Plant growth-promoting rhizobacteria as a soil-based strategy to improve nutrient acquisition and drought tolerance in sugarcane

Abstract

Drought is an increasingly important constraint to sustainable sugarcane production because prolonged water deficit can impair root development, nutrient acquisition, photosynthesis, biomass accumulation, sucrose accumulation, and ultimately cane and sugar yield. Plant growth-promoting rhizobacteria (PGPR) have emerged as a potential soil-based strategy for improving sugarcane performance under water-limited conditions through multiple direct and indirect mechanisms. This review critically evaluates the current evidence regarding PGPR-mediated drought mitigation in sugarcane, with special emphasis on nutrient acquisition, root-system development, phytohormonal regulation, ACC deaminase activity, biological nitrogen fixation, siderophore and extracellular polymeric substance production, antioxidant responses, and pathogen suppression. Available evidence suggests that PGPR can improve intermediate traits such as root growth, nutrient status, photosynthetic performance, water relations, and oxidative-stress regulation; however, the extent and consistency of these responses are highly variable among microbial strains, sugarcane genotypes, drought severity and duration, soil properties, climatic conditions, and management practices. Importantly, positive responses observed under laboratory, pot, and greenhouse conditions cannot be assumed to translate directly into stable yield benefits under field conditions. Severe or prolonged drought may restrict microbial survival, metabolic activity, root exudation, nutrient diffusion, and rhizosphere colonization, thereby reducing the effectiveness of inoculants. Evidence for several proposed mechanisms, including microbial hormone production, zinc, silicon, and sulphur mobilization, pathogen suppression under combined drought, and post-drought recovery, remains limited or indirect in sugarcane. Similarly, microbial consortia should not be assumed to be synergistic without appropriate comparative and factorial validation. Major priorities, therefore, include multi-location and multi-season field trials, standardized quantitative measurements of microbial persistence and plant responses, genotype- and soil-specific inoculant selection, longitudinal assessment across drought and recovery phases, and evaluation of economic feasibility and formulation stability. Overall, PGPR represents a promising but context-dependent component of integrated drought management in sugarcane rather than a universally reliable standalone solution.