Plant cells companion

Kuhn, C., Quick, W.P., Schulz, A., Riesmeier, J.W., Sommewald, U., and Frommer, W.B., Companion cell-specific inhibition of the potato sucrose transporter SUT1, Plant Cell Environ., 19, 1115-1123, 1996. 

Roberts, K. (1974). The Structural Development of Plant Cells in Companion to... 

The plasmodesmata may be aggregated in primary pit fields or in the pit membranes between pit pahs. The plasmodesmata appear as narrow canals (2 pm) lined by a plasma membrane and are traversed by a des-motubule, a tubule of endoplasmic reticulum. The plasmodemata are dynamic altering their dimensions and are functionally diverse. For example, whereas some transport endogenous plant transcription factors, others transport numerous proteins from companion cells to enucleated sieve elements. 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

The apparent biosynthesis and accumulation of sanguinarine within a single cultured opium poppy cell is in contrast to the requirement for different cell types to facilitate biosynthetic gene expression, alkaloid biosynthesis, and product accumulation in the plant 251). The movement of proteins from companion cells to sieve... 

The phloem tissue of plants is composed mainly of the sieve elements, companion cells, and the phloem parenchyma. The sieve elements are specialized elongated cells lacking a vacuole and connected at each end to other sieve elements by special pores. However, as in all cells, the cytoplasm is contained within the plasmalemma, with connections to other cells via the plasmodesmata. The phloem carries the products of photosynthesis, the assimilates, from the mature leaves to the meristematic and actively growing parts of the plant. The mechanism of phloem transport has long been the... 

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