Immunity and bacterial recruitment in plant leaves are parallel processes that together shape sensitivity to temperature stress

Rising global temperatures necessitate developing resilient crops with better adaptability to changing climates. Under elevated temperatures, plant immunity is downregulated, increasing risk of foliar pathogen attack. Manipulating plant defense hormones is one way to mitigate this detrimental effect. However, it is unclear how plant immunity interacts with plant microbiome assembly and how temperature will thus affect overall plant health and stability. In this study, we compared two Arabidopsis thaliana genotypes that feature divergent strategies for recruitment of commensal bacteria from natural soil. NG2, an A. thaliana ecotype we collected from Jena, Germany, was grown in its native soil and compared to CLLF, a genotype that recruits higher bacterial loads and higher bacterial diversity but without any dysbiotic phenotype. CLLF hyperaccumulates salicylic acid (SA) and jasmonates, has constitutively upregulated innate defenses, and shows increased resistance to necrotrophic fungal and hemi-biotrophic bacterial pathogens, indicating that pathogen immunity and non-pathogen recruitment function in parallel. Some of its leaf bacteria can utlize SA as a carbon source, suggesting that immunity and recruitment may even be linked by chemical hormones. CLLF exhibits high tolerance to heat stress in comparison to the NG2, with SA-associated defense processes remaining active under heat. Synthetic community (SynCom) experiments revealed that when the taxonomic diversity of bacteria available to CLLF is artificially reduced, resilience to heat stress is compromised, leading to dysbiosis. However, this dysbiosis does not occur in CLLF with a full SynCom or in the NG2 with any SynCom. These findings suggest that the downregulation of defenses in response to heat may contribute to the avoidance of dysbiosis caused by certain leaf bacteria, while full bacteriome taxonomic diversity can help maintain balance.