The Potential Function of SiLOX4 on Millet Discoloration during Storage in Foxtail Millet

Millet color is an important index for consumers to assess foxtail millet quality. The yellow color of millet is mainly because of the accumulation of carotenoids, which are essential for human nutrition. However, the discoloration of millet during storage due to carotenoid degradation seriously reduces the nutritional and commercial value of millet products. The essential quality traits of millet discoloration during storage were analyzed using two foxtail millet varieties, namely 9806-1 and Baomihunzi. We observed that the millet discoloration was caused by carotenoid degradation during storage, and different genotypes exhibited different discoloration rates. The carotenoid reduction rate in 9806-1 (32.2%) was higher than that in Baomihunzi (10.5%). The positive correlation between carotenoid reduction and the expression of SiLOX protein indicated that SiLOX from foxtail millet played a major role in carotenoid reduction during storage. The expression profiles of the SiLOX gene family were analyzed at different grain maturing stages, from S1 to S3, in these two varieties to determine the key SiLOX genes responsive to millet discoloration in foxtail millet. The consecutively low expression of SiLOX2, SiLOX3, and SiLOX4 contributed to the low level of SiLOX protein in Baomihunzi. Furthermore, the undetectable expression of SiLOX4 in the later stage of maturation in Baomihunzi was associated with low discoloration, indicating that SiLOX4 might be a key gene in regulating the discoloration of millet. This study provided critical information on the mechanism of carotenoid degradation during millet storage and laid the foundation for further understanding of carotenoid metabolism in foxtail millet.