Endophytic <i>Bacillus subtilis</i> <scp>GUCC4</scp> Enhances Passion Fruit Seedling Tolerance to Low‐Temperature Stress

ABSTRACT Passion fruit ( Passiflora edulis Sims ) is a popular fruit favored by consumers due to its unique taste and health properties around the world. Moreover, it serves as an important source of bioactive compounds, making it highly valuable for industrial exploitation. However, its growth and development are highly susceptible to cold stress, which can result in significant economic losses. Protection against abiotic stress is a significant function of the plant microbiota. In this work, we systematically evaluated the potential of the plant growth‐promoting bacterium (PGPB) Bacillus subtilis GUCC4, an endophyte isolated from healthy passion fruit leaves, to enhance the cold stress tolerance in passion fruit. The results demonstrated that inoculation with B. subtilis GUCC4 significantly enhances the cold tolerance of passion fruit. The strain was shown to rapidly colonize and migrate into both roots and leaves within 48 h of inoculation. Compared to the control, B. subtilis GUCC4 inoculation markedly enhanced total chlorophyll content, photosynthetic parameters, antioxidant enzyme activities, osmoregulatory substances, and phytohormone levels in passion fruit seedlings. Notably, B. subtilis GUCC4 upregulated genes associated with plant growth and transcription factors responsive to low‐temperature stress at 5°C. The mode of action is multifaceted and includes efficient colonization, growth promotion, hormone balancing, and stress protection. Overall, this study demonstrates that B. subtilis GUCC4 effectively mitigates the adverse effects of low‐temperature stress on passion fruit seedlings, offering a novel and promising strategy for enhancing plant resilience under adverse climate conditions.