterial communities in its rhizosphere, especially the abundance of plant growth-promoting bacteria. Physiological changes associated with adult individuals of E. platyacanthus during the rainy and dry seasons were analyzed [chlorophyll fluorescence, photosynthetic pigments, nocturnal acidity, water potential, reactive oxygen species, and root exudates (amino acids)]. Rhizosphere samples were taken to determine the soil's physicochemical properties and bacterial communities. During the dry season, E. platyacanthus makes significant physiological adjustments to respond to the decrease in soil water and nutrient availability, mainly increasing chlorophyll b and carotenoids. The physiological regulation cascade modifies the profile of amino acids exuded by its roots. In turn, the rhizosphere bacterial community changed. Beneficial bacteria to the plant from the phylum Armatimonadota, classes Actinobacteria and Dehalococcoidia, and families Gemmataceae and Nitrosococcaceae, were more abundant in the dry season. Based on a regularized generalized canonical correlation analysis, we showed a model to explain how E. platyacanthus tolerance to the dry season could be conferred by an interaction between its physiological regulatory mechanisms and the symbiotic relationships with soil bacteria mediated through root exudation. Furthermore, this work is the first description of the amino acids exuded by cacti that stimulate the increase in the populations of beneficial bacteria in the rhizosphere.