Differential expression within an SPS gene family

We have characterised the expression of a subfamily of genes that encode sucrose phosphate synthase (SPS), the plant enzyme responsible for sucrose synthesis. cDNA fragments from four genes were cloned from diploid Actinidia chinensis Planch. var. chinensis. Three genes are highly similar (94-96% nucleotide identity in the overlapping region of the reading frame) and all belong to Family A, with the fourth gene (67% nucleotide identity) belonging to Family B. The A2 gene encodes a longer open reading frame than the other two A family genes, with a C-terminal extension of 102 amino acids. We compared transcript abundance of these four genes using RT-PCR. The three A genes were expressed in all tissues, but only A1 was present at high abundance and responded to developmental and environmental stimuli. The A1 transcript increased in fruit in response to treatment with exogenous ethylene and again during the ripening climacteric, but was not affected by low temperature. A1 expression was also high in senescent leaves, stems and in flower buds. The B family transcript was detectable in roots, albeit at very low levels and in all other tissues examined. It also increased in low temperature and in fruit at the climacteric (but not with applied ethylene). This is the first demonstration of differential expression patterns for highly homologous SPS genes in any plant species. (C) 2003 Elsevier Science Ireland Ltd. All rights reserved.