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 (A2 or LSG) with a signal peptide, was designed to target the stem vacuole in grain sorghum inbred line (Tx430) and sweet x grain hybrids (Rio x Tx430). Results: The study demonstrated that transgenic lines of grain sorghum, containing 50-60% isomaltulose, accumulated sevenfold (804 mmol/L) more total sugar than the control Tx430 did (118 mmol/L) in stalks. Subsequently, the best-engineered line (LSG9) was crossed with an elite sweet sorghum variety (Rio). The total sugar concentrations (over 800 mmol/L) measured by HPLC (high-performance liquid chromatography), were significantly higher in both F1 and F2 progenies than the sweet sorghum Rio, representing a 57% and 69% increase respectively. Those total sugar contents in engineered sorghum lines are higher than in the field-grown sugarcane (normal range 600-700 mmol/L). Physiological characterization demonstrated that the superior progenies had notably increased rates of photosynthesis, sucrose transport, and sink strength. Conclusion: The genetic engineering approach has significantly enhanced total sugar concentration in grain sorghum and hybrids of (grain x sweet) 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 accumulation is inheritable 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 sorghum.