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.