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Review
. 2023 Aug 23;12(17):3033.
doi: 10.3390/plants12173033.

The Interplay between Enucleated Sieve Elements and Companion Cells

Angel J Matilla  1
Affiliations
Review

The Interplay between Enucleated Sieve Elements and Companion Cells

Angel J Matilla. Plants (Basel). .

Abstract

In order to adapt to sessile life and terrestrial environments, vascular plants have developed highly sophisticated cells to transport photosynthetic products and developmental signals. Of these, two distinct cell types (i.e., the sieve element (SE) and companion cell) are arranged in precise positions, thus ensuring effective transport. During SE differentiation, most of the cellular components are heavily modified or even eliminated. This peculiar differentiation implies the selective disintegration of the nucleus (i.e., enucleation) and the loss of cellular translational capacity. However, some cellular components necessary for transport (e.g., plasmalemma) are retained and specific phloem proteins (P-proteins) appear. Likewise, MYB (i.e., APL) and NAC (i.e., NAC45 and NAC86) transcription factors (TFs) and OCTOPUS proteins play a notable role in SE differentiation. The maturing SEs become heavily dependent on neighboring non-conducting companion cells, to which they are connected by plasmodesmata through which only 20-70 kDa compounds seem to be able to pass. The study of sieve tube proteins still has many gaps. However, the development of a protocol to isolate proteins that are free from any contaminating proteins has constituted an important advance. This review considers the very detailed current state of knowledge of both bound and soluble sap proteins, as well as the role played by the companion cells in their presence. Phloem proteins travel long distances by combining two modes: non-selective transport via bulk flow and selective regulated movement. One of the goals of this study is to discover how the protein content of the sieve tube is controlled. The majority of questions and approaches about the heterogeneity of phloem sap will be clarified once the morphology and physiology of the plasmodesmata have been investigated in depth. Finally, the retention of specific proteins inside an SE is an aspect that should not be forgotten.

Keywords: P-proteins; companion cell; endoplasmic reticulum; enucleation; lipophilic compounds; plasmodesmata; sap; sieve element; transcription factors.

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Conflict of interest statement

The author declares no conflict of interest.

Figures

Figure 1
Figure 1
Parenchyma phloematic cell (PPC), sieve element (SE), and companion cell (CC), and distribution of mitochondria (mi), nucleus (n) and plastids (pl) in photosynthetic leaves. In the CCs, plastids are aligned and occupy a large proportion of the cell volume. Numerous mitochondria (mi) are also found in each cell type. The nuclei (nu) and the vacuole (vac) have unusual shapes and positions in the CCs. Plastid size differs with cell type (1 μm in SE, 3 μm in CC and 4 μm in PPC), whereas mitochondrial size variations are more limited (0.6 μm in SE, 0.6 μm in CC and 0.7 μm in PPC). Numerous plasmodesmata (pd) exist in the SE/companion cell physiological unit. This figure is reproduced from [20].
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