Suppression of the Expression of C Type Sucrose Phosphate Synthase via RNAi Decreases Sucrose Synthesis without Altering Photosynthesis in Apple Leaves

Both sorbitol and sucrose are synthesized in the cytosol of mesophyll cells as end-products of photosynthesis in apple leaves. Our previous work has shown that antisense inhibition of sorbitol synthesis leads to an increased starch synthesis in the chloroplast during the day and enhanced starch degradation for sucrose synthesis at night, thereby maintaining leaf photosynthesis. It was hypothesized that an analogous mechanism operates with regards to sucrose synthesis in apple leaves. In this study, we decreased the expression of C type sucrose phosphate synthase (SPS), the main isoform of SPS, in apple leaves via RNAi to test this hypothesis. The expression level of C type SPS was decreased to approximately 30% to 40% of that in the untransformed control. Correspondingly, leaf sucrose concentrations were decreased to 30% to 40% of the control level. Leaf starch levels were elevated in the leaves of transgenic trees at the end of the day and starch degradation was enhanced at night. Leaf sorbitol concentration was similar between transgenic plants and the control during the day, but was higher in the transgenic plants at night. Leaf photosynthesis and plant growth were not altered. These data along with the results obtained previously on transgenic plants with decreased sorbitol synthesis indicate that presence of two branches for end-product synthesis in the cytosol of apple leaves, along with starch synthesis in the chloroplast, enables the system to compensate for an interruption of either branch without affecting photosynthetic carbon acquisition.