Vacuoles release sucrose via tonoplast-localised SUC4-type transporters.

Arabidopsis thaliana has seven genes for functionally active sucrose transporters. Together with sucrose transporters from other dicot and monocot plants, these proteins form four separate phylogenetic groups. Group-IV includes the Arabidopsis protein SUC4 (synonym SUT4) and related proteins from monocots and dicots.

Conserved cis-regulatory elements for DNA-binding-with-one-finger and homeo-domain-leucine-zipper transcription factors regulate companion cell-specific expression of the Arabidopsis thaliana SUCROSE TRANSPORTER 2 gene.

The transition from young carbon-importing sink leaves of higher plants to mature carbon-exporting source leaves is paralleled by a complete reversal of phloem function. While sink-leaf phloem mediates the influx of reduced carbon from older source leaves and the release of this imported carbon to the sink-leaf mesophyll, source-leaf phloem catalyzes the uptake of photoassimilates into companion cells (CCs) and sieve elements (SEs) and the net carbon export from the leaf. Phloem loading in source leaves with sucrose, the main or exclusive transport form for fixed carbon in most higher plants, is catalyzed by plasma membrane-localized sucrose transporters.

Technical Advance: A rapid method for detection of plant gene transcripts from single epidermal, mesophyll and companion cells of intact leaves.

A protocol for the detection of gene transcripts from single plant cells in living, undamaged plant tissue is described. Samples of leaf epidermal, mesophyll and companion cells were extracted by using glass microcapillaries and directly subjected to RT-PCR without any purification steps nor time consuming construction of cDNA libraries. The procedure is not restricted to surface cells or outer cell layers.

Simple, but not branched, plasmodesmata allow the nonspecific trafficking of proteins in developing tobacco leaves.

Leaves undergo a sink-source transition during which a physiological change occurs from carbon import to export. In sink leaves, biolistic bombardment of plasmids encoding GFP-fusion proteins demonstrated that proteins with an Mr up to 50 kDa could move freely through plasmodesmata. During the sink-source transition, the capacity to traffic proteins decreased substantially and was accompanied by a developmental switch from simple to branched forms of plasmodesmata.

Cell-to-cell and long-distance trafficking of the green fluorescent protein in the phloem and symplastic unloading of the protein into sink tissues.

Macromolecular trafficking within the sieve element-companion cell complex, phloem unloading, and post-phloem transport were studied using the jellyfish green fluorescent protein (GFP). The GFP gene was expressed in Arabidopsis and tobacco under the control of the AtSUC2 promoter. In wild-type Arabidopsis plants, this promoter regulates expression of the companion cell-specific AtSUC2 sucrose-H+ symporter gene.