Stem vacuole-targetted sucrose isomerase enhances sugar content in sorghum

Abstract

Background Sugar content is critically important in determining sugar crop productivity. However, improvement in sugar content has been stagnant among sugar crops for decades. Sorghum, especially sweet sorghum with high biomass, shown great potential for biofuel, has lower sugar content than sugarcane. To enhance sugar content, the sucrose isomerase ( SI ) gene, driven by stem-specific promoters ( A2 or LSG ) with a vacuole-targetted signal peptide, was transformed into the sorghum inbred line (T×430). Results The study demonstrated that transgenic lines of grain sorghum, containing 50–60% isomaltulose, accumulated up to eightfold (1000\xa0mM) more total sugar than the control T×430 did (118\xa0mM) in stalks of T 0 generation. Subsequently, the elite engineered lines (A5, and LSG9) were crossed with sweet sorghum (Rio, and R9188). Total sugar contents (over 750\xa0mM), were notably higher in F 1 , and F 2 progenies than the control Rio (480\xa0mM). The sugar contents of the engineered lines (over 750\xa0mM), including T 0 , T 1 , F 1 , and F 2 , are surprisingly higher than that of the field-grown sugarcane (normal range 600–700\xa0mmol/L). Additionally, analysis of physiological characterization demonstrated that the superior progenies had notably higher rates of photosynthesis, sucrose transportation, and sink strength than the controls. Conclusions The genetic engineering approach has dramatically enhanced total sugar content in grain sorghum (T 0 , and T 1 ) and hybrid sorghum (F 1 , and F 2 ), demonstrating that sorghum can accumulate as high or higher sugar content than sugarcane. This research illustrates that the SI gene has enormous potential on improvement of sugar content in sorghum, particularly in hybirds and sweet sorghum. The substantial increase on sugar content would lead to significant financial benefits for industrial utilization. This study could have a substantial impact on renewable bioenergy. More importantly, our results demonstrated that the phenotype of high sugar content is inheritable and shed light on improvement for other sugar crops.