Sucrose Metabolism in the Culm of Transgenic Sugarcane with Reduced Soluble Acid Invertase Activity

Transgenic derivatives of sugarcane cultivar Q117, with reduced acid invertase activity, were used to investigate the role of this enzyme in sucrose metabolism in the sugarcane culm. Despite more than 70% reduction in the level of acid invertase activity in the immature internodes, no significant changes in either sucrose load or purity were evident. Radiolabelling experiments indicated no significant influence on sucrose hydrolysis as a result of the reduced acid invertase activity. Introduction Materials and methods After numerous studies on carbohydrate metabolism in sugarcane, the metabolic control of sucrose accumulation is still poorly understood (Moore, 1995). Sucrose accumulation occurs despite continuous cleavage of sucrose, when synthesis exceeds breakdown (Batta and Singh, 1986; Hatch et al., 1963; Whittaker and Botha, 1997; Wendler et al., 1990). Many studies have been done on the sucrose breakdown activities, invertases and sucrose synthase (SuSy) (reviewed by Moore, 1995); and more recently on the synthesis activity, sucrose phosphate synthase (SPS) (Ebrahim et al., 1998; Wendler et al., 1990). Several studies showed a strong correlation between sucrose content and the difference between SPS and soluble acid invertase (SAI) activity, with decreased SAI in tissues accumulating sucrose. Analysis of progeny from crosses between highand low-sucrose Saccharurn clones also indicated a critical role of SAI in regulating sucrose accumulation (Ebrahim et al., 1998; Zhu et al., 1997). However, a decrease in SAI is not always evident during sucrose accumulation (Dendsay et al., 1995; Vorster and Botha, 1999). It is not clear whether there is potential to enhance sucrose accumulation by reduction in SAI below the low levels indirectly selected in current elite sugarcane cultivars. The efficient sugarcane transformation system allows the production of multiple independent lines altered in the activity of a target gene or enzyme, in the same genetic background. This is a powerful tool to test the metabolic significance of suspected key enzymes in commercial sugarcane varieties. Here we compare sucrose metabolism in the growing culms of a series of transgenic sugarcane lines with reduced acid invertase activity. Although SAI activity was reduced by more than 70% in some lines, no change in sucrose accumulation or rates of sucrose hydrolysis was evident.