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.