Callose

Callose (also callose ) is a polysaccharide, the plant cells is used as a universal sealing material in a variety of situations.

Construction

For callose is a vegetable polysaccharide. β -D-glucose monomers in this case are linked via β -1 → 3 glycosidic bonds to give a polymer or glucan, which can partly also have β -1 ,6- branches. This form of linkage results in a long, helical chain of the glucan, which is always present in a compact form.

Detection of callose in plant tissue can be done by staining with the dye aniline blue. This becomes embedded in the helical structure of callose and is under UV light by fluorescence detectable. The general chemical formula ( C6H10O5 ) n

Synthesis

Not the synthesis of the callose contrary to the original assumption by the rosette-shaped cellulose synthase, but also by a membrane-bound Callosesynthase ( CALS ) (EC 2.4.1.34 ). CALs catalyzes a so-called vectorial reaction, wherein the substrate ( UDP-glucose ) is bound to the cytoplasmic side of the plasma membrane and are deposited on the bonding to the outside of the cell membrane into the apoplast, between the membrane and the cell wall.

CALS is a family glucan synthase -like genes: coding which has a strong homology to involved in the Glucanbiosynthese of yeast genes ( engl. gluccan synthase like GSL ). The primary sequence of the catalytic subunits of CALs (2000 amino acids) and cellulose synthase (1000 amino acids) are a clear indication of the separately held synthesis of callose. In Arabidopsis thaliana previously could twelve such, the subunits encrypted GSL genes were identified that were CalS1 named to CalS12. In Arabidopsis, this twelve genes are distributed over the entire five existing chromosomes.

The CALS is a protein from multiple transmembrane domains, which are composed together into two regions. These two regions are connected to the cytoplasm through a hydrophilic loop, the catalytic subunit, eg CalS1 contains. The catalytic site can be divided into UDP -glucose -binding and glycosyltransferase domain. This hydrophilic loop associated also are still various other proteins which serve to support the synthesis and the regulation of the enzyme complex. A crucial role of sucrose synthase generated by cleavage of sucrose UDP -glucose. According to a hypothesis Furthermore you can find here also an annexin -like membrane protein, Rop protein, a proline- rich domain, a glycosylation site and two phosphorylation sites.

Functions

Callose serves plant cells as a universal sealing material and comes in both regular development and growth processes as well as in response to biotic and abiotic stress factors used. If the demand for cell callose can be rapidly synthesized in large quantities, on the other hand, a rapid drop is possible.

Typical functions of callose " in regular operation " of the plant are, for example, the appearance in the cell plate of dividing cells during cytokinesis. In pollen mother cells, the main component of the callose dar. also in growing pollen tubes sealed the advancing protoplast the abandoned portion of the tube with Callosepfropfen. Callose also closes the pores of the sieve plates of phloem dormant ( hibernation ) or defective phloem and as efficiently separates from the rest of the tissue. In plasmodesmata ( cell-cell connection), the callose represents a structural component that can regulate through a jacket around the neck region of the channel whose diameter and so restrict the intercellular transport.

The closure of sieve pores and in addition also the regulation of the size of plasmodesmata are functions that come into play even in stressful situations. So callose prevents inter alia the further spread of viruses through plasmodesmata. In attacks by fungal hyphae or wounds callose is extremely fast synthesized as the first physical barrier ( papillae ) or wound closure by the plant. Such papillae that coexist callose and lignin, other polysaccharides, phenolic compounds, RO intermediates and proteins are made of resistant plants almost immediately after recognition of the pathogen and deposited on the penetration points. They are so prevent the impending Pathogeneintritt or ensure at least a slowdown or demobilization of the pathogen. The besieged cell is helped in this way in any case to a postponement of their time to the initiation of further defensive measures such as the oxidative burst, or for the use of anti- pathogenic substances such as PR proteins ( engl. pathogen- related protein, Pathogenzellwand - degrading enzymes ) phytoalexins or offers. If there is yet to Pathogeneintritt or to infection of the cell, these seals off with the help of other Calloseablagerungen from the plasmodesmata. This foreclosure results in the accumulation of carbohydrates and about the so-called sugar sensing for activation of sink metabolism ( Forcella, Schoen & Weis, 2005; Schön 2005). The defense now set on cell metabolism starts a whole range of other measures, at the end of the programmed cell death (PCD: engl Programmed Cell Death. ) Can stand.

Regulation

The traversed in the regulation of CALS signaling pathways are still largely unknown. However, Ca2 ions is ascribed an important role. For example, it is used for resistant plants immediately after the beginning of the cell to the aperture in Pathogenattacke elicitor -dependent membrane channel, whereby influx of Ca2 ions, and in parallel by the H ions occurs. The Ca2 concentration increase stimulates presumably via the aforementioned annexin -like membrane protein that Callosesynthese.

A Ca2 - independent isoform of Callosesynthase appears to be involved during the formation of the cell plate. The activation here runs Expected a Phragmoplastin ROP interaction (ROP - protein to the hydrophilic loop).

Glycosylation ( glycosylation ), phosphorylation ( phosphorylation ), the influence of phytohormones (eg abscisic acid ) and the achievement of the optimum pH for the CALS addition be, for example, G- proteins, as discussed regulatory mechanisms.