Role of Autophagy-related Proteins ATG8f and ATG8h in the Maintenance of Autophagic Activity in Roots Under Phosphate Starvation

Nutrient starvation-induced autophagy is a conserved process in eukaryotes. Plants defective in autophagy show hypersensitivity to carbon and nitrogen limitation. However, the role of autophagy in plant phosphate (Pi) starvation response is relatively less explored. Among the core autophagy-related () genes, encodes a ubiquitin-like protein involved in autophagosome formation and selective cargo recruitment. The genes, and , are notably induced in roots under low Pi. In this study, we show that such upregulation correlates with their promoter activities and can be suppressed in the () mutant. Yeast one-hybrid analysis failed to attest the binding of the PHR1 transcription factor to the promoter regions of and . Dual luciferase reporter assays in mesophyll protoplasts also indicated that PHR1 could not transactivate the expression of both genes. Loss of ATG8f and ATG8h leads to decreased root microsomal-enriched ATG8 but increased ATG8 lipidation. Moreover mutants exhibit reduced autophagic flux estimated by the vacuolar degradation of ATG8 in the Pi-limited root but maintain normal cellular Pi homeostasis with reduced number of lateral roots. While the expression patterns of and overlap in the root stele, is more strongly expressed in the root apex and root hair and remarkably at sites where lateral root primordia develop. We hypothesize that Pi starvation-induction of ATG8f and ATG8h may not directly contribute to Pi recycling but rely on a second wave of transcriptional activation triggered by PHR1 that fine-tunes cell type-specific autophagic activity.