Relationship of Cytokinin to Green Stem Syndrome of Soybean.

In recent years, the occurrence of green stem syndrome (GSS) of soybean is increasing in Japan. GGS is that only pods are matured, stem and leaf maintain high water contents in harvest time. Combine harvesting should be delayed by GSS and sometimes it causes dirty seeds during harvesting. The cause of GGS is to lose balance of sink / source, however, the mechanism has not yet been well understood. Cytokinins (CKs), one of the plant hormone, play important roles in plant growth and development, and inhibit senescence. The biosynthesis of CKs is mainly taken place in root, and exports to shoot via xylem sap (Heindl et al. 1982). Therefore, we hypothesized that the increase of CKs contents causes GGS. There are two treatment groups, control (not depodded) and 2/3 depodded in R5 stage to cause GGS. In harvest time, all of tissues decreased water contents in non-treated group; while, depodded treatment group maintained high water content in stem. Depodded treatment induced GGS of soybean. CKs contents of xylem sap were measured 3 times weekly after R5 stage. As a result, CKs contents in xylem sap increased at seven days after depodding. As for CK metabolism genes, 14 CK biosynthetic (isopentenyltransferase, GmIPT) and 17 CK degradative (CK dehydrogenase, GmCKX) genes were identified (Le et al. 2012). We examined these genes expression in root at seven days after depodding. As a result, about CK biosynthesis genes, GmIPT1 and GmIPT7 are higher expression in root in depodded treatment. Additionally, GmCKX8 and GmCKX12 are lower expression in root in depodded treatment. In conclusion, it is indicated that increase CKs contents in xylem sap were caused by these genes, and they are specific in GGS.