Hydrophyte root microbiome: a novel reservoir of plant growth-promoting bacteria for enhancing lettuce (Lactuca sativa L.) growth and nutritional quality under soilless cultivation.

Soilless cultivation is a major component of modern protected agriculture; however, often lack the beneficial microbial communities that support plant health in natural soils. This study explored hydrophyte root microbiomes as an eco-friendly and novel source of plant growth-promoting bacteria (PGPB) for engineering beneficial microbial communities in soilless systems. Bacteria associated with the roots of Eichhornia crassipes, Pistia stratiotes, and Alternanthera philoxeroides, expressing PGP traits were assessed for their growth-promoting potential on lettuce (Lactuca sativa L. cv. 'Lolo Red') using cocopeat-based soilless media and soil under polyhouse conditions. Hydrophyte associated rhizobacteria exhibited diverse PGP functions, including nutrient solubilization and phytohormone production, similar to those exhibited by rhizobacteria associated with terrestrial plants. Under soil and soilless conditions, lettuce crop inoculated with hydrophyte associated rhizobacteria significantly enhanced germination, plant biomass, root architecture, photosynthetic pigments and leaf quality traits, including TSS (total soluble solids), total phenolics, vitamin C, anthocyanins, antioxidant enzyme activities, DPPH (2,2-Diphenyl-1-picrylhydrazyl) radical scavenging and FRAP (Ferric Reducing Antioxidant Power) responses in both the cultivation systems. Two strains viz. Bacillus aerius Aq35 and Pseudomonas protegens Aq45 were particularly very promising. Under soilless cultivation, Aq35 enhanced root fresh weight and surface area by 90.0% and 83.5%, respectively, while Aq35 and Aq45 increased lettuce yield by 39.62% and 28.70%, respectively. Bacterial inoculation significantly enhanced the availability of soil and plant macro (N, P, and K) and micronutrients (Fe, Mn, Zn, and Cu) along with a marked increase in enzymatic activities and microbial biomass carbon (MBC). Under soilless substrate, dehydrogenase activity increased from 43.40 to 64.65 µg TPF g⁻¹ day⁻¹, while alkaline phosphatase activity increased from 261.53 to 380.87 µg PNP g⁻¹h⁻¹ inoculated (Aq35) treatment over uninoculated treatment indicating enhanced substrate biological quality. These results demonstrate that hydrophytes as rich, pre-adapted reservoirs of potent PGPB, and strains such as B. aerius Aq35 and P. protegens Aq45 hold strong potential as biostimulants for sustainable soilless agriculture.