Streptomyces rhizobacteria enhance growth and drought tolerance in maize (Zea mays L.) under greenhouse conditions.

Drought is among the most severe limitations to crop productivity, impairing plant development. Although plant growth-promoting rhizobacteria have been reported to alleviate drought stress in several crops, little is known about how Streptomyces affect vegetative growth of maize under water deficit. This study investigated rhizospheric Streptomyces isolates as sustainable tools to mitigate drought-related constraints in maize. Isolates CLV16, CLV100, CLV115, and CLV179 were screened under PEG6000-induced water stress (-0.6, -1.0, and - 1.7 MPa, representing mild, moderate, and severe water deficit, respectively) to characterize their growth and plant growth-promoting traits under stress. All isolates remained metabolically active at water potentials down to -1.0 MPa, while phosphate solubilization and siderophore production were maintained across the tested. Under severe water stress, all isolates retained their PGPR- attributes, although showing distinct functional responses: CLV100 showed threefold greater colony growth than the other isolates, whereas CLV115 exhibited the highest production of indolic compounds, with levels 8.9-fold higher than those of the other isolates. CLV179 produced threefold more ammonia than the other isolates, while CLV16 showed the lowest expression of the tested traits. Under greenhouse conditions, Streptomyces-colonized maize plants grown at 30% field capacity (soil moisture 5.4 to12.8%) showed increased root elongation and biomass accumulation compared with well-watered control at 100% field capacity (soil moisture 19.2 to 28.2%). Specifically, CLV100, CLV115 enhanced maize biomass and shoot growth under drought stress. CLV179, besides these improvements on maize growth, resulted in increased root length (22%) in stressed plants compared with non-bacterized control. These findings suggest the potential of Streptomyces isolates as bioinoculants for improving maize performance under water deficit.