Expression of alpha-expansins and cell growth in tobacco and cucumber

The goals of this research were to assess the physiological roles and functions of expansins. I used molecular genetic techniques to alter the expression levels of expansins in tobacco suspension cells (BY2) and looked for changes in important physiological properties of plant cells, such as cell size, wall strength, and resistance to shear stress in a bioreactor environment. I also used molecular biology techniques to identify and characterize changes in expression levels of expansins in the root and peg of developing cucumber seedlings. The peg is a polar outgrowth at the root-shoot transition zone of cucumber seedlings that is used to help remove the seed coat.;To work with BY2 suspension cells, I first needed to show that they had an expansin mediated growth response and to identify expansin cDNAs. BY2 cells were shown to have an acid growth response, grow in response to treatment with exogenous expansins, express expansin cDNAs, and to produce their own active expansin proteins. I transformed BY2 cells with a construct that constitutively overexpessed the NtExp1 expansin cDNA, and found that transformed cell lines ruptured more easily in explosive decompression tests, and that they were more sensitive to higher shear growth environments than controls. This indicated that their cell walls were weaker.;The work on cucumber pegs showed that peg development was not dependent on gravity as was previously reported. Pegs developed in over 95% of seedlings grown on clinostats or in space. Pegs from space grown seedlings were morphologically different from those of ground controls. We identified seven new alpha-expansin cDNAs in cucumber and characterized the expression patterns of six of the nine known cucumber expansins. Two expansins (CsExp3 and CsExp4) were preferentially expressed in the peg and root tissue. We checked for alterations in their expression levels in response to gravity and found that expression levels decreased when seedlings were grown on clinostats, but that no change occurred when seedlings were germinated in space.;This work shows that expansins do have important effects on cell wall strength and that expansin gene family members are differentially expressed throughout the developing cucumber seedling.