Comparison of SWEET gene family between maize and foxtail millet through genomic, transcriptomic, and proteomic analyses.

Foxtail millet [Setaria italica (L.) P. Beauv.] does not show high yield and biomass compared with maize (Zea mays L.) although it is a C crop with the potential for high productivity. Because SWEET genes, which are important for sugar transport in plants, play critical roles in biomass production and seed filling in crops, genome-wide, transcriptomic, and proteomic comparison on SWEET gene family between these two species would provide some clues for unlocking this issue. In our study, 24 SWEET genes were identified in foxtail millet and maize. Sequence-based bioinformatics combined with gene expression analyses identified several candidate functional orthologs in these two species. A comparative analysis on expression characteristics of SWEET genes and proteins between maize and foxtail millet indicate that not only some critical major SWEET proteins show significant upregulation in maize compared with their orthologs in foxtail millet, but also there are more quantities of maize SWEET genes showing high expressions than that of foxtail millet genes, suggesting that compared with foxtail millet, maize possesses higher capacity of sugar transport, the crucial determinant for crop yield and biomass. These results provide a basis on revealing why foxtail millet exhibits low yield and biomass although it is a C crop with the potential for high productivity.