High Light Intensity Potentiates Bacillus amyloliquefaciens PMB05-Associated Plant Immunity and Bacterial Soft Rot Control in Arabidopsis thaliana

Bacterial soft rot, caused by Pectobacterium carotovorum subsp. carotovorum, affects numerous vegetable crops. Beneficial microbes like Bacillus amyloliquefaciens PMB05 offer a sustainable alternative to traditional copper- or antibiotic-based controls, which risks the development of pathogen resistance. Bacillus amyloliquefaciens PMB05 enhances disease resistance to soft rot by intensifying PAMP-triggered immunity (PTI). Although our previous research showed that PMB05-intensified PTI correlates with a reduced photosynthetic electron transport rate under high light conditions, it remains unclear whether increasing light intensity can strengthen PMB05-mediated immune signaling to further enhance disease resistance in plant leaves. In this study, Arabidopsis plants were grown under varying light intensities to address this question. Furthermore, expression of the type III elicitor, HrpN protein, was applied to evaluate the impact of PMB05 treatment on immune activation. HrpN acts as a PAMP that can trigger plant defense responses. The results revealed that higher growth light intensity was associated with PMB05-intensified PTI response by increasing reactive oxygen species generation but suppresses callose deposition. In addition, PMB05-facilitated PTI-mediated stomatal closure occurred more rapidly and tightly under high light, thereby restricting pathogen invasion into leaf tissues. Disease assays further confirmed that higher light intensity significantly reduced soft rot severity in A. thaliana, while the combination of high light and PMB05 treatment achieved superior control efficacy. Furthermore, PMB05-associated disease protection under high growth light was not detected in sid2, npr1, and nahG plants, indicating that intact SA-associated functions are required for the detectable protective phenotype under the conditions tested. These findings demonstrate that environmental light management can strategically potentiate immunity-intensifying microbes, establishing a novel paradigm for sustainable disease control through the coordinated manipulation of light conditions and microbial application.