New mechanism of strigolactone-regulated cold tolerance in tomato

In this study, proteomic analysis revealed that cold treatment in tomato induced more ubiquitinated proteins compared with normal conditions, which is consistent with previous findings that stress-induced ubiquitination occurs in damaged proteins (Wang et al., 2019). In a previous study by their team, they reported that cold stress could induce SL biosynthesis in tomato (Chi et al., 2021). Intriguingly, they found that the number of ubiquitinated proteins under cold stress increased significantly in the SL-deficient mutant ccd7 compared with the wild-type (WT). By contrast, the treatment with synthetic SL analog GR24^5DS could inhibit cold stress-induced protein ubiquitination. Then, the authors further investigated the effect of SLs on autophagy and employed monodansylcadaverine staining and transmission electron microscopy analysis to identify autophagic vesicles and bodies in WT and ccd7 mutants, under cold conditions. Compared with the WT, the numbers of autophagic vesicles and bodies in the ccd7 mutant obviously reduced after cold treatment, whereas treatment with GR24^5DS induced the formation of autophagic vesicles and bodies to activate autophagy activity. These data demonstrated that SLs triggered autophagy to degrade cold-induced ubiquitinated proteins, revealing a new regulatory pathway for SL and cold responses in plants.

So, how do SLs modulate cold-induced autophagy? ELONGATED HYPOCOTYL 5 (HY5) could: (1) inhibit autophagy under nitrogen-sufficient or light conditions in Arabidopsis (Yang et al., 2020); (2) accumulate at protein levels after GR24 treatment in Arabidopsis (Tsuchiya et al., 2010; Jia et al., 2014); and (3) act as a master regulator in light signaling-mediated cold responses in Arabidopsis (Catalá et al., 2011). The authors focused on HY5 to test whether HY5 mediated strigolactone-induced autophagy under cold conditions. The authors found that SLs positively regulated HY5 protein levels under normal and cold conditions. Meanwhile, they found that survival rates of HY5-OE or HY5 RNAi plants after cold stress were significantly increased or decreased, whereas the GR24^5DS treatment did not increase the survival rate of HY5-RNAi plants. Moreover, autophagy activities also decreased in HY5-RNAi and increased in HY5-OE plants, and GR24^5DS treatment could not increase autophagy activities in HY5-RNAi, which is consistent with HY5 protein levels under these conditions, strongly supporting that SLs trigger autophagy in a manner dependent of HY5 under cold conditions.

Autophagy-related genes (ATGs) play important roles in enhancing autophagy to increase stress tolerance (Sun et al., 2018; Chen et al., 2019). The authors found that among a series of ATGs induced by cold stress in tomato, ATG18a was also induced by GR24^5DS treatment. The mutant of ATG18a exhibited sensitivity to cold treatment, along with decreased autophagosome formation and increased accumulation of ubiquitinated proteins. HY5 could directly bind to and activate the promoter of ATG18a, indicating that ATG18a functions downstream of HY5 to trigger autophagy formation under cold stress in tomato.

In summary, the work by Chi et al. (2025) revealed a novel regulatory mechanism between SLs and cold stress in tomato. SLs promoted autophagy to counteract cold-induced protein aggregation in an HY5-dependent manner in tomato, indicating great potential in the breeding of cold-tolerant varieties. More importantly, this study established a new link between SLs and autophagy. As SLs and autophagy have various functions in regulating drought stress, oxidative stress, and nutrient starvation (Wang et al., 2015, 2019; Signorelli et al., 2019), it raises a general interest in whether SLs also control other stress responses in an autophagy-dependent manner. In addition to the autophagy pathway, the ubiquitin-proteasome system also plays indispensable roles in mediating a variety of signaling pathways. For instance, the E3 ligase DWARF 3 (D3) triggers ubiquitination and degradation of D53 to activate SL signaling (Jiang et al., 2013; Zhou et al., 2013). It is worth further investigating whether the ubiquitin-proteasome system also participates in the SL-regulated protein ubiquitination and degradation under different stress conditions.