The mechanisms of plant response to nutrient limitation and utilisation are of great interest for agricultural purposes. Phosphate is a non-renewable resource and is one of the most important nutrients required for plant growth. Recently a new family of plant protein kinases, composed of 10 members, were discovered because of their involvement in stresses and their responses to the hormone abscisic acid (ABA). In Arabidopsis, all of these SnRK2 protein kinases have been shown to be activated by drought or hyperosmostic stress, with the exception of SnRK2.9. Five members are also activated by ABA treatment. Recently SnRK2.8 was linked to metabolic processes by being down regulated in low nutrient level conditions. In the present study, SnRK2.9 was investigated and shown to play a role in metabolic pathways, but in an opposite manner. Contrarily to SnRK2.8, transcripts level of SnRK2.9 is induced in response to phosphate, nitrogen, and sulphur deprivation. Interestingly, opposite to most phosphate-starvation inducible genes, sucrose decreases SnRK2.9's transcripts level. Transgenic plants that overexpress SnRK2.9 do not appear to be affected in terms of growth. On the other hand, overexpressing antisense SnRK2.9 or mutated snrk2.9 at residue Asp-123 by conversion to Glu (D123E), showed reduced plant growth. This phenotype was more pronounced in the absence of phosphate. A T-DNA knockout line for SnRK2.9 showed a 45% decrease in root and shoot biomass compared to wild-type Arabidopsis when grown under phosphate deprivation. Similar trends were observed when the Arabidopsis gene was introduced in Medicago sativa (alfalfa) under the control of the CaMV 35S promoter. Overexpressing D123E Atsnrk2.9 had a serious inhibitory effect on growth and the plants were no longer responsive to changes in phosphate levels. In Arabidopsis, the D123E snrk2.9 overexpressors had a 66% reduction in total seed yield when grown under +Pi conditions and a 33% reduction under -Pi conditions. These Arabidopsis transgenic lines do not share similar traits to the known phosphate metabolic mutants Pho1, Pho2, and Siz1. SnRK2.9 appears to play a key role in biomass and seed production. / Thesis (Ph.D, Biology) -- Queen's University, 2007-09-26 12:35:00.626