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

Transgenic potato tubers that overexpressed either a cytosolic or an apoplastic 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 transgenic plants did not develop a visible phenotype in aerial regions the size and number of tubers were significantly modified in the various lines. Transmission electron and light microscopy were performed to monitor starch grain size and number, cell size and cell wall thickness. Water potential, osmotic 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 grains per cell was almost identical in all transgenic lines. However, the amount of starch was modified in the transgenics as compared to the wild type. As expected, the size of starch grains was reduced in all lines that expressed an AGPase antisense mRNA. These results indicate that invertase activity 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.