Sucrose Transporters in Selaginella moellendorffi

Abstract

Vascular tissue in plants functions in long distance transport of water, nutrients and metabolites. The evolution of vascular tissue is thought to be important for the development of large land plants. Phloem is one type of vascular tissue, it mainly transports sugars from photosynthetic leaves to the rest of the plant. Selaginella (a Lycophyte) is one of the earliest vascular plants and the Selaginella moellendorffi genome has been recently sequenced by the Department of Energy Joint Genome Institute (JGI). Selaginella has phloem tissue and now we know that it also has five sucrose transporter (SUT) genes. SUTs transport sucrose into the phloem in higher plants and we are interested to know if SUTs have the same function in Selaginella. By phylogenetic analysis, 4 of the Selaginella SUTs are type III and 1 is type II. This is interesting because type I SUTs are present and essential for dicot plants but also are not present in monocots. To study the function of Selaginella SUTs we attempted to clone cDNAs for all five genes. This was done by reverse transcription PCR and TA cloning into Gateway vector pCR8. PCR products of the right length were obtained for 4 of the SUT genes. After TA cloning, plasmids from E. coli 74 transformants were checked by restriction digest, and 11 clones were sequenced. However, only one correct cDNA was obtained (SmSUT4.3). This represents the first cloning of a sucrose transporter from a Lycophyte. Interestingly, the predicted SmSUT4.3 cDNA sequence incorrectly predicted the presence of one intron. Now that we have cloned SmSUT4.3, it will be subcloned into oocyte and yeast expression vectors to test the function of the protein.