Metabolic Changes In Roots Associated with Improved Drought Tolerance In Agrostis Stolonifera Expressing SAG12-Ipt Gene Controlling Cytokinin Synthesis.

Drought is one of the most widespread abiotic stresses limiting plant growth and has been shown to reduce endogenous cytokinin (CK) content and limit further synthesis in roots.  Previous studies have demonstrated that elevated cytokinin production through over-expression of SAG12-ipt gene encoding adenine isopentenyl phosphotransferase controlling cytokinin synthesis contributes to the enhancement of drought resistance by increasing total root length, root to shoot biomass ratio, and root viability.  This study was designed to elucidate whether changes in membrane lipid composition and saturation levels are associated with improved root growth for creeping bentgrass (Agrostis stolonifera) with increased CK synthesis during water deficit.  Transgenic and non-transgenic plants were exposed to drought stress for fifteen days followed by a five day recovery period. Plants were destructively sampled at 8 and 15d drought and 5d recovery to evaluate the following morphological and metabolic parameters: visual evaluation of turf quality (TQ), relative water content of leaves (RWC), photochemical efficiency of leaves (Fv/Fm), chlorophyll content of leaves (Chl.), root to shoot ratio of fresh and dry biomass, root viability via electrolyte leakage (EL) and triphenyl tetrazolium chrloride (TTC) reduction, changes to membrane fatty acid levels via lipid methylation and subsequent GC-MS analysis, and cytokinin content of leaves and roots via net iPA-content.  During drought, transgenic plants expressing the SAG12-ipt gene maintained significantly better TQ than both non-transgenic and null transformant plants, having higher leaf RWC and maintaining adequate Fv/Fm values.  Membrane stability of roots was best maintained in SAG12 plants, shown by lower EL and increased viability in all root zones (upper, middle, lower).  This ability to maintain root efficacy during severe water deficit was correlated to changes in fatty acid composition and saturation levels.  Plants expressing SAG12-ipt gene maintained higher levels of unsaturated fatty acids during drought compared to non-transgenic plants.  Unsaturated fatty acids containing more double bonds help to maintain efficacy of plasma membranes and therefore functional integrity during periods of water withholding.