Work

Bacteria from extreme environments as candidate bioinoculants for drought tolerance in legumes: a critical appraisal of mechanisms and evidence

Year: 2026

Type: review

Abstract: Drought is one of the major constraints limiting legume productivity, impairing plant growth, biological nitrogen fixation, and seed nutritional quality. The increasing frequency and intensity of water scarcity have intensified the search for sustainable strategies to improve crop resilience. Bacteria isolated from extreme environments have been proposed as candidate bioinoculants because adaptations to desiccation, salinity, or extreme temperatures may provide traits relevant to plant performance under water deficit. However, most of these isolates are extremotolerant rather than extremophilic sensu stricto, and much of the supporting evidence derives from stresses other than drought. The objective of this review is to synthesize current knowledge on the mechanisms through which extremophilic and extremotolerant bacteria may contribute to drought tolerance in legumes, using explicit operational definitions for these microbial categories and for drought, and a four-tier evidence classification applied at the level of individual studies. Candidate mechanisms include exopolysaccharide- and biofilm-mediated rhizosphere water retention, modulation of root architecture and auxin/ethylene balance through ACC deaminase activity, osmolyte accumulation, antioxidant responses, nutrient mobilization, maintenance of nodulation and biological nitrogen fixation, and regulation of host stress-responsive genes. Direct evidence, defined as extremophilic or extremotolerant strains evaluated in legumes under soil water deficit, is restricted to a small number of studies, whereas a large proportion of the literature relies on salinity, heat, or PEG-induced osmotic stress, non-legume hosts, genomic potential, or in vitro assays. We also discuss constraints that may limit the use of these bacteria, including rhizosphere colonization, persistence, and competitiveness against native microbiota under non-extreme conditions. Current findings provide plausible mechanisms and proof-of-concept results, but they do not yet demonstrate reliable field performance of extremophile-derived inoculants in legume production. Strain-level characterization, standardized drought protocols, and multi-environment field validation are required to establish their agronomic value.

Source: Frontiers in Plant Science

DOI: 10.3389/fpls.2026.1944380