Plant growth-promoting rhizobacteria alleviate drought-stress in upland rice

Drought poses a substantial risk to global agriculture and food security. Plant growth-promoting rhizobacteria (PGPR) offer a promising biotechnological alternative to enhance crop resilience to drought. This study investigated the morphophysiological and agronomic responses of Samambaia Branco, a drought-sensitive upland rice landrace, inoculated with Bacillus toyonensis (BRM 32110), Burholderia sp. (BRM 32111), and Serratia marcescens (BRM 63523), under drought stress in a phenotyping platform. Drought significantly impaired overall crop performance, reducing grain yield by 27.7% and increasing spikelet sterility up to 18.4% in non-inoculated plants. However, PGPR-inoculated plants underwent robust belowground architectural restructuring, showing average increase in root length (29.4%), surface area (34.1%), volume (36.6%) and diameter (8.7%). This enhanced root system provided a mechanistic basis for superior water status. PGPR-inoculated plants sustained higher leaf water potential under drought stress (-0.67 MPa on average) than non-inoculated plants (-1.3 MPa). Hydraulic preservation was coupled with a profound shift in non-structural carbohydrate partitioning, characterized by depletion of starch reserves (26.5%) and total soluble carbohydrates (16.7%), alongside accumulation of low-molecular-weight compatible solutes (glucose, fructose, and sucrose) osmotic adjustment. During drought and recovery, PGPR-inoculated plants, particularly BRM 32110 and BRM 63523, optimized the tradeoff between carbon gain and transpirational loss, achieving a 12.4% increase in water use efficiency and partially lowering the physiological threshold for stomatal reopening. Multivariate principal component analysis confirmed clear treatments separation, highlighting distinct microbe-mediated metabolic pathways for drought tolerance. Overall, bioagent-based technologies can bolster the climate resilience of drought-sensitive landraces, improving crop performance.