In many plants, translocation of sucrose from mesnsophyll to phloem for long‐distance transport is carrier‐mediated. The sucrose H+‐symporter gene SUT1 from potato is expressed at high levels in the phloem of mature, exporting leaves and at lower levels in other organs. Inhibition of SUT1 by expression of an antisense gene in companion cells under control of the rolC promoter leads to accumulation of high amounts of soluble and insoluble carbohydrates in leaves and inhibition of photosynthesis. The distribution of in situ localized starch does not correspond with areas of reduced photosynthesis as shown by fluorescence imaging. Dissection of antisense effects on sink and source organs by reciprocal grafts shows that inhibition of transporter gene expression in leaves is sufficient to produce chlorosis in leaves and reduced tuber yield. In contrast to the arrest of plasmodesmal development found in plants that express yeast invertase in the apoplast, in mature leaves of sucrose transporter antisense plants plasmodesmata are branched and have median cavities. These data strongly support an apoplastic mode of phloem loading in potato, in which the sucrose transporter located at the plasma membrane of the sieve element/companion cell complex represents the primary route for sugar uptake into the long‐distance translocation pathway.

The structure of the shrunken gene of Zea mays encoding sucrose synthase (EC 2.4.1.13) was determined by (i) sequencing the transcription unit and ˜1.2 kb of 5′ ‐upstream sequences from a genomic clone, (ii) by sequencing a nearly full length cDNA clone and (iii) by determining the transcription start site by a combination of primer extension experiments with synthetic oligodeoxynucleotide primers and S1 mapping. The sucrose synthase gene is 5.4 kb long, of which 2746 bp are found in the mature mRNA. The gene is interrupted by 15 introns. The first two introns are ˜1 kb and ˜0.5 kb in length, respectively, while the other introns are much smaller. A TATA box is located 30 bp upstream from the transcription start site. Approximately 610 bp upstream of the transcription start site a direct repeat of 16 nucleotides, separated by a 4‐fold repetition of the sequence GGTGG is detected. The 16‐bp sequence has similarities to a sequence repeat found between two promoters of a maize zein gene also expressed in the endosperm tissue. The transposable element Ds in the mutant sh‐m5933 and sh‐m6233 alleles is inserted in the seventh and first intron, respectively. The genomic and cDNA clones were obtained from different maize lines. This allows the determination of polymorphic sites which are frequent in 3rd codon position and absent in 1st and 2nd codon positions. In addition, the 3′ ‐untranslated sequence shows two duplications that may have arisen by the insertion and subsequent excision of transposable elements.

Summary. In developing seeds of Viciafaba, transfer ceils line the inner surface of the seed coat and the juxtaposed epidermal surface of the cotyledons. Circumstantial evidence, derived from anatomical and physiological studies, indicates that these cells are the likely sites of sucrose efflux to, and influx from, the seed apoplasm, respectively. In this study, expression of an H+/sucrose symporter-gene was found to be localised to the epidermal-transfer cell complexes of the cotyledons. The sucrose binding protein (SBP) gene was expressed in these ceils as well as in the thin-walled parenchyma transfer cells of the seed coat. SBP was immunolocalised exclusively to the plasma membranes located in the wall ingrowth regions of the transfer cells. In addition, a plasma membrane H+-ATPase was most abundant in the wall ingrowth regions with decreasing levels of expression at increasing distance from the transfer cell layers. The observed co-localisation of high densities of a plasma membrane tt+-ATPase and sucrose transport proteins to the wall ingrowths of the seed coat and cotyledon transfer cells provides strong evidence that these regions are the principal sites of facilitated membrane transport of sucrose to and from the seed apoplasm.Keywords: Gene expression; Plasma membrane transport; Seed; Sucrose efflux/influx; Transfer ceils; Vicia faba.Abbreviations: BCIP 5-bromo-4-chloro-3-indolyl phosphate; DIG digoxigenin; H+-ATPase plasma membrane H+-translocating adenosine triphosphatase; Ig immunoglobulin; LeSUTt tomato H+/sucrose symporter; SBP sucrose binding protein.

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