Apple responses to frost are affected by inoculations with cold-tolerant bacteria

Climate warming is responsible for mild winters and warm springs, causing premature plant development and increasing the risk of exposure of vulnerable plant tissues to spring frost. Cold stress triggers changes in membrane fluidity and reactive oxygen species (ROS) levels, causing damage to plant tissues. Wild plants growing in alpine regions are associated with complex microbial communities that may support plant growth and survival under cold conditions. However, limited information is available on the beneficial plant-microbe interactions in the mitigation of freezing stress. The aim of this project was to characterize the efficacy and mode of action of cold-tolerant endophytic bacteria isolated from alpine Rosaceae plants in mitigating freezing stress in apple plants. Three cold-tolerant bacteria, belonging to Pseudomonas and Duganella genera, were selected according to their ability to decrease electrolyte leakage in freezing-stressed strawberry seedlings. Apple seedlings (cultivar Golden Delicious) were grown under controlled conditions, treated (bacterium-inoculated) or not (mock-inoculated) with cold-tolerant bacteria, and exposed to frost simulation. Preliminary results showed that bacterial inoculation decreased ROS content in leaf tissues after freezing stress. Transcriptomic analysis revealed the upregulation of genes related to gene ontology (GO) biological processes of carbohydrate metabolism, stress response, and signal transduction in mock-inoculated and bacterium-inoculated plants after freezing stress. Moreover, plants inoculated with coldtolerant bacteria upregulated the expression of genes related to nucleotide metabolism and cell cycle. These findings highlighted the potential contribution of coldtolerant bacteria isolated from alpine Rosaceae plants in enhancing apple resilience to freezing stress and created the basis to improve agricultural sustainability under climate change.