Comprehensive analysis of the Ppatg3 mutant reveals that autophagy plays important roles in gametophore senescence in Physcomitrella patens

Abstract

Background Autophagy is an evolutionarily conserved system for the degradation of intracellular components in eukaryotic organisms. Autophagy plays essential roles in preventing premature senescence and extending the longevity of vascular plants. However, the mechanisms and physiological roles of autophagy in preventing senescence in basal land plants are still obscure. Results Here, we investigated the functional roles of the autophagy-related gene PpATG3 from Physcomitrella patens and demonstrated that its deletion prevents autophagy. In addition, Ppatg3 mutant showed premature gametophore senescence and reduced protonema formation compared to wild-type (WT) plants under normal growth conditions. The abundance of nitrogen (N) but not carbon (C) differed significantly between Ppatg3 mutant and WT plants, as did relative fatty acid levels. In vivo protein localization indicated that PpATG3 localizes to the cytoplasm, and in vitro Y2H assays confirmed that PpATG3 interacts with PpATG7 and PpATG12. Plastoglobuli (PGs) accumulated in Ppatg3, indicating that the process that degrades damaged chloroplasts in senescent gametophore cells was impaired in this mutant. RNA-Seq uncovered a detailed, comprehensive set of regulatory pathways that were affected by the autophagy mutation. Conclusions The autophagy-related gene PpATG3 is essential for autophagosome formation in P. patens. Our findings provide evidence that autophagy functions in N utilization, fatty acid metabolism and damaged chloroplast degradation under non-stress conditions. We identified differentially expressed genes in Ppatg3 involved in numerous biosynthetic and metabolic pathways, such as chlorophyll biosynthesis, lipid metabolism, reactive oxygen species removal and the recycling of unnecessary proteins that might have led to the premature senescence of this mutant due to defective autophagy. Our study provides new insights into the role of autophagy in preventing senescence to increase longevity in basal land plants.

A deeply conserved protease, acylamino acid-releasing enzyme (AARE), acts in ageing in Physcomitrella and Arabidopsis

“…Intriguingly, this double KO results in stunted gametophores and a reduced life span. This phenotype partially resembles a loss-of-function mutant of a central component of autophagy (PpATG3) in Physcomitrella (Chen et al, 2020). In PpATG3, gametophores exhibit a reduced life span and colonies are smaller than in WT.…”

“…In contrast, gametophore size was not affected. Further, photosynthetic capacity in ΔPpATG3 was also reduced, an effect which is apparently not caused by AARE depletion (Nakai et al, 2012) and PpAARE genes were not differentially expressed in ΔPpATG3 (Chen et al, 2020). Thus, we conclude that the reduced life span observed in ΔPpAARE1/2 and ΔPpAARE1/3/2 is not due to an impaired autophagy system causing nitrogen starvation.…”

Autophagy modulates growth and development in the moss Physcomitrium patens

“…The recent availability of sequenced genomes from several lineages of streptophyte algae and bryophyte species has allowed comprehensive comparative evolutionary analyses moving forward in our understanding of plant evolution ( Rensing, 2017 ; Fürst-Jansen et al., 2020 ). Studies on autophagy in the algae Chlamydomonas reinhardtii ( Pérez-Pérez et al., 2017 ; Kajikawa and Fukuzawa, 2020 ), the liverwort Marchantia polymorpha ( Norizuki et al., 2019 ), and the moss Physcomitrium (Physcomitrella) patens ( Mukae et al., 2015 ; Sanchez-Vera et al., 2017 ; Chen et al., 2020 ; Kanne et al., 2021 ) have started to emerge, providing progress on evolutionary aspects of plant autophagy as well as species-specific roles of this process.…”

“…On the other hand, PpATG8 overexpression enhanced the ability to reprogram somatic gametophore cells into chloronema stem cells when subjected to severe wounding ( Kanne et al., 2021 ), whereas the absence of autophagy negatively affects this process ( Rodriguez et al., 2020 ). Moreover, atg3 loss-of-function lines showed premature gametophore senescence under non-stress conditions, and accumulated plastoglobuli suggesting that autophagy degradation of damaged chloroplasts in senescent gametophore cells is impaired in this mutant, ( Chen et al., 2020 ). Using atg5 loss-of-function lines Mukae et al., ( Mukae et al., 2015 ) reported an early senescent phenotype under carbon and nitrogen starvation conditions, which was partly explained by amino acid imbalance because of the lack of cytoplasmic degradation by autophagy in P. patens .…”

Autophagy modulates apical growth and development in the moss Physcomitrium patens

“…The recent availability of sequenced genomes from several lineages of streptophyte algae, and bryophyte species, the first group of plants that conquered land, has allowed comprehensive comparative evolutionary analyses moving forward our understanding of plant evolution (Rensing, 2017, 2020). Studies on autophagy in the algae Chlamydomonas reinhardtii (Kajikawa and Fukuzawa, 2020; Pérez-Pérez et al ., 2012; Pérez-Pérez et al ., 2017), the liverwort Marchantia polymorpha (Norizuki et al ., 2019), and the moss Physcomitrium (Physcomitrella) patens (Chen et al ., 2020; Kanne et al ., 2021; Mukae et al ., 2015; Sanchez-Vera et al ., 2017) have started to emerge, providing progress on evolutionary aspects of plant autophagy as well as species-specific roles of this process.…”