Sieve element and companion cell: the story of the comatose patient and the hyperactive nurse

Abstract

Sieve elements and companion cells constitute the modules of the conducting elements in the phloem ofAngiosperms. Consequently, phloem transport basically relies on the concerted action of the sieve element/companion cell complexes. Sieve elements and companion cells are highly interactive units and show an extreme division of labour as exemplified by their state of life. Whereas the sieve element is almost ‘clinically’ dead, the companion cell is a paragon of bubbling activity. In the course of evolution, the sieve element has sacrificed all of its genetic and most of its metabolic equipment to serve photoassimilate translocation. A small part of the structural and metabolic outfit has been retained for a proper accomplishment of its function. In contrast, the cells bordering the sieve element have gained metabolic weight during evolution. With reference to their evolutionary descent, the peculiarities of sieve elements and companion cells are discussed in the light of recent cell-biological and molecular-biological findings. Emphasis is focused on their interaction, which is the secret of the success of the sieve element/companion cell complex.

CHOLINE TRANSPORTER-LIKE1 is required for sieve plate development to mediate long-distance cell-to-cell communication

“…As the SE differentiates, it undergoes a selective breakdown and all the remaining components become distributed along the ARTICLE wall 47,48 . Studies of SE formation in various plant species revealed that this process is accompanied by plasmodesmata enlargement and disintegration of the desmotubules, which leads to the formation of mature sieve plate pores 49,50 (Fig.…”

Collection and Analysis of <em>Arabidopsis</em> Phloem Exudates Using the EDTA-facilitated Method

“…However, most greenhouse or growth chamber managers will not support use of a pathogen. In addition, aphids introduce several proteins into the phloem with their saliva 19,33 . This leads to a limited transcriptional reprogramming 30 and has the potential to change the phloem composition 26 .…”

“…The sieve element is the site of long-distance movement. To allow for unobstructed longitudinal flow, sieve elements are missing most organelles as well as nuclei and are thought to contain at best a limited translational machinery 21,33 . It is believed that companion cells synthesize proteins and other compounds, which travel in the phloem stream.…”

A Plasma Membrane-Anchored Fluorescent Protein Fusion Illuminates Sieve Element Plasma Membranes in Arabidopsis and Tobacco

“…Membrane proteins expressed in immature SEs that are still nucleate, such as the early nodulin (ENOD)-like protein of Arabidopsis (Arabidopsis thaliana; Khan et al, 2007), are likely diluted or degraded as the SEs expand and mature, making them potentially poor targets for fluorescence tagging. Mature SEs lose the capacity for protein expression, delegating it to their sister cells, the companion cells (CCs;van Bel and Knoblauch, 2000). Thus, CC-expressed membrane-anchored proteins remain the strongest candidate for continuous SE membrane labeling in mature phloem.…”