Expression analysis of a sucrose carrier in the germinating seedling of Ricinus communis

This study describes the expression of a sucrose carrier at various developmental stages in Ricinus communis. A partial-length cDNA clone, RcSUT1, was isolated by RT-PCR from Ricinus seedling RNA. This is almost identical to a sucrose carrier cDNA, Rscr1, which has previously been isolated by library screening. However, we have observed a very different expression pattern in the seedling to that previously reported. Northern analysis, with RcSUT1 as a probe, revealed high expression of a 2 kb transcript in the cotyledons of the germinating seedling; transcript levels were similar in cotyledons harvested 3-6 days after germination. A much lower level of this transcript was detected in the root, hypocotyl and endosperm RNA of the seedling and very low levels were also present in the sink and source leaves of the mature plant. This pattern of expression was also reflected at the protein level with an antipeptide antibody raised to part of the RcSUT1 deduced amino acid sequence. Tissue print hybridisation analysis of the hypocotyl revealed that the sucrose carrier transcripts were localised to the phloem cells of the vascular bundles. A more detailed analysis of sucrose carrier gene expression in the cotyledons of the germinating seedling was carried out by in situ hybridisation; the strongest signals were observed from the lower epidermal layer and the phloem, consistent with an active loading role for these cells. An ultrastructural study of the cells in the lower epidermis showed that they have wall ingrowths which are characteristic of transfer cells. The results are discussed in relation to the physiological role of the sucrose carrier in the Ricinus seedling and to the pathways of sucrose movement from endosperm to the sieve elements in the cotyledons.