Characterization of sucrose phosphate synthase, sucrose synthase and invertase in roots of two nearly isogenic carrot lines that differ in their capacity to accumulate sucrose

Activities of sucrose phosphate synthase (SPS), sucrose synthase (SuSy) and acid invertase (Inv) were assayed in two carrot lines: one with high- sucrose, low-reducing-sugar contents (HS) and the other with low-sucrose, high-reducing-sugar contents (LS). In general, SPS and SuSy exhibited lower activities in the LS vs. HS lines, except for the high SuSy activity in the LS xylem. SPS and SuSy activities were much higher in the phloem than in the xylem of the HS line. The kinetics of SuSy activity, in the degrading direction, were similar in HS and LS lines. K(m) values were 4-5 mmol/L sucrose and V(max) values were 23-26 μmol fructose · g FW^-1·h^-1. In the synthesizing direction, K(m) and V(max) of SuSy activity were higher in the LS than in the HS line at pH 6.5, but identical at pH 8.0. In the presence of exogenous UDP-glucose, sucrose synthesis by SuSy rose 1.5-fold in the HS line and 4.25-fold in the LS line. Roots of both lines showed similar acid Inv activities. Our results suggest that each carrot line has specific Inv isozymes because they responded differently to increased sucrose concentration and pH. Inv activity of the HS line was induced by increasing substrate concentrations, whereas it decreased in the LS line. Inv activity in the LS line did not change between pHs 4 and 8, whereas in the HS line activity decreased at pHs higher than 6.5.