FIGURE 4 Heat map of exogenous melatonin regulating the expression of OsGR3 ,OsGPX1 , OsNR2 , OsGS1 , and OsGOGAT1 in seedlings under NaCl stress. (a) indicates the expression of theOsGR3 and OsGPX1 , (b) indicates the expression of theOsNR2 , OsGS1 and OsGOGAT1 .
3.5 | Expression regulation, GO annotation, and KEGG metabolic pathway of exogenous melatonin for enzymes involved in the AsA-GSH cycle and nitrogen metabolism in seedlings under NaCl stress
GO functional annotations can be divided into three categories: biological processes, molecular functions, and cell composition (Dong et al., 2021). Based on the results of GO annotations and KEGG metabolic pathways of rice OsGR3 , OsGPX1 , OsNR2 , OsGS1 , andOsGOGAT1 , these genes are involved in biological processes, such as redox and cytosolic composition; functions, such as oxidoreductase activity; and three metabolic pathways, including nitrogen metabolism, alanine, aspartate and glutamate metabolism, and glutathione metabolism (Figure 5a, b). Exogenous melatonin regulated gene expression in the AsA-GSH cycle and nitrogen metabolism pathway in seedlings under NaCl stress (Figure 6). NR reduces nitrate to nitrite, and then converts it to ammonia, which is
converted under the action of GS and GOGAT for Gln and Glu. Further, exogenous melatonin regulates the activities of NR, GS, and GOGAT by upregulating the expression of OsNR2 , OsGS1 , andOsGOGAT1 , ultimately affecting the nitrogen metabolism whole process. Glu can produce GS through the action ofglutamate cysteine ligase (GCLM) and glutathione synthetase (GS II). In contrast, GSH can synthesize Glu via γ-glutamyltranspeptidase (γ-GT), which connects the AsA-GSH cycle with nitrogen metabolism, forming a complete metabolic channel network (Fig. 6). Under the action of GR and GPX, a balance between GSSG and GSH can be achieved, and the circulation and regeneration of GSH can be promoted. Exogenous melatonin may upregulated the expression of OsGR3 and OsGPX1 , therefore, upregulated the activity of GR and GPX, and participated in the AsA cycle. The purpose of the AsA-GSH cycle is to eliminate excessive ROS and reduce the toxic effects of NaCl stress on seedlings.