Tie-dyed1 Encodes a Novel, Phloem-Expressed Transmembrane Protein That Functions in Carbohydrate Partitioning      

Abstract

Carbon is partitioned between export from the leaf and retention within the leaf, and this process is essential for all aspects of plant growth and development. In most plants, sucrose is loaded into the phloem of carbon-exporting leaves (sources), transported through the veins, and unloaded into carbon-importing tissues (sinks). We have taken a genetic approach to identify genes regulating carbon partitioning in maize (Zea mays). We identified a collection of mutants, called the tie-dyed (tdy) loci, that hyperaccumulate carbohydrates in regions of their leaves. To understand the molecular function of Tdy1, we cloned the gene. Tdy1 encodes a novel transmembrane protein present only in grasses, although two protein domains are conserved across angiosperms. We found that Tdy1 is expressed exclusively in phloem cells of both source and sink tissues, suggesting that Tdy1 may play a role in phloem loading and unloading processes. In addition, Tdy1 RNA accumulates in protophloem cells upon differentiation, suggesting that Tdy1 may function as soon as phloem cells become competent to transport assimilates. Monitoring the movement of a fluorescent, soluble dye showed that tdy1 leaves have retarded phloem loading. However, once the dye entered into the phloem, solute transport appeared equal in wild-type and tdy1 mutant plants, suggesting that tdy1 plants are not defective in phloem unloading. Therefore, even though Tdy1 RNA accumulates in source and sink tissues, we propose that TDY1 functions in carbon partitioning by promoting phloem loading. Possible roles for TDY1 are discussed.

Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance

“…This is consistent with a decrease in phloem loading of sucrose as exemplified by the effects of antisense inhibition of SUT1 in potato (Solanum tuberosum; Riesmeier et al, 1994), SUC2 insertional mutants in Arabidopsis (Gottwald et al, 2000), and tdy1 mutants of maize (Zea mays; Ma et al, 2009). The ability of nakr1-1 to translocate 14 C sucrose was tested in intact plants.…”

“…Previous work has shown that reduced phloem loading leads to starch accumulation in leaves (Riesmeier et al, 1994;Gottwald et al, 2000;Ma et al, 2009). Leaves of short-day-grown plants were collected at the beginning and end of the day and stained with I 2 /KI solution.…”

Genetic Control of Carbon Partitioning in Grasses: Roles of Sucrose Transporters and Tie-dyed Loci in Phloem Loading

“…To understand the function of Tdy1, we recently cloned the gene (Ma et al, 2009). Tdy1 encodes a novel transmembrane protein found only in grasses, although two conserved protein subdomains are present in monocots and dicots.…”

Maize SUT1 functions in phloem loading

“…[13][14][15] mutants. [16][17][18][19][20][21][22][23][24][25] Guttation fluid was collected from untreated wild-type sibling and sut1-m1 mutant plants and assayed for soluble sugar concentrations. Wildtype guttation fluid contained very low sugar concentrations, whereas guttation fluid from sut1-m1 leaves contained over 700 mg of sucrose/g of fluid and 20-25 mg each of glucose and fructose/g of fluid ( Fig.…”