Abscisic acid enhances tolerance to spring freeze stress and regulates the expression of ascorbate–glutathione biosynthesis-related genes and stress-responsive genes in common wheat

The present study showed that exogenous abscisic acid (ABA) pre-treatment remarkably increases the freeze tolerance of common wheat (Triticum aestivum L.) seedlings at the anther connective tissue formation phase (ACFP) and tetrad developmental phase. After 3-day freeze stress, ABA pre-treatment significantly increases the content of soluble sugar, soluble protein, and proline, activities of superoxide dismutase (SOD) and peroxidase (POD) and decreases hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels. Under freeze stress conditions, ABA treatment at the ACFP and tetrad developmental phase resulted in a marked increase in leaf ascorbic acid (ASA) and glutathione (GSH) levels. Temporal and spatial expression patterns of eight genes encoding ASA–GSH synthesis-related enzymes and eight stress-responsive genes were measured using quantitative real-time PCR (qRT-PCR). The results showed that ABA temporally regulated the transcript levels of these genes. Moreover, these genes were differentially expressed in ABA-treated wheat seedlings between the ACFP and tetrad developmental phase during freeze stress. These results implied that exogenous ABA increases the levels of GSH and ASA in freeze stressed wheat seedlings in time- and developmental phase-specific manners. Furthermore, we found that the ABA-independent and ABA-dependent pathways were not completely independent in this study and that ABA-independent stress-responsive genes were also induced by exogenous ABA in at least one stress exposure time point. Comparison of our results with the findings of previous studies showed that plant hormones induce an increase in GSH and ASA levels, possibly by differentially regulating the expression levels of genes encoding ASA–GSH synthesis enzymes. Our results provide the first insights on the molecular mechanism of ABA-induced spring freeze tolerance in wheat.