Overexpression of maize SDD1 (ZmSDD1) improves drought resistance in Zea mays L. by reducing stomatal density

Drought is one of major factors limiting the high and stable yields of maize, and thus, improvement of water use efficiency (WUE) and cultivation of drought-resistant maize cultivars have become very urgent. Previous research shows that SDD1 from Arabidopsis thaliana, a key gene during stomatal development, negatively regulates stomatal density, reduces transpiration, and improves WUE and drought resistance without reducing biomass. In this study, maize SDD1 (ZmSDD1) containing intron was cloned, and the phylogenetic tree was constructed, revealing a close genetic relationship with the SDD1 of Oryza sativa. ZmSDD1 was expressed in the leaves, shoots, and roots of maize. Under drought, cold, and salt stress conditions, the expression of ZmSDD1 in B73 maize leaves is reduced. To reveal the functions of ZmSDD1, we built a plant expression vector pGM626-Ubi-SDD1-ABt containing ZmSDD1 which was driven by the maize Ubiquitin promoter. Transgenic plants were obtained via shoot-tip transformation. The stomata number on leaf surfaces of the plants within Ubiquitin::ZmSDD1 overexpression plants was 30 % lower than that in the wildtype ones. After repeated drought treatments the transgenic maizes demonstrated a much higher survival rate with 6.68-fold ZmSDD1 overexpression, which indicated the expression of the gene negatively regulating stomata density and caused the reduction of stomatal conductance and transpiration rate after drought-treatment. In addition, ZmSDD1 enhanced the drought resistance in the transgenic maizes through improving WUE and photosynthetic rate which was considered as the result of the higher PEPCase activity, CO2 utilization rate and the lower CO2 compensation point.