Constitutive down-regulation of SiSGR gene is related to green millet in Setaria italica

Abstract

Millet colour is an important index to evaluate the quality of dehulled foxtail millet (Setaria italica (L.) Beauv.). Most varieties are yellow, due to the accumulation of carotenoids. However, there are some foxtail millet germplasms producing dark green millet. To elucidate the molecular mechanism of the chlorophyll retention phenotype, Daqinggu with green millet colour and Jingu 21 with yellow millet colour were selected as research material in this study. The total carotenoid level in dehulled millet of Daqinggu was about 0.024 mg/g, and 0.038 mg/g in Jingu 21. The transcript levels of carotenoid structural genes were investigated at three stages of grain development in both millet varieties. The expression levels of carotenoid biosysnthesis-related genes SiPSY3, SiPDS, SiZ-ISO, SiLCYB and SiCYP97C were significantly higher in Daqinggu than in Jingu 21, which was not consistent with the difference in the carotenoid levels between these two varieties. Interestingly, SiSGR, a homologue to the STAY-GREEN gene in Arabidopsis, tomato, and rice, was constitutively down-regulated during maturation in Daqinggu. In addition, the total chlorophyll content was consistently higher in Daqinggu than in Jingu21 during grain maturation. These evidences suggest that SiSGR is a key gene in regulating chlorophyll retention for dark green foxtail millet.