In order to investigate the role of melatonin in the drought tolerance, we examined pigments, gas exchange, osmolytes, and reactive oxygen radical metabolism in soybean plants. Drought declined photosynthetic pigments and caused irreversible reduction in net photosynthesis, which was followed by stomatal limitation for 5 and 10 d and nonstomatal limitation for 15 d. Soluble sugar, soluble proteins, and proline concentrations were higher in drought-stressed seedlings compared with the control. The contents of superoxide anion, hydrogen peroxide, and malondialdehyde increased during drought stress indicating oxidative stress. Drought stress also increased superoxide dismutase, peroxidase, and catalase activities. Melatonin treatment improved the tolerance of drought-treated plants, which was possibly due to the enhanced content of osmolytes and higher antioxidant enzyme activities that retard dehydration and lipid peroxidation.

Gibberellins (GAs) regulate diverse aspects of growth and development, but their role in root development and lateral root (LR) formation is poorly understood. In this study, GA3 was applied to soybean [Glycine max (L.) Merr] by seed soaking. The results showed that root length and root surface area were significantly inhibited in early stages after GA3 treatment. Microscopic examination showed that GA3 treatment changed the cortex thickness, the pericycle diameter, and cell size in main root. Interestingly, exogenous GA3 increased the quantity of lateral root primordia (LRP), but LR number decreased in this period. Moreover, the content of GAs, auxin and abscisic acid in root was altered. RNA-seq results revealed that application of GA3 not only changed the expression of genes in GA biosynthesis pathway, including GA20ox and GA2ox, but also the GA regulation genes and signalling pathway genes. The changes in expression of gene concerning other hormones were also detected. In addition, GA3 altered cell wall biogenesis and degradation genes which might be related to the changes of root morphology. In response to increased GA3, 103 transcription factors were detected. Thus, exogenous GA3 changed the content of hormones in roots and affected the root development by regulating the expression of respective genes.