Annual Plant Reviews, Membrane Transport in Plants
Smaller molecules e. Larger molecules, including proteins for example green fluorescent protein and RNA, can also pass through the cytoplasmic sleeve diffusively. One mechanism of regulation of the permeability of plasmodesmata is the accumulation of the polysaccharide callose around the neck region to form a collar, thereby reducing the diameter of the pore available for transport of substances. This increase in plasmodesmata pore permeability allows for larger molecules, or macromolecules , such as signaling molecules, transcription factors and RNA-protein complexes to be transported to various cellular compartments.
The desmotubule is a tube of appressed flattened endoplasmic reticulum that runs between two adjacent cells. Around the desmotubule and the plasma membrane areas of an electron dense material have been seen, often joined together by spoke-like structures that seem to split the plasmodesma into smaller channels. If this is the case these proteins could be used in the selective transport of large molecules between the two cells.
Plasmodesmata have been shown to transport proteins including transcription factors , short interfering RNA , messenger RNA , viroids , and viral genomes from cell to cell. One example of a viral movement proteins is the tobacco mosaic virus MP MP is thought to bind to the virus's own genome and shuttle it from infected cells to uninfected cells through plasmodesmata. Plasmodesmata are also used by cells in phloem , and symplastic transport is used to regulate the sieve-tube cells by the companion cells.
The size of molecules that can pass through plasmodesmata is determined by the size exclusion limit. This limit is highly variable and is subject to active modification. Several models for possible active transport through plasmodesmata exist. A similar mechanism may be involved in transporting viral nucleic acids through the plasmodesmata. Plasmodesmata link almost every cell within a plant, which can cause negative effects such as the spread of viruses.
In order to understand this we must first look at cytoskeletal components, such as actin microfilaments, microtubules, and myosin proteins, and how they are related to cell to cell transport. Actin microfilaments are linked to the transport of viral movement proteins to plasmodesmata which allow for cell to cell transport through the plasmodesmata.
Fluorescent tagging for co-expression in tobacco leaves showed that actin filaments are responsible for transporting viral movement proteins to the plasmodesmata. When actin polymerization was blocked it caused a decrease in plasmodesmata targeting of the movement proteins in the tobacco and allowed for kDa rather than kDa components to move between tobacco mesophyll cells. This also impacted cell to cell movement of molecules within the tobacco plant. Viruses break down actin filaments within the plasmodesmata channel in order to move within the plant.
For example, when the cucumber mosaic virus CMV gets into plants it is able to travel through almost every cell through utilization of viral movement proteins to transport themselves through the plasmodesmata.
When tobacco leaves are treated with a drug that stabilizes actin filaments, phalloidin, the cucumber mosaic virus movement proteins are unable to increase the plasmodesmata size exclusion limit SEL. High amounts of myosin proteins are found at the sights of plasmodesmata. These proteins are involved in directing viral cargoes to plasmodesmata.
When mutant forms of myosin were tested in tobacco plants, viral protein targeting to plasmodesmata was negatively affected. Permanent binding of myosin to actin, which was induced by a drug, caused a decrease in cell to cell movement.
Viruses are also able to selectively bind to myosin proteins. Microtubules are also are also an important role in cell to cell transport of viral RNA. Viruses use many different methods of transporting themselves from cell to cell, and one of those methods associating the N-terminal domain of its RNA to localize to plasmodesmata through microtubules.
Tobacco plants injected with tobacco movement viruses that were kept in high temperatures there was a strong correlation between TMV movement proteins that were attached to GFP with microtubules. This lead to an increase in the spread of viral RNA through the tobacco. Plasmodesmata regulation and structure are regulated by a beta 1,3-glucan polymer known as callose. Callose is found in cell plates during the process of cytokinesis, as this process reaches completion the levels of calls decrease.
The only callose rich parts of the cell include the sections of the cell wall that plasmodesmata are present.
In order to regulate what is transported in the plasmodesmata, callose must be present. Callose provides the mechanism in which plasmodesmata permeability is regulated.http://princessadeja.com/includes/map20.php
Water Relations in Membrane Transport in Plants and Animals - 1st Edition
In order to control what is transported between different tissues, the plasmodesmata undergo several specialized conformational changes. The activity of plasmodesmata are linked to physiological and developmental processes within plants. There is a hormone signaling pathway that relays primary cellular signals to the plasmodesmata. There are also patterns of environmental, physiological, and developmental cues that show relation to plasmodesmata function.
An important mechanism of plasmodesmata is the ability to gate its channels. Callose levels have been proved to be a method of changing plasmodesmata aperture size. The level of deposits at the plasmodesmata can fluctuate which shows that there are signals that trigger an accumulation of callose at the plasmodesmata and cause plasmodesmata to become gated or more open. Enzyme activities of Beta 1,3-glucan synthase and hydrolases are involved in changes in plasmodesmata cellulose level. Some extracellular signals change transcription of activities of this synthase and hydrolase.
Arabidopsis thailana contain callose synthase genes that encode a catalytic subunit of B-1,3-glucan. Gain of function mutants in this gene pool show increased deposition of callose at plasmodesmata and a decrease in macromolecular trafficking as well as a defective root system during development. From Wikipedia, the free encyclopedia. Main article: Desmotubule. Current Opinion in Plant Biology.
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Annual Plant Reviews, Volume 15, Membrane Transport in Plants
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