A rhizosphere interplay between cypress trees and root-associated bacteria during drought

The biotic environment of trees can largely affect their drought tolerance in the field. Specifically, root-associated bacterial communities play a main and vital role in maintaining health of the tree host. These communities are in complex relationships with their host, where the composition and abundance of community members are also dependent on the same environment of the trees. Here we explored the enhanced root exudation under drought in Cupressus sempervirens, we inoculated young cypress trees with the Gram-positive Bacillus Subtilis and the Gram-negative Pseudomonas stutzeri, beneficial soil bacteria. We followed the soil chemistry, tree physiological responses to drought and alterations in the interaction between cypress roots and bacteria during a drought period. We found that trees recruited B. Subtills and P. stutzeri, as observed by 3-10 fold-increased colonization in rhizosphere than in bulk soil. Using Confocal microscopy and bacteria colony counting showed that P. stutzeri accumulated and enhance colonization on roots of irrigated trees, whereas B. subtilis colonization increased on roots of drought-exposed trees. Furthermore, there were simultaneous changes in root exudate profiles of trees inoculated with bacteria compared to trees not inoculated. Inoculated trees had 60% higher photosynthesis rate during heatwave, and recovered faster post-drought. We propose that trees benefit from recruitment of specific bacterial lineages under environmental changes, in turn allowing them to improve their drought tolerance. The ongoing project further characterizes the effects of this bacteria-root interaction on tree water and nutrition status.