Flooding resilience is determined by leaf age in Arabidopsis thaliana

Flooding events lead to major crop losses every year, this is expected to increase due to climate change. The resilience to flooding stress is not uniform throughout a plant: the older leaves are typically the first to die whereas the young leaves and the meristem survive for longer. In this thesis I describe some of the mechanisms that control this age-dependent flooding tolerance using the model organism Arabidopsis thaliana. During a flooding event, old leaves went into a state of senescence faster than young leaves. This process was found to be in part controlled by the transcription factor ORESARA1 (ORE1), which induced the expression of senescence-related genes primarily in old leaves. Although ORE1 protein accumulated in both old and young leaves during flooding, it was activated by phosphorylation specifically in old leaves. Knocking out ORE1 reduced the senescence of old leaves during flooding and improved flooding tolerance. When floodwaters subside, this typically poses a second round of stress for plants as they are suddenly exposed to light and air. Leaves can rapidly dehydrate during the post-flooding phase, and this was found to be more severe in old leaves than young leaves. Although old leaves recovering from flooding stress showed a strong transcriptional response to dehydration and abscisic acid (ABA), their stomata had a reduced sensitivity to ABA which could contribute to their increased water loss. The results presented in this thesis highlight how different processes lead to variation with leaf age in flooding resilience. Understanding these mechanisms can help in developing novel crop varieties that are more tolerant to flooding stress across all of their tissues.