Discussion
In this study, we identified a fungal volatile, TMTM, as the main
component in the headspace of the beneficial fungus M. hyalina .
Application of TMTM participated in maintaining the sulfur homeostasis
in Arabidopsis seedlings under sulfur deficiency. At low
concentrations (10 – 100 µg), TMTM compensated sulfur-limitation
responses of the seedlings: the volatile restored growth and root
development which were inhibited under sulfur-limiting conditions,
restricted the upregulation of sulfur deficiency marker genes
(SULTRs , SDI1 and SDI2 ), or the breakdown of GSL
and the accumulation of GSH. On medium with HS, these TMTM effects were
not detectable. TMTM shifted the measured parameters in LS plants to
those found in seedlings grown on HS medium without TMTM application.
Higher concentration induced toxic or inhibitory effects, altered the
sulfur homeostasis, and restricted plant growth. However, TMTM was not
directly incorporated into cysteine by OASTLs, and is not inhibiting
their function. This suggests that cysteine might not be a direct
product of TMTM incorporation, or TMTM must be processed before its
sulfur atoms can be incorporated into cysteine.