What Is the Definition of Sieve Tube in Biology
If you think about it, there must be a proper connection between the sieve tubes and the accompanying cells in the phloem tissue. The former needs the support of the latter. Looking closely, you will find small connections or channels on the adjacent walls of these plant cells. They are called plasmodesma. A sieve tube is connected to neighboring companion cells via these channels to maintain proper nutrition, protein, and other organic compounds for function. These channels eventually become sieve plates over time. Element of sieve in vascular plants, elongated living cells of the phloem, whose nuclei have fragmented and disappeared and whose transverse walls are pierced by groups of pores in the form of sieves (sieve plates). These are the channels for transporting food (mainly sugar) from the leaves to the rest of the plant. Formation of plugs of protein P (phloem protein). In addition to the plasma membrane of sieve elements, the P-protein filaments form a fine network. When the screening tube is damaged, the P protein (along with other phloem contents) flows to the cut end due to internal hydrostatic pressure. The tangled mass of protein filaments and protein bodies forms a “protein P” plug that helps seal the cut end of the screening tube. However, not all flowering plants have P proteins.
A sieve plate is a screen tube element formed from the maturity of plasmodesma, the connection between sieve tubes and accompanying cells. You will find these plaques in mature phloem tissues that go vertically from one adjacent cell to another. It is generally believed that remote translocation in the phloem is driven by a mass flow generated by a pressure gradient between the sink tissue and the source tissue. In the classical concept, the screen tubes that form the translocation path between the swelling and descending ends were considered osmotically isolated. However, this model has been adapted to a more dynamic concept, suggesting that the screening tubes essentially leak and that the transported substances are released and recovered along the phloem pathway (van Bel, 2003a). Sucrose, for example, is lost from phloem at considerable rates, but is constantly recovered from apoplast by the activity of sucrose transporters that adorn the phloem ad (Kühn et al., 1997). Recent experimental data from Ayre et al. (2003) support the view that small solutes in CC enter the translocation stream indiscriminately, but are then subjected to mechanisms that control retention and/or reclamation along the transport route (for details, see van Bel, 2003b). The sieve elements were first discovered in 1837 by forest botanist Theodor Hartig. Since this discovery, the structure and physiology of phloem tissue has been given more prominence, as more emphasis has been placed on its specialized components such as sieve cells. Phloem was introduced in 1858 by Carl Nägeli after the discovery of sieve elements.
Since then, several studies have been conducted on the functioning of phloem sieve elements in terms of working as a transport mechanism. [2] An example of phloem analysis by sieve elements was performed in the study of Arabidopsis leaves. By studying leaf phloem in vivo by laser microscopy and using fluorescent markers (both in companion cells and in sieve elements), the accompanying cell network with the compact sieve tubes was highlighted. Markers of sieve elements and accompanying cells were used to study the network and organization of phloem cells. [4] In contrast, the brown funnel (Nilaparvata lugens) uses a different mechanism to overcome the callosis of the sieve tube. While genes encoding (1,3)-β-glucan synthases are upregulated and screen-tube callosis is deposited during insertion into resistant and susceptible rice plants, genes encoding (1,3)-β-glucan endohydrolases are upregulated only in susceptible plants (Hao et al., 2008). This suggests that the limited or absent expression of these endohydrolase (1,3)-β-glucan genes allows the maintenance of sieve tube closure in resistant plants and is probably the main reason for their resistance (Hao et al., 2008). Thus, instead of preventing callose synthesis as in aphids, brown funnels use (1,3)-β-glucan endohydrolases to hydrolyze sieve plate callosis. Since any damage to the sieve tubes leads to the formation and blockage of callosis, phloem feeding insects, in order to successfully feed on sieve element juice, developed special mechanisms for overcoming the stress deposition of callosis. When aphids are inserted into the screen tubes, vaginal saliva is secreted, preventing the influx of calcium from the wall through the puncture site (Will and van Bel, 2006). Since salivary proteins contain calcium-binding domains (Will et al., 2007), it has been suggested that aphid saliva may act as a chemical calcium scavenger to prevent an increase in calcium concentration and thus the shield tube sealing reaction.
The screen tube elements are arranged lengthwise from one end to the other to form screen tubes. Formed by these vertical connections between several elements of screen tubes, the screen tubes are directly responsible for transport through the minimum resistance around their walls. [8] Using these pores, which make up a large part of the structure of the screen plates, the diameter of the screening tubes can be regulated. This regulation is necessary for the screening tubes to react to changes in the environment and conditions in the body. [5] In host plants, mature phloem sieve tubes generally contain the highest concentration of phytoplasmas (Figure 3). Since phloem cells are living cells, this can be considered intracellular. In addition, intracellular phytoplasmas of different morphologies, some of which were probably caused by budding or multiplication, have also been found in the cytoplasm of an immature phloem element. In addition, phytoplasmas have been detected in the cytoplasm of phloemparenchyma cells alongside sieve elements, in parenchymal cells in or near the vascular system of cascuta shoots, and in coconut embryos. Although phytoplasmas are detected in seeds, there is no evidence that phytoplasmas can be transferred to next-generation plants. A large number of proteins are described in angiosperm sieve tubes, including at least 82 non-redundant RNA-binding proteins and the ubiquitin/26S proteasome proteolysis pathway machinery (Box 9.2) detected in pumpkin phloem juice (Lin et al., 2009). Some of these are host factors that have been identified as influencing the transport of viruses over long distances.
A sieved tube is a major component of the phloem tissue present in angiosperms. These cells are accompanied by companion cells that form an elegant transport system to deliver carbohydrates and other organic compounds to the rest of the plant parts. Sieve tubes and the cells that accompany them are dominant in this tissue. These cells are alive but have no nucleus. All space is used for the transport of nutrients. Here, the accompanying cells act to support these tubes. The tubes do not have ribosomes to synthesize proteins. Therefore, all functions managed by the ribosomes and nucleus are performed by the accompanying cells next to the sieve tubes.
