Root Nodule-associated Plant Probiotics Modulate Growth and Drought Stress Responses in Horse Gram Macrotyloma uniflorum (Lam.)

Nodule-associated plant probiotics (NAPPs) are a representative group of plant growth-promoting microorganisms that establish a mutualistic relationship with leguminous plants. The present study aimed to unravel the NAPPs of horse gram and to evaluate its potential plant growth attributes. A total of ten NAPPs were isolated and identified, which belonged to bacterial strains such as Pseudomonas aeruginosa HGB1, Pseudomonas indoloxydans HGB2, Acinetobacter rhizosphaerae HGB4, Enterobacter bugandensis HGB5, Klebsiella michiganensis HGB6, and Flavobacterium anhuiense HGB7, and yeast strains such as Malassezia restricta HGY1 and HGY2, and actinobacteria ( Leucobacter aridicollis HGB3), along with Rhizobium sp. HGR1. The highest levels of indole acetic acid (IAA) (39.12 µg ml ^–1 ), ACC deaminase (541.2 nmol α-Ketobutyrate mg protein ^− 1 h ^− 1 ), siderophores (93.681 µg ml ^–1 ), phosphorus solubilization, and potassium availability were positively correlated with the isolates HGR1, HGB1, HGB2, HGY1, and HGB5. Zinc solubilization was strongly correlated with HGR1, HGY1, and HGB1. Organic acid profiling of the strains revealed their potential to enhance nutrient solubilization and drought tolerance. Rhizobium sp. HGR1 and the potential nodule-associated non-rhizobial endophytes (NREs), namely P. indoloxydans HGB2 and M. restricta HGY1, were selected for their performance and non-pathogenicity. Furthermore, the interaction between Rhizobium sp. HGR1 and multi-trait NAPPs, HGB2 and HGY1, were evaluated under 20% PEG-induced moisture-deficit stress by root exudate profiling. Metabolite profiling revealed 28 bioactive metabolites, including steroids and their derivatives, saturated hydrocarbons, quinolones and their derivatives, pyridines and their derivatives, among others. Thus, this study highlights the significant potential of NAPPs to enhance plant growth, improve drought-stress resilience, and modulate signalling pathways, thereby contributing to the sustainable production of horse gram.