Stem vacuole-targetted sucrose isomerase enhances sugar accumulation in sorghum

Abstract Background: Sugar accumulation is critically important in determining sugar crop productivity. Sorghum, especially high biomass sweet sorghum has shown great potential for biofuel. However, improvement in sugar content has been stagnant among sugar crops for decades. In this study, sorghum was investigated as a C4 diploid model for more complicated genomes such as maize and sugarcane. To promote sugar accumulation in sorghum, the sucrose isomerase (SI) gene, driven by stem-specific promoters A1 (A) or LSG2 (L) with a signal peptide, was designed to target the stem vacuole in grain sorghum inbred line (Tx430).Results: The study demonstrated that transgenic lines of grain sorghum can accumulate isomaltulose which accounted for 50-60% of total sugar (up to 1012 mM) in stalks. While the average sugar content is 118 mM in the control Tx430. Subsequently, the best-engineered line (L9) was crossed with an elite sweet sorghum variety (Rio). The total sugar contents were significantly higher in both F1 (up to 763 mM) and F2 (up to 821 mM) progenies than the sweet sorghum Rio (485 mM), representing 57% and 69% increase respectively. Those total sugar contents in those engineered sorghum lines are higher than in the field-grown sugarcane (600-700 mM). Physiological characterization demonstrated that the superior progenies of F2 hybrids had notably higher rates of photosynthesis, sucrose transport, and sink strength than controls.Conclusion: The genetic engineering approach has significantly enhanced total sugar content in grain sorghum and hybrids of (sweet X grain) sorghum. This research has put sorghum in the spotlight and frontier as a biofuel crop. More importantly, our results prove that the phenotype of high sugar content is heritable in the grain sorghum as well as hybrids. The massive increase in sugar accumulation would lead to enormous financial benefits for industrial and biofuel use. This study would have a substantial impact on renewable energy due to the supreme capacity of total sugar accumulation in transgenic sorghum.