Temporal and Spatial Expression Pattern of Sucrose Synthase during Tomato Fruit Development

Sucrose synthase is proposed to play an important role in the early stages of tomato fruit (Lycopersicon esculentum Mill.) growth. In this work, the temporal and spatial expression patterns of sucrose synthase during tomato fruit development were investigated. Fruit contained the majority of the sucrose synthase protein and mRNA relative to other organs. Only trace levels of sucrose synthase protein and mRNA were detected in the stem, petiole, and roots. Sucrose synthase mRNA was detected in pistils prior to anthesis, reached peak levels in fruit 5 to 7 d after anthesis (DAA), and was not detectable after 35 DAA. Sucrose synthase protein levels reached a maximum at 20 to 25 DAA and then declined to nondetectable levels after 45 DAA. The lack of coordination between protein and mRNA levels suggests that sucrose synthase expression may be controlled at the levels of both transcription and translation. Sucrose synthase mRNA was differentially localized in the fruit, being most abundant in the mesocarp cells adjacent to the placenta, the columella, and the cells surrounding the vascular bundle. Except around the vascular tissue, the localization of sucrose synthase mRNA positively correlates with starch granule accumulation at the cellular level.