Sucrose Starvation Induces Microautophagy in Plant Root Cells.

Shino Goto-Yamada,Shuji Shigenobu,Shoji Mano,Katsushi Yamaguchi,Makoto Hayashi,Mikio Nishimura,Haruko Ueda,Ikuko Hara-Nishimura,Kazusato Oikawa,Jakub Bizan,Kenji Yamada

doi:10.3389/fpls.2019.01604

Abstract

Autophagy is an essential system for degrading and recycling cellular components for survival during starvation conditions. Under sucrose starvation, application of a papain protease inhibitor E-64d to the Arabidopsis root and tobacco BY-2 cells induced the accumulation of vesicles, labeled with a fluorescent membrane marker FM4-64. The E-64d–induced vesicle accumulation was reduced in the mutant defective in autophagy-related genes ATG2, ATG5, and ATG7, suggesting autophagy is involved in the formation of these vesicles. To clarify the formation of these vesicles in detail, we monitored time-dependent changes of tonoplast, and vesicle accumulation in sucrose-starved cells. We found that these vesicles were derived from the tonoplast and produced by microautophagic process. The tonoplast proteins were excluded from the vesicles, suggesting that the vesicles are generated from specific membrane domains. Concanamycin A treatment in GFP-ATG8a transgenic plants showed that not all FM4-64–labeled vesicles, which were derived from the tonoplast, contained the ATG8a-containing structure. These results suggest that ATG8a may not always be necessary for microautophagy.

Highlights

Autophagy is one of the cellular degradation systems in eukaryotes that removes unwanted or toxic cellular components
The wild-type root formed a huge aggregate in each cell after treatment with E-64d and FM4-64 under sucrose starvation for 24 h; the formation of aggregates was as hard at the tip of the root and moderate toward the top, and the mock treatment with FM4-64 rarely produced aggregates in the root tip (Figure 1A)
To assess whether the formation of E-64d vesicles is related to clathrin-dependent endocytosis, we treated the ap2m-2 and ap2s-1 mutants, which are defective in the components of the AP2 clathrin adaptor complex, μ- and σ-subunits, respectively (Supplementary Figure 3, Fan et al, 2013; Yamaoka et al, 2013)

Summary

Introduction

Autophagy is one of the cellular degradation systems in eukaryotes that removes unwanted or toxic cellular components. Cytosolic components are surrounded by a double-layered isolation membrane to form an autophagosome. The isolated cytosolic components in the autophagosomes are transported to vacuoles (yeasts and plants) or fused to lysosomes (animals), and degraded by enzymes in these lytic organelles (Ohsumi, 2001). Macroautophagy was initially identified as a starvationinduced response in yeast in which macroautophagy recycles carbon and nutrients to survive starvation (Takeshige et al, 1992). In the macroautophagy process occurring in starvation, cellular components are non-selectively engulfed by autophagosome membranes for degradation. Many autophagy-related (ATG) genes have been identified from

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Conclusion