Plasmodesma
Plasmodesmata evolved independently in several lineages,[3] and species that have these structures include members of the Charophyceae, Charales, Coleochaetales and Phaeophyceae (which are all algae), as well as all embryophytes, better known as land plants.
This is due to the fact that as a cell wall expands, the abundance of the primary plasmodesmata decreases.
In order to further expand plasmodesmal density during cell wall growth secondary plasmodesmata are produced.
[12][failed verification] Plasmodesmata are approximately 50–60 nm in diameter at the midpoint and are constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule.
[13] The cytoplasmic sleeve is a fluid-filled space enclosed by the plasmalemma and is a continuous extension of the cytosol.
Smaller molecules (e.g. sugars and amino acids) and ions can easily pass through plasmodesmata by diffusion without the need for additional chemical energy.
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.
[10] 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.
[22] Also, increasing calcium concentrations in the cytoplasm, either by injection or by cold-induction, has been shown to constrict the opening of surrounding plasmodesmata and limit transport.
[25][26][27] Plasmodesmata link almost every cell within a plant, which can cause negative effects such as the spread of viruses.
Fluorescent tagging for co-expression in tobacco leaves showed that actin filaments are responsible for transporting viral movement proteins to the plasmodesmata.
[28] 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).
When mutant forms of myosin were tested in tobacco plants, viral protein targeting to plasmodesmata was negatively affected.
Enzyme activities of Beta 1,3-glucan synthase and hydrolases are involved in changes in plasmodesmata cellulose level.
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.
