Background: Autophagy is a protein degradation process by which cells recycle cytoplasmic contents under stress conditions or during senescence; a basal level of housekeeping autophagy also occurs under non-stressed conditions. Although a number of genes that function in autophagy (ATG genes) have been identified in plants, the upstream components that regulate the plant autophagy pathway are still obscure. Target of rapamycin (TOR) is a negative regulator of autophagy in both yeast and animals, and homologs of TOR in plants control plant growth and protein synthesis. However, a role for TOR in regulation of autophagy in plants has not been demonstrated previously.

Methodology/Principal Findings: In this paper we used RNA interference (RNAi) to generate transgenic plants with reduced AtTOR transcript level. By observing monodansylcadaverine- (MDC) and GFP-AtATG8e-labeled autophagosomes, these plants were demonstrated to have constitutive AtATG18a-dependent autophagy. Reverse transcriptase-PCR also showed increased expression of some AtATG genes in the RNAi-AtTOR plants. Unlike autophagy induced by starvation or salt stress, an NADPH oxidase inhibitor did not inhibit the constitutive autophagy in the RNAi-AtTOR lines, indicating that AtTOR is either downstream of or in a parallel pathway to NADPH oxidase.

Conclusions/Significance: Together, our results provide evidence that TOR is a negative regulator of autophagy in plants.