Redox poise and metabolite changes in bread wheat seeds are advanced by priming with hot steam

Fast and uniform germination is key to agricultural production and can be achieved by seed "priming" techniques. Here, we characterised the responses of bread wheat ( Triticum aestivum L.) seeds to a hot steam treatment ("BioFlash"), which accelerated water uptake, resulting in faster germination and seedling growth, typical traits of primed seed. Before the completion of germination, metabolite profiling of seeds revealed advanced accumulation of several amino acids (especially cysteine and serine), sugars (ribose, glucose), and organic acids (glycerate, succinate) in hot steam-treated seeds, whereas sugar alcohols (e.g., arabitol, mannitol) and trehalose decreased in all seeds. Tocochromanols (the "vitamin E family") rose independently of the hot steam treatment. We further assessed shifts in the half-cell reduction potentials of low-molecular-weight (LMW) thiol-disulphide redox couples [i.e., glutathione disulphide (GSSG)/glutathione (GSH) and cystine/cysteine], alongside the activities of the reactive oxygen species (ROS)-processing enzymes superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase. Upon the first 4 h of imbibition, a rapid conversion of LMW disulphides to thiols occurred. Completion of germination was associated with a re-oxidation of the LMW thiol-disulphide cellular redox environment, before more reducing conditions were re-established during seedling growth, together with an increase in all ROS-processing enzyme activities. Furthermore, changes in the thiol-disulphide cellular redox state were associated to specific stages of wheat seed germination. In conclusion, the priming effect of the hot steam treatment advanced the onset of seed metabolism, including redox shifts associated with germination and seedling growth.