To ascertain the effect of exogenously applied hydrogen peroxide (H 2O 2) on drought stress, we examined whether the spraying of soybean leaves with H 2O 2 would alleviate the symptoms of drought stress. Pre-treatment by spraying leaves with H 2O 2 delayed foliar wilting caused by drought stress compared to leaves sprayed with distilled water (DW). Additionally, the relative water content of drought-stressed leaves pre-treated with H 2O 2 was higher than that of leaves pre-treated with DW. Therefore, we analyzed the effect of H 2O 2 spraying on photosynthetic parameters and on the biosynthesis of oligosaccharides related to water retention in leaves during drought stress. Under conditions of drought stress, the net photosynthetic rate and stomatal conductance of leaves pre-treated with H 2O 2 were higher than those of leaves pre-treated with DW. In contrast to DW spraying, H 2O 2 spraying immediately caused an increase in the mRNA levels of d-myo-inositol 3-phosphate synthase 2 (GmMIPS2) and galactinol synthase (GolS), which encode key enzymes for the biosynthesis of oligosaccharides known to help plants tolerate drought stress. In addition, the levels of myo-inositol and galactinol were higher in H 2O 2-treated leaves than in DW-treated leaves. These results indicated that H 2O 2 spraying enabled the soybean plant to avoid drought stress through the maintenance of leaf water content, and that this water retention was caused by the promotion of oligosaccharide biosynthesis rather than by rapid stomatal closure.
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