Reduced neutral invertase activity in the culm tissues of transgenic sugarcane plants results in a decrease in respiration and sucrose cycling and an increase in the sucrose to hexose ratio

Transgenic sugarcane plants ( Saccharum officinarum L. interspecific hybrids) were regenerated from previously described cell lines with reduced neutral invertase (NI) activity. The effects that were observed in the differentiated culm tissues at different stages of maturity paralleled those observed across the growth cycle of the suspension cultures. Reduced NI activity correlated with an increase in sucrose and a decrease in hexose levels. However, the magnitude of the reduction in enzyme activity and the accompanying changes in carbohydrate metabolism were not as pronounced as in the suspension cultures. Feeding experiments with radio-labelled fructose provided evidence that the cycling of sucrose as well as the total respiration rate correlated directly with NI activity. Sucrose synthase activity was upregulated in the transgenic plants, possibly to compensate for the reduction in invertase activity. Despite this partial compensation, the respiratory rates of the transgenic lines were still significantly lower than those of the untransformed control lines. This study clearly demonstrates the importance of NI in directing carbon towards respiratory processes in the sugarcane culm. In addition, this is the first report in which data obtained from genetically modified sugarcane suspension cell cultures and their regenerated, whole-plant counterparts are compared. The observed correlations support the use of cell cultures as a model system for sugarcane internodes, which could significantly accelerate reverse genetic studies on sugarcane carbohydrate metabolism in the future.