Ongoing anthropogenic climate change is projected to substantially increase regional frequency, duration, and intensity of drought events over the coming decades. Among the systems most vulnerable to these intensifying drought conditions are agricultural crops. Vicia faba (Fava Bean), an important staple crop cultivated worldwide, is particularly susceptible to drought stress, which significantly impairs growth, yield, and nitrogen fixation. In addition to plant genetics and irrigation methods, root-associated microbial communities (the rhizosphere microbiome) are increasingly recognized as critical mediators of plant drought responses. Many plant species can modulate the composition of their rhizosphere microbiome, potentially enhancing resilience to water limitation. Investigating drought-induced shifts in the V . faba rhizosphere microbiome may therefore provide insight into microbially-mediated mechanisms of stress tolerance. A controlled greenhouse experiment was conducted to examine how acute drought stress influences (1) the diversity of culturable rhizosphere bacteria and (2) community-level carbon use profile of the microbial community. Twenty V . faba plants were grown from seed under regularly watered conditions for the first two weeks and subsequently assigned to either a well-watered control or a drought treatment. Rhizosphere samples were collected at three time points two weeks apart from different plants. Suspensions of the rhizosphere were plated, and the microbial colonies were distinguished by morphotype using a custom Cell Profiler workflow to quantify richness and diversity of each rhizosphere community. Biolog EcoPlates were also used to generate community-level physiological profiles of carbon use. 16S rRNA-gene-based molecular fingerprinting was used to assess the change in diversity and richness of bacterial taxa. Across all samples, colony counts and morphotype richness varied by time and treatment. A significant reduction in morphotype richness under drought was observed after two weeks, followed by a return to richness levels comparable to controls at four weeks. EcoPlate analyses indicated broad and consistent carbon-substrate utilization across treatments. TRFLP community fingerprinting showed that bacterial diversity and richness in drought treatment plants had significantly decreased by the end of the drought period. Together, these findings suggest that short-term drought leads to a loss of bacterial diversity in the V . faba rhizosphere, without impacting the total abundance of cultivable bacteria or the functional capacity of the rhizosphere community.
