https://orcid.org/0000-0002-5447-643X
SNARE protein complexes are key regulators of membrane vesicle trafficking and are conserved among eukaryotes. Among the more than 60 SNARE proteins in the Arabidopsis genome, SYNTAXIN OF PLANTS61 (SYP61), a resident of the trans-Golgi network/early endosome (TGN/EE), is involved in vacuolar trafficking and secretion of cell wall components (Drakakaki et al., 2012). In this issue of The Plant Cell, Yoko Hasegawa and colleagues (Hasegawa et al., 2022) used various genetic and biochemical assays in combination with confocal microscopy to investigate the interaction of SYP61 with the ubiquitin ligase ARABIDOPSIS TOXICOS EN LEVADURA31 (ATL31). They show that SYP61 interacts with ATL31 and is necessary for the proper localization of ATL31. They also show that SYP61 is ubiquitinated in plants and that its ubiquitination level is regulated in response to carbon (C)/nitrogen (N)-nutrient status, suggesting that SNARE regulation plays a role in nutrient signaling.
To determine the subcellular localization of ATL31, the authors used stable transgenic plants harboring ATL31-GFP and various subcellular markers. Using confocal microscopy, ATL31-GFP signals were detected in the plasma membrane, TGN/EE, and late endosome (LE). Because SYP61 and ATL31 both target the TGN/EE and mediate the secretion of cell wall components, the authors examined whether these two proteins interact and whether ATL31 ubiquitinates SYP61. Using co-immunoprecipitation (CO-IP) and split ubiquitin yeast-two hybrid assays, they showed that ATL31 interacts with SYP61. ATL31-GFP localization was altered in syp61 mutants, indicating that SYP61 is required for the proper localization of ATL31. In addition, they showed that SYP61 protein is ubiquitinated and that its ubiquitination is, at least in part, mediated by ATL31.
In addition to being essential macronutrients, C and N metabolites are important signaling molecules during plant growth and development. For example, a high C/N ratio during early development inhibits seedling growth, while during late development, it promotes senescence (Coruzzi and Zhou, 2001). Hasegawa et al. (2022) wanted to know if SYP61 also plays a role in C/N signaling, as previous work (Sato et al., 2009) revealed a connection between ATL31 and C/N-mediated nutrient signaling. The syp61 mutants were hypersensitive to high C/low N conditions and this hypersensitivity was not rescued by overexpression of ATL31, suggesting that SYP61 is essential for plant responses to various C/N nutrient conditions (see Figure).
syp61 knockdown mutants are hypersensitive to C/N-nutrient stress. Shown are representative images of WT (Col-0) and syp61 knockdown mutant (syp61 amiRNA) seedlings grown for 8 days on media with different concentrations of glucose or mannitol and nitrogen. Adapted from Hasegawa et al. (2022), Figure 4.
In summary, Hasegawa et al. (2022) provide new insights into the role of the SNARE SYP61 in C/N-nutrient signaling. They further show that SYP61, a regulator of the cellular trafficking machinery, is subjected to ubiquitination in response to C/N-nutrient conditions. In the future, it would be intriguing to identify other regulatory components of SYP61 and the biological relevance of their modifications.
