Light regulates alternative splicing of hydroxypyruvate reductase in pumpkin

Hydroxypyruvate reductase (HPR) is a leaf peroxisomal enzyme that functions in the glycolate pathway of photorespiration in plants. We have obtained two highly similar cDNAs for pumpkin HPR (HPR1 and HPR2). It has been revealed that two HPR mRNAs might be produced by alternative splicing from a single type of pre-mRNA. The HPR1 protein, but not the HPR2 protein, was found to have a targeting sequence into leaf peroxisomes at the C-terminus, suggesting that alternative splicing controls the subcellular localization of the two HPR proteins. Immunoblot analysis and subcellular fractionation experiments showed that HPR1 and HPR2 proteins are localized in leaf peroxisomes and the cytosol, respectively. Moreover, indirect fluorescence microscopy and analyses of transgenic tobacco cultured cells and Arabidopsis thaliana expressing fusion proteins with green fluorescent protein (GFP) revealed the different subcellular localizations of the two HPR proteins. Both mRNAs were induced developmentally and by light, but with quantitative differences. Almost equal amounts of the mRNAs were detected in pumpkin cotyledons grown in darkness, but treatment with light greatly enhanced the production of HPR2 mRNA. These findings indicate that light regulates alternative splicing of HPR mRNA, suggesting the presence of a novel mechanism of mRNA maturation, namely light-regulated alternative splicing, in higher plants.