Boosting drought resilience: the role of endophytic Bacillus safensis in enhancing melatonin production in chickpea cultivars

Drought stress significantly affects global crop yields, necessitating innovative strategies to enhance plant resilience. This study explored the role of the endophytic bacterium Bacillus safensis COBR7 in promoting melatonin biosynthesis and drought tolerance in two chickpea (Cicer arietinum L.) cultivars, Samin (drought-tolerant) and ILC3279 (drought-sensitive). Melatonin biosynthesis genes in chickpeas were identified by comparing homologous sequences from closely related species in the NCBI database using the BLASTP method. Four TDC (Tryptophan decarboxylase), two SNAT (Serotonin N-acetyltransferase), seven COMT (Caffeic acid O-methyltransferase), and one ASMT (N-acetylserotonin O-methyltransferase) genes were identified in chickpeas. We examined their expression by real-time PCR under two conditions (inoculated and non-inoculated with bacteria) and three irrigation conditions (control, moderate drought, and severe drought). Inoculation with B. safensis significantly enhanced the expression of ASMT, COMT, and TDC genes, particularly under severe drought stress, with the highest expression levels observed in the inoculated plants. Gas chromatography-mass spectrometry (GC-MS) analyses revealed that melatonin concentrations increased under drought conditions, with inoculated samples exhibiting higher levels than non-inoculated controls. In this study, drought stress- and treatment-induced changes were observed in other metabolites (amino acids, sugars, and organic acids) in both cultivars. OrthoVenn2-based comparative genomic analysis identified 544 conserved orthologous gene clusters shared between C. arietinum and B. safensis, including stress-responsive genes (e.g., superoxide dismutase, ACC deaminase) and metabolic pathways critical for drought adaptation, such as tryptophan biosynthesis—a precursor for melatonin. These shared clusters suggest a synergistic interaction in which B. safensis COBR7 primes melatonin-mediated drought tolerance in chickpea, potentially through the bacterial provision of tryptophan-derived intermediates or the modulation of plant stress signaling. This highlights COBR7’s role as a bioinoculant to enhance resilience in water-limited agroecosystems.