Metabolism of TMTM may not be a single-step process
It is known that plants are able to assimilate gaseous sulfur compounds, such as SO2 and H2S (Randewiget al. , 2012; Lee et al. , 2017; Ausma and De Kok, 2019). They are also able to assimilate other sulfur-containing organic volatiles produced by microbes. One example is dimethyl disulfide (Meldauet al. , 2013). Nevertheless, how organosulfides are metabolized inside the plant remains unknown.
Diallyl disulfide (DADS), a volatile from garlic, is perhaps the best studied organosulfide, due to its anticancer ability (Yi and Su, 2013; Xiong et al. , 2018; Agassi et al. , 2020; Liet al. , 2018). It increases GSH levels and regulates antioxidant enzyme activity, leading to reduced oxidative stress in animal models (Demeuleet al. , 2004; Hassanein et al. , 2021; Wei et al. , 2021). In plants, DADS also affects sulfur metabolism genes (Chenget al. , 2016; Cheng et al. , 2020; Yang et al. , 2019). Metabolism of DADS and other organosulfides generates H2S (Kimet al. , 2019; Cai and Hu, 2017; Bolton et al. , 2019; Lianget al. , 2015). Studies on how DADS and other organosulfides are metabolized suggest the involvement of GSH and cysteine (Boltonet al. , 2019; Liang et al. , 2015; Cai and Hu, 2017). The reaction between DADS and GSH produces S-allyl GSH and a short-lived intermediate allyl perthio through α-carbon nucleophilic substitution. The allyl perthio reacts with a second GSH, resulting in the release of H2S and S-allyl GSH disulfide (Lianget al. , 2015).
We found that 1000 µg TMTM increased both the GSH and GSL levels, and the GSH level responded faster to the volatile treatment (Figure 6A and 6B). A possible explanation could be that incorporation of sulfur from TMTM into the plant metabolism is connected to GSH. We tested if TMTM can be a direct substrate for OASTLs and found that this is not the case (Figure 7). Therefore, unlike sulfate assimilation, TMTM might first interfere with the GSH/GSSG system. This might lead to the cleavage of the C-S bonds and sulfur incorporation into plant. A detailed metabolome analysis of early sulfur-containing compounds after TMTM treatment might elucidate early steps in the role of this novel volatile.