Effects of Prolonged Washing on Primary and Secondary Transport Processes at the Plasma Membrane in Red Beet Storage Tissue

Valine uptake in the tap root of sugar beet: a comparative analysis with sucrose uptake

Given the lack of data on the absorption of amino acids in the tap root of Beta vulgaris, we studied the uptake of valine and compared it with that of sucrose at the same concentration (1 mM). The uptake of both substrates shared some similar characteristics. In particular, the absorption in both cases was controlled by an active process as evidenced by the inhibitory effect of CCCP and inhibitors of ATPases (DES, DCCD, orthovanadate). Both absorptions also involved the thiol and histidyl groups of protein carriers included in the plasmalemma as shown by treatment with specific compounds (PCMBS, mersalyl, NEM) inhibiting the transport of the nutrients in tissues and in purified PMV. However, it was shown that these uptakes present major differences. Firstly, unlike sucrose uptake, valine uptake was very sensitive to transmembrane electrical potential. Indeed, hyperpolarizing treatment with FC increased valine uptake but did not modify sucrose uptake. By contrast, treatment with high concentrations of KCl, which should result in depolarization of the cells, considerably decreased valine uptake and activated sucrose uptake. Secondly, ion mobilizations were different in the two types of transport. Unlike sucrose, application of valine to tissues strongly modified the time course of H+ influx. By contrast, sucrose uptake was controlled by K+ involvement as shown by effects either of modulators of K+ mobilization (LiCl, TEA) or of treatments inducing K+ starvation from the external medium.

Regulation of sugar, amino acid and peptide plant membrane transporters

During the past few years, various cDNAs encoding the proton cotransporters which mediate the uptake of sucrose, hexoses, amino acids and peptides across the plant plasma membrane have been cloned. This has made possible some preliminary insight into the regulation of the activity of these transporters at various levels. The paper summarises the present status of knowledge and gaps relative to their transcriptional control (organ, tissue and cell specificity, response to the environment) and post-transcriptional control (targeting and turnover, kinetic and thermodynamic control, lipidic environment, phosphorylation). This outline and the description of a few cases (the sink/source transition of the leaf, the pollen grain, the legume seed) serve as a basis for suggesting some directions for future research.

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.