Marchantia polymorpha, a New Model Plant for Autophagy Studies.

Takuya Norizuki,Hirokazu Tsukaya,Takashi Ueda,Naoki Minamino,Takehiko Kanazawa

doi:10.3389/fpls.2019.00935

Abstract

Autophagy is a catabolic process for bulk and selective degradation of cytoplasmic components in the vacuole/lysosome. In Saccharomyces cerevisiae, ATG genes were identified as essential genes for autophagy, and most ATG genes are highly conserved among eukaryotes, including plants. Although reverse genetic analyses have revealed that autophagy is involved in responses to abiotic and biotic stresses in land plants, our knowledge of its molecular mechanism remains limited. This limitation is partly because of the multiplication of some ATG genes, including ATG8, in widely used model plants such as Arabidopsis thaliana, which adds complexity to functional studies. Furthermore, due to limited information on the composition and functions of the ATG genes in basal land plants and charophytes, it remains unclear whether multiplication of ATG genes is associated with neofunctionalization of these genes. To gain insight into the diversification of ATG genes during plant evolution, we compared the composition of ATG genes in plants with a special focus on a liverwort and two charophytes, which have not previously been analyzed. Our results showed that the liverwort Marchantia polymorpha and the charophytes Klebsormidium nitens and Chara braunii harbor fundamental sets of ATG genes with low redundancy compared with those of A. thaliana and the moss Physcomitrella patens, suggesting that multiplication of ATG genes occurred during land plant evolution. We also attempted to establish an experimental system for analyzing autophagy in M. polymorpha. We generated transgenic plants expressing fluorescently tagged MpATG8 to observe its dynamics in M. polymorpha and produced autophagy-defective mutants by genome editing using the CRISPR/Cas9 system. These tools allowed us to demonstrate that MpATG8 is transported into the vacuole in an MpATG2-, MpATG5-, and MpATG7-dependent manner, suggesting that fluorescently tagged MpATG8 can be used as an autophagosome marker in M. polymorpha. M. polymorpha can provide a powerful system for studying the mechanisms and evolution of autophagy in plants.

Highlights

Autophagy is a highly conserved catabolic process in eukaryotes for degrading and recycling cytoplasmic components
To gain insight into this phenomenon, we searched the genome sequences of charophytes (K. nitens and C. braunii) and bryophytes (M. polymorpha and P. patens) for homologs of ATG genes, a number of which were compared with those of A. thaliana and C. reinhardtii
ATG11 is not required for starvation-induced bulk autophagy but is essential for selective autophagy in S. cerevisiae (Kim et al, 2001); Atg11 interacts with Atg1 and cargo receptors, which is crucial for selective autophagy (Yorimitsu and Klionsky, 2005; Farre et al, 2008; Okamoto et al, 2009)

Summary

Introduction

Autophagy is a highly conserved catabolic process in eukaryotes for degrading and recycling cytoplasmic components. During macroautophagy (hereafter referred to as “autophagy”), a cup-shaped membrane sac called the isolation membrane or phagophore elongates and sequesters cytoplasmic components, and its edge is closed to form a double-membranebounded structure called the autophagosome (Baba et al, 1994). In Saccharomyces cerevisiae, autophagy-related processes are involved in biosynthetic delivery; the newly synthesized precursor form of aminopeptidase I is transported into the vacuole by small double-membrane-bounded vesicles (Baba et al, 1997), which are formed through a mechanism similar to autophagy (Harding et al, 1996; Scott et al, 1996)

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Conclusion