TMTM maintains the sulfur homeostasis under sulfur limitation
Plants respond to sulfur limitation in various ways. The first response
is up-regulation of sulfate transporters (SULTRs ) to increase
sulfate uptake from root
(Takahashiet al. , 2011; Shibagaki et al. , 2002; Yoshimoto et
al. , 2002; Kataoka et al. , 2004; Takahashi et al. , 1997).
On the other hand, genes for GSL biosynthesis (e.g., BCAT4 ,CYP79B2 and CYP79F2 ) are down-regulated, while those
repressing GSL biosynthesis (SDI1 and SDI2 ) are
up-regulated. These responses help plants to remobilize sulfur to
sustain growth
(Lewandowska
and Sirko, 2008; Frerigmann and Gigolashvili, 2014; Borpatragohainet al. , 2016). Among the inspected genes, the sulfur starvation
genes SULTR1;1 , SULTR1;2 , SULTR2;1 and bothSDI1 and SDI2 were down-regulated in a TMTM dose-dependent
manner in plants which suffer from sulfur limitation (Figure 5A and 5B).
Since the sulfur in TMTM can be incorporated into the plant material,
the expression of the above-mentioned genes and those involved in GSL
and GSH metabolism is similar to seedlings grown under HS condition
without TMTM. Moreover, excessive TMTM results in the upregulation of
these genes, indicating that these plants are actively moving excess
sulfur to secondary metabolites. This is in accordance with a recent
study by
Sugiyamaet al. (2021), showing a retrograde sulfur flow from
glucosinolates to cysteine in Arabidopsis. Interestingly, the
mRNA levels for GSL and GSH metabolism genes was higher in seedlings
after 2 days on LS medium without TMTM compared to seedlings grown on HS
medium. This might be caused by higher sulfate influx from the medium
due to up-regulation of SULTRs . The plants actively metabolize
the assimilated sulfate into various metabolites and utilize this as a
store to sustain growth under sulfur limitations.
The effect is also observed at the metabolic level. Under sulfur
limitation, the GSH and GSL levels decreased. However, 100 µg TMTM
maintained the levels high under sulfur starvation conditions (Figure
6). Again, besides maintaining sulfur homeostasis, excess sulfur from
the high TMTM dose is largely metabolized into secondary metabolites.