sievetube

Sievetube, often referred to as a sieve tube or sieve tube element, denotes the conducting component of the phloem in vascular plants. It forms a long, continuous channel through which photosynthetically produced nutrients are transported from source tissues to sinks such as growing shoots, roots, and developing fruits.

Structure and organization

Sieve tube elements are elongated living cells that align end to end to create a continuous conduit. During maturation they typically lose a nucleus, vacuole, and a portion of organelles, yet they remain metabolically active with support from adjacent companion cells. The end walls between sieve tube elements are perforated by sieve plates, which contain numerous pores that allow phloem sap to pass. Callose can be deposited around these pores in response to injury to limit leakage. Companion cells, closely associated with sieve tube elements via numerous plasmodesmata, provide metabolic support and are involved in loading and unloading cargo into the sieve tubes.

Function and transport

The primary function of sievetubes is the bulk transport of organic solutes, especially sucrose, along with amino acids, hormones, and RNA. Transport is driven by turgor pressure differences generated by osmotic loading and unloading of solutes at source and sink tissues, a mechanism commonly described by the pressure-flow hypothesis. Loading of sugars into sieve tubes at sources is often an energy-dependent process mediated by companion cells, while unloading at sinks completes the cycle.

Development and variation

Sieve tube elements differentiate from provascular tissue in the phloem and, despite their reduced organelle content, remain living cells supported by companion cells. Variations exist among plant groups, with some gymnosperms possessing sieve cells rather than fully developed sieve tubes.

Sievetube function is essential for plant growth and resource allocation, linking photosynthetic production to expanding tissues throughout the plant.