Lack or excess of autophagy leads to premature leaf senescence probably owing to unbalanced nutrient management

Abstract

Background and Aims Macroautophagy is essential for the degradation and recycling of various macromolecules in eukaryote cells. In plants, autophagy is involved in the degradation of damaged chloroplasts in response to stress. Autophagy is a key player in nitrogen management at the whole-plant level, and autophagy mutants display strong defects in nitrogen remobilization and early leaf senescence phenotypes especially under nitrogen source limitation. It is known that leaf senescence is associated to nutrient remobilization processes and is induced by nitrate limitation. However, it remains to be determined which actors are involved in this interplay and whether nutrient remobilization acts as a signal that enhances leaf senescence or whether senescence-associated chloroplast degradation promotes nutrient remobilization. In this context, our aim is to demine whether the level of autophagy activity controls leaf longevity and influences the progress of leaf senescence balancing resources and waste management. Methods In this study, we used Arabidopsis autophagy knock-out mutants (atg) and plants overexpressing autophagy associated genes (ATG8-OE) to compare their phenotypes to wild type under both sufficient nitrate condition and nitrate starvation. Key Results The transfer from nitrate-sufficient conditions to nitrate starvation accelerated leaf senescence in all the genotypes. Unexpectedly, under both sufficient and nitrate-starved conditions, both atg mutants and ATG8 over-expressors exhibited earlier leaf senescence phenotypes compared to wild type. Given that autophagy is a longevity factor, the more severe leaf senescence phenotype of over-expressors was puzzling. This study highlights a relationship between autophagy, nitrogen remobilization and leaf senescence and shows how the fine tuning of nutrient management can influence senescence onset. Conclusions The fine-tuning of autophagy is necessary to control leaf senescence.