GmSGR1, a stay-green gene in soybean (Glycine max L.), plays an important role in regulating early leaf-yellowing phenotype and plant productivity under nitrogen deprivation

Chlorophyll (Chl) degradation is closely associated with leaf senescence and plays an important role in mediating photosynthetic capacity of the plants. In this study, GmSGR1, a SGR1 type of stay-green (SGR) gene in soybean, has been functionally characterized for the role in mediating leaf senescence, photosystem II (PSII) efficiency, and plant productivity. GmSGR1 is 3621 bp long that contains four exons and three introns at genome level, encoding a 261 aa-long polypeptide with molecular weight (mW) of 29.41 kDa. GmSGR1 shares high similarity to genes encoding D2 and D1, the components of PSII complex, and those encoding senescence-inducible proteins in soybean. The GmSGR1::GFP fusion was specificly detected in chloroplast of transgenic tobacco leaves, suggesting that GmSGR1 targets onto plastids where exerts distinct functions after sorted by endosplasmic reticulum. The transcripts of GmSGR1 were gradually increased in leaves along with the leaf senescent progression. This gene expression pattern was validated by assay of histochemical GUS staining and GUS activity of leaves, in which the reporter GUS was driven by the GmSGR1 promoter. These results indicated that the GmSGR1 expression is tightly regulated by the senescence signaling. After both of N-sufficient and -deprived treatments, the tobacco plants overexpressing GmSGR1 exhibited early leaf-yellowing phenotype, reduced Chl content and PSII efficiency, and lowered biomass compared with the wild type plants. These findings confirm that GmSGR1 acts as one critical regulator in modulation of leaf senescence and plant productivity through regulation of Chl degradation and PSII capacity. Together, our investigation provides novel insights into mechanisms underlying the distinct SGR-mediated stay-green phenotype of plants. GmSGR1 is potential for generation of soybean cultivars with improved productivity under the N-starvation conditions.