Marchantia polymorpha, a New Model Plant for Autophagy Studies

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.

Friend or Enemy: A Dual Role of Autophagy in Plant Virus Infection

“…For instance, atg5 and atg7 mutants were inoculated with cauliflower mosaic virus (CaMV) and exhibited more severe symptoms than the wild type, indicating that the process of autophagy may involve an antiviral mechanism (Hafrén et al, 2017). In recent years, the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) (CRISPR/Cas9) system was developed for genome editing (Doudna and Charpentier, 2014;Ma et al, 2016), allowing the scientific community to obtain atg mutants in different species and therefore promoting the study of the function and mechanism of autophagy (O'Prey et al, 2017;Norizuki et al, 2019).…”

Role of Autophagy in Male Reproductive Processes in Land Plants

“…In land plants (embryophytes), core ATG genes are highly conserved, and their functions are shown to be similar to homologs in yeast and mammals (Avin-Wittenberg et al, 2012;Yoshimoto, 2012;Norizuki et al, 2019). Studies of Arabidopsis thaliana atg mutants have demonstrated that autophagy is involved in responses to abiotic and biotic stressors such as nutrient starvation and pathogen attacks (Marshall and Vierstra, 2018).…”

Bryophyte spermiogenesis occurs through multimode autophagic and nonautophagic degradation

“…28,29 This drastic organelle remodeling during spermiogenesis could be completed through activated degradation pathways such as autophagy; therefore, we examined whether spermiogenesis in M. polymorpha involves autophagy. We previously reported that Mpatg mutants exhibit chlorosis and hypersensitivity to nutrient starvation, 17 but the involvement of autophagy in reproductive processes remained unassessed. In this study, we investigated whether mutants defective in autophagy also exhibit any defects in spermiogenesis.…”

“…In this study, we investigated whether mutants defective in autophagy also exhibit any defects in spermiogenesis. In contrast to wild-type spermatozoids with elongated helical cell bodies containing thin cylindrical nuclei with trace amounts of cytoplasm, spermatozoids of Mpatg5-1 ge , which was generated using the CRISPR/Cas9 system, 17 harbored abundant cytoplasm and abnormally shaped nuclei that were significantly more circular than wild-type spermatozoids (Figures 4A-4C).…”

“…[13][14][15] Several core autophagy-related (ATG) proteins mediate autophagosome formation and are highly conserved in eukaryotic lineages, including plants. [16][17][18][19] Autophagy plays a critical role in bulk degradation of the cytoplasm for nutrient recycling and participates in the selective degradation of various cytoplasmic components, which is important for homeostasis and/or cell reorganization. [20][21][22] Mitochondria are a target of selective degradation by autophagy, which underpins proper mitochondrial functions in various organisms, including plants.…”