A calcium-regulated gatekeeper in phloem sieve tubes.

The phloem is a highly specialized long distance transport tissue in vascular plants. Despite detailed knowledge of the development and ultrastructure of phloem tissue in a wide variety of plant species, surprisingly little is known about the mechanisms and regulation of phloem transport. The main function of the phloem is transport of the immediate products of photosynthesis (e.g., sugars) from “source” tissue (actively photosynthesizing leaves) to “sink” tissue (immature leaves, growing root tips, and developing flowers, fruit, and seed). It is sometimes overlooked that a wide variety of other material is also transported through the phloem, including proteins, amino acids, solutes, viruses, and various signaling molecules.