Desert rhizosphere microbiota and plant resistance to water stress

INTRODUCTION: Soil salinity and drought are among the most severe environmental stresses affecting plant growth and production around the world. While mycorrhiza have been widely studied for their contribute to plant resistance to water stress, few information are available about the role of bacteria. OBJECTIVES: aim of the work was to study diversity and growth promotion potential of bacterial plant microbiome, evaluating its contribute in alleviating water stress. MATERIALS AND METHODS: Two model plants, naturally subjected to water stress, were chosen: pepper (Capsicum annuum L.), a drought-sensitive crop cultivated in traditional Egyptian farm and Salicornia (S. strobilacea), a wild halophyte plant growing in Tunisian hypersaline soils (Chott). DGGE fingerprinting was used to describe structure and diversity of rhizosphere and endosphere bacterial communities, in comparison with plant-free soils. Wide collections of bacterial isolates were established from the two plants, and the strains were screened in vitro and in vivo for plant growth promotion (PGP) activities and for the resistance to environmental stresses typical of desert environments, i.e. temperature, salinity and osmotic pressure. RESULTS: The results showed that the plant determined a selective pressure on the phylogenetic diversity of the associated populations. The screening of the bacterial collection showed that a significant fraction of the cultivable bacterial populations associated to desert plants display multiple PGP activities and are adapted to survive under severe environmental conditions. In vivo experiments with selected strains demonstrated, moreover, the capacity of PGP rhizobacteria to colonize the root surface and to enhance plant photosynthetic activity and biomass synthesis (up to 40%) under drought stress. CONCLUSIONS: in the sight of “reverse desertification”, crop cultivation in arid, saline and degraded lands is a key practice for the preservation of soil stability and fertility, with the root system acting as a “resource island” able to attract and select microbial communities endowed with multiple PGP traits that sustain plant development under water stress conditions.