The Molecular Mechanism Of Abscisic Acid Enhances Contents Of Ascorbate, Glutathione And Starch In Wheat Seedlings Under Polyethylene Glycolstimulated Drought Stress

In the present study, the molecular mechanism of ABA in increased contents of ascorbate, glutathione and starch in PEG-stimulated wheat seedlings is explored. The important findings of this study are as follows:(1) Compared to the control, drought stress dramatically reduced wheat seedling growth. However, in the presence of exogenous ABA, the effect of drought stress on wheat seedling growth was significantly abrogated. This was demonstrated by the significantly increased plant heights, fresh and dry weights. These results showed that ABA improved the growth of wheat seedlings under drought stress conditions. In the presence of ABA(ABA + drought treatment), hydrogen peroxide(H2O2) and malondialdehyde(MDA) were significantly reduced. And ABA markedly increased contents of GSH and ASA in both leaves and roots and induced starch biosynthetic in the leaves of ABA-treated wheat seedlings during drought stress.(2) Temporal and spatial expression patterns of eight genes encoding ASA and GSH synthesis-related enzymes were measured using quantitative real-time reverse transcription polymerase chain reaction(q PCR). The results indicated that ABA temporally regulated the transcript levels of genes encoding ASA-GSH cycle enzymes. Moreover, these genes exhibited differential expression patterns between the root and leaf organs of ABA-treated wheat seedlings during drought stress. In roots of PEG-treated wheat seedlings, exogenous ABA significantly enhanced the transcription levels of the genes encoding ASA-GSH cycle enzymes at different time points, including glutathione-S-transferase 1(GST1) at 4 d, glutathione-S-transferase 2(GST2) at 1 and 5 d, glutathione peroxidase(GPX1) at 2, 4 and 5 d, phospholipid hydroperoxide glutathione peroxidase(GPX2) at 1 and 5 d, glutathione reductase(GR) at 1 and 2 d, dehydroascorbate reductase(DHAR) at 3, 4 and 5 d, monodehydroascorbate reductase(MDHAR) and glutathione synthetase(GS) at 1, 4 and 5 d. During PEG-induced stress, however, transcription levels of eight genes encoding ASA-GSH synthesis-related enzymes in leaf were significantly different with those in root tissue. During PEG treatment period, exogenous ABA significantlyincreased the transcript levels of the GST1 at 1, 4 and 5 d, GST2 at 4 and 5 d, GPX1 at 2 and 3 d, GPX2 and GS at 2, 3 and 4 d, GR at 1, 3, and 4 d, DHAR at 1 and 4 d, MDHAR at 1 and 3 d after PEG treatment. These results implied that ABA increased the contents of ASA and GSH by differentially regulating the transcription levels of ASA-GSH synthesis enzymes gens under drought stress conditions.(3) The transcript levels of the genes encoding transitory starch biosynthesis-related enzymes were further measured using quantitative real-time PCR. Under PEG-stimulated drought stress condition, exogenous ABA significantly enhanced the transcription levels of the genes encoding many starch biosynthesis-related enzymes at different time points, including ADP-glucose phrophosphorylase S2(Ta AGPS2) at 1, 2, 3 and 4 days, ADP-glucose phrophosphorylase L2(Ta AGPL2) at 1, 4 and 5 days, granule-bound-starch-synthase II(Ta GBSSII) at 2, 3 and 4 days, starch synthase I(Ta SSI) at 1, 3 and 4 days, starch synthase IIIb(Ta SSIIIb) at 1 and 5 days, starch synthase IV(Ta SSIV) at 1, 2 and 3 days, branching enzymes IIa(Ta BEIIa) at 1 and 5 days, starch phosphorylase L(Ta PHOL) at 1, 3 and 4 days, and debranching enzymes 2(Ta DPE2) at 3, 4 and 5 days. This indicated that, under PEG-stimulated drought stress, exogenous ABA increased starch contents in leaves of wheat seedlings possibly by temporally regulating transcript levels of the genes encoding starch synthesis-related enzyme genes.