Analysis of growth, composition and thickness of the cell walls of transgenic tobacco plants expressing a yeast-derived invertase

Transgenic tobacco (Nicotiana tabaccum L. cv. Samsun NN) expressing a yeast invertase in the vacuole provides a novel tool for studying the role of turgor, osmotic pressure, and cell wall properties during cell expansion. The plants used showed increased osmolarity and an increased cell size in young leaves. Their advantage is that they allow long-term analysis and undisturbed conditions. Cell expansion rate was maximal in leaf six of the transgenic plants and in leaf eleven of wild-type plants. Turgor rose to 0.52 +/- 0.04 MPa (n = 45) and 0.35 +/- 0.03 MPa (n = 45) in transgenic and wild-type plants, respectively. It was maximal where elongation rates were highest. Thus, elevated cell expansion rate was, at least in part, related to an enhancement in turgor. However, comparison between turgor and relative expansion rates showed that higher turgor pressures were required to achieve similar cell expansion rates in transformed plants as in the wild-type. This finding underlines the importance of the yield threshold and, thus, of the cell wall in growth regulation. This conclusion is further supported by the observation that the cell walls of transgenic plants were up to 77% thicker than the wild-type, but not qualitatively modified.