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Sucrose-phosphate synthase (SPS) is one of the key regulatory enzymes in carbon assimilation and partitioning in plants. SPS plays a central role in the production of sucrose in photosynthetic cells and in the conversion of starch or fatty acids into sucrose in germinating seeds. To explore the mechanisms that regulate the tissue-specific and developmental distribution of SPS, the expression pattern of rice (Oryza sativa) sps1 (GenBank accession no. U33175) was examined by in situ reverse transcriptase-polymerase chain reaction and the expression directed by the sps1 promoter using the beta-glucuronidase reporter gene. It was found that the expression of the rice sps1 gene is limited to mesophyll cells in leaves, the scutellum of germinating seedlings, and pollen of immature inflorescences. During leaf development, the sps1 promoter directs a basipetal pattern of expression that coincides with the distribution of SPS activity during the leaf sink-to-source transition. It was also found that during the vegetative part of the growth cycle, SPS expression and enzymatic activity are highest in the youngest fully expanded leaf. Additionally, it was observed that the expression of the sps1 promoter is regulated by light and dependent on plastid development in photosynthetic tissues, whereas expression in scutellum is independent of both light and plastid development.

Direct support for the concept that RNA molecules circulate throughout the plant, via the phloem, is provided through the characterisation of mRNA from phloem sap of mature pumpkin (Cucurbita maxima) leaves and stems. One of these mRNAs, CmNACP, is a member of the NAC domain gene family, some of whose members have been shown to be involved in apical meristem development. In situ RT-PCR analysis revealed the presence of CmNACP RNA in the companion cell-sieve element complex of leaf, stem and root phloem. Longitudinal and transverse sections showed continuity of transcript distribution between meristems and sieve elements of the protophloem, suggesting CmNACP mRNA transport over long distances and accumulation in vegetative, root and floral meristems. In situ hybridization studies conducted on CmNACP confirmed the results obtained using in situ RT-PCR. Phloem transport of CmNACP mRNA was proved directly by heterograft studies between pumpkin and cucumber plants, in which CmNACP transcripts were shown to accumulate in cucumber scion phloem and apical tissues. Similar experiments were conducted with 7 additional phloem-related transcripts. Collectively, these studies established the existence of a system for the delivery of specific mRNA transcripts from the body of the plant to the shoot apex. These findings provide insight into the presence of a novel mechanism likely used by higher plants to integrate developmental and physiological processes on a whole-plant basis.