Role of the Arabidopsis Glucose Sensor Hexokinase1 (HXK1) in seedling establishment
Ganpudi, Ashwin Laxminaranyan
Light energy serve as primary substrates for photosynthetic sugar production in plant metabolic maintenance. In addition, light also functions as a crucial environmental cue initiating a series of signal transduction cascades downstream to the photoreceptors. Limited knowledge exists on the molecular connections integrating the metabolic and photoreceptor signalling pathways. The primary focus of this thesis is the plant glucose receptor Hexokinase1 (HXK1). Arabidopsis HXK1 performs a dual function 1) an enzymatic role in glycolysis catalysing glucose phosphorylation to generate glucose-6-phosphate (G6P) and 2) a transcriptional repressor role in response to exogenous sugar. While the enzymatic role required for ATP production during respiration is conserved across biological systems, the sugar induced nuclear signalling role has only been identified in yeast and Arabidopsis. In this thesis, I show that HXK1 operates during nutrient limiting conditions such as extended periods of darkness or suboptimal light in seedlings. A first-ever RNAseq enabled us to gain insights into the mutant transcriptome. Energy demanding pathways were downregulated and carbon starvation induced Branched Chain Amino Acid (BCAA) degradation pathway is upregulated as an alternate energy source. Supplying Glucose-6-Phosphate (G6P – HXK1 enzymatic endproduct) restored the mutant phenotype and the C starvation response during nutrient limited conditions. This highlights the requirement of HXK1 enzyme rather than the signalling component during seedling establishment. Nuclear HXK1 operates as a transcriptional repressor in response to exogenous sugar. It is postulated to function during feedback inhibition of photosynthetic genes. However, our data indicates that this does not appear to be the case when endogenous sugars are naturally elevated in seedlings. Further, I present preliminary data on the potential feedback regulation by HXK1Overpression (HXK1OX) in blue and red light signalling pathway. HXK1 exerts a negative control on blue light mediated photomorphogenesis. Red light negates this effect in a PHYB dependent manner. Although the transcriptomes are reflective of the phenotype, the molecular mechanism behind this response is unknown. Taken together, my thesis discovers novel facets of HXK1 during seedling establishment.