Differential expression of three sucrose-phosphate synthase isoforms during sucrose accumulation in citrus fruits ( Citrus unshiu Marc.)

Abstract

The role of sucrose-phosphate synthase (SPS) in sucrose metabolism during citrus fruit development was investigated through sucrose content and enzyme activity in relation to the transcription of three SPS cDNA isoforms. The profiles of sucrose accumulation in edible tissue (juice sacs and segment epidermis) of satsuma mandarin ( Citrus unshiu Marc.) agreed with those of SPS activity, that is, an increase in sucrose content accompanied high SPS activity. However, sucrose accumulation in edible tissue preceded the increase in SPS activity. The sucrose accumulation and the activity of SPS in peel tissue (albedo and flavedo) were lower than those in edible tissue. The expressions of three SPS genes (previously isolated and designated as CitSPS1, CitSPS2, and CitSPS3) were analyzed during the fruit developmental stage. Transcripts of the CitSPS1 in edible tissue were expressed at all stages analyzed and the transcript level was highest at the mature stage (223 days after flowering (DAF)). No transcripts of the CitSPS2 were found in edible tissue before 187 DAF, but thereafter transcripts were detected and increased until the mature stage. However, transcripts of CitSPS3 were undetected in edible tissue throughout the developmental stage. The expression of CitSPS1 in peel tissue was low in young fruit, then increased notably at 148 DAF, retaining its level until the mature stage. Transcripts of CitSPS2 in peel tissue showed the same pattern as that of edible tissue. The expression of CitSPS3 in peel tissue was detected at 148, 187 and 223 DAF, although its level was weak compared with CitSPS1 and CitSPS2. The activity of SPS in edible tissue paralleled the levels of transcription of CitSPS1 and CitSPS2, suggesting that the expression of these two genes might have an important role in determining the sucrose composition or accumulation in citrus fruit. In peel tissue, the patterns of SPS activity mostly agreed with the transcripts of CitSPS1, CitSPS2 and CitSPS3. These results also indicate that SPS genes are independently regulated, resulting in distinct temporal and spatial expression patterns.