Plant growth-promoting effect of Kocuria sp. and Brevibacillus sp. strains on tomato under different irrigation conditions

With global agriculture increasingly challenged by water scarcity and the need to reduce chemical inputs, plant growth-promoting bacteria (PGPB) offer a sustainable means to enhance crop productivity and stress tolerance. By improving nutrient availability and uptake efficiency, PGPB can reduce the reliance on synthetic fertilizers without compromising crop yield or quality, delivering both economic and environmental benefits. Despite this potential, most existing studies have focused on short-term effects in seedlings or plantlets under laboratory conditions using artificial substrates, with relatively few investigations addressing the full crop life cycle under realistic field conditions. In this study, we assessed the effects of two rhizosphere-derived bacterial isolates from arid grasslands— Kocuria rhizophila FSP120 and Brevibacillus porteri FSP5—on tomato ( Solanum lycopersicum “Uno Rosso”) performance under both full and deficit irrigation regimes. The isolates were applied either as seed coatings or post-emergence inoculants. Seed coating treatments negatively impacted germination and early seedling development. In contrast, post-emergence application of FSP120 significantly enhanced plant height, leaf number, and advanced the onset of flowering by more than 1 week. Importantly, FSP120 inoculation increased marketable fruit yield by 16% under full irrigation and 11% under deficit conditions compared to untreated controls. Lycopene and total carotenoid concentrations were improved by 23–29%, while tocopherol and vitamin C contents remained unaffected. Metagenomic analysis confirmed successful rhizosphere colonization by FSP120, indicating its ecological competitiveness and potential for enhancing plant resilience. Conversely, FSP5 exhibited limited colonization capacity and lower agronomic benefits. These results underscore the importance of selecting effective bacterial strains and optimizing delivery methods in the design of PGPB-based bioinoculant strategies. K. rhizophila FSP120 strain emerges as a promising candidate for improving the sustainability and nutritional quality of tomato production throughout the full plant life cycle under field-relevant irrigation conditions.