Title : Identifying and characterising putative aldose 6-phosphate reductases in Arabidopsis thaliana
Abstract:
In Rosaceae (apple, stone fruits) and Plantaginaceae (plantain) species, sugar alcohols like sorbitol are phloem-translocated and allow more efficient use of carbon, act as compatible solutes in abiotic stress (cold, drought, salt) and facilitate boron mobilisation. The key regulatory step in the synthesis of this acyclic polyol is catalysed by aldose-6-phosphate-reductase (A6PR) in source organs, which reduces glucose-6-phosphate. The resulting sorbitol-6-phosphate is then hydrolysed to form sorbitol, via sorbitol-6-phosphatase. In sink organs like roots and fruits, sorbitol is oxidised by NAD+-dependent sorbitol dehydrogenase (SDH) to fructose. Curiously, A6PR- and SDH-like enzyme activity is found in families that synthesise and transport sucrose, and in Arabidopsis (a sucrose-translocating Brassicaceae), we have identified two proteins with the structural features and >65% amino acid identity with known plant A6PRs; we call these AtA6PR1 and AtA6PR2. Unlike in Rosaceae, we demonstrate that AtA6PR1 and AtA6PR2 are ubiquitously-, but differentially-expressed in different Arabidopsis organs. By transient transformation of tobacco, we show that GFP-fusion proteins of both reductases are localised in the cytosol. Potential mutant lines have been genotyped, and along with studies of the relative expression of both genes in wild-type plants grown under different abiotic stresses (cold, saline), we are determining their physiological role in this non-sorbitol translocating species. Additionally, when AtA6PR1 is over-expressed in wild-type and sdh- mutant Arabidopsis lines, the starch content increases. Currently, similar experiments are underway with AtA6PR2 with the overall aim of analysing the effects that a potential mis-balance in sorbitol metabolism has on the plant. Progress in the biochemical characterisation of these proteins will also be presented. Funding: Fondecyt 1140527.
