Impact of invertase overexpression on cell size, starch granule formation and cell wall properties during tuber development in potatoes with modifiedcarbon allocation patterns

Transgenic potato tubers that overexpressed either a cytosolic or an apopla stic invertase in the wild type or AGPase antisense background were used to analyse the effect of invertase activity on cell expansion, starch granule formation and turgor pressure during tuber development. Although the trans genic plants did not develop a visible phenotype in aerial regions the size and number of tubers were significantly modified in the various lines. Tra nsmission electron and light microscopy were performed to monitor starch gr ain size and number, cell size and cell wall thickness, Water potential, os motic pressure, and, indirectly, turgor pressure were determined during the final stages of tuber development. Glucose levels were high in transgenic tubers that overexpressed a yeast-derived invertase. The number of starch g rains per cell was almost identical in all transgenic lines. However, the a mount of starch was modified in the transgenics as compared to the wild typ e. As expected, the size of starch grains was reduced in all lines that exp ressed an AGPase antisense mRNA, These results indicate that invertase acti vity and glucose levels do not affect initiation of starch grain formation during the early stages of tuber development, but growth of starch corns in the later stages of tuber maturation.