Autophagy Sustains the Arabidopsis Root Meristem during Phosphate Starvation

The ability of organisms to survive during stress conditions relies on several cellular responses. Among these responses, autophagy is known as a multi-functional process which can alleviate various cellular stresses including nutrient starvation. Although many studies have elucidated the crucial role of autophagy in response to carbon and nitrogen starvation, none of them demonstrated any essential function of autophagy during other types of starvation. Here, we use Arabidopsis as a model to show that autophagy is induced during phosphate (Pi) starvation. Time course monodansylcadavarine (MDC) staining revealed numerous autophagosome spots in the primary root tip of seedlings 4 days after transfer to media without Pi. QRT-PCR of atATG8s, a gene family involved in autophagosome formation, showed that relative mRNA abundance of some ATG8s increased at time points consistent with the observed autophagosome spots. In addition, characterization of autophagy mutant atg5-4 revealed conditional lateral root growth reduction phenotype under low Pi condition. Accelerated root meristem consumption suggested by early loss of CYCB1::GUS marker in atg5-4 root meristem explains this defect in lateral root growth. Furthermore, time course FDA-PI viability staining results and low primary root recovery rate indicate that loss of root meristem in atg5-4 is irreversible. Based on this data, we conclude that autophagy is a part of Pi starvation responses which have crucial roles in maintaining the root meristem