Rhamnogalacturonan-II

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Rhamnogalacturonan II

Rhamnogalacturonan-II (RG-II) is a complex polysaccharide found in the primary cell wall of plants. It is one of the four main components of pectin, alongside homogalacturonan, rhamnogalacturonan-I, and substituted galacturonans. RG-II is notable for its highly conserved structure across different plant species, suggesting a critical role in plant physiology and cell wall architecture.

Structure

RG-II is distinguished by its unique and intricate structure, which consists of a galacturonic acid-rich backbone with four distinct side chains, labeled A, B, C, and D. These side chains are composed of more than 12 different types of sugars, including rare sugars such as apiose, kdo (2-keto-3-deoxyoctonate), and aceric acid (3-C-carboxy-5-deoxy-L-xylose). The complexity of RG-II's structure is further enhanced by the presence of borate diester cross-links that form between two RG-II molecules, creating a borate-diester-linked dimer. This dimerization is essential for the structural integrity of the plant cell wall.

Function

The primary function of RG-II is to contribute to the mechanical properties and porosity of the plant cell wall. The borate-diester cross-linking of RG-II molecules plays a crucial role in cell wall strengthening by connecting different RG-II molecules, thereby stabilizing the cell wall matrix. This cross-linking is believed to be important for plant growth, as it affects cell wall expansion and rigidity.

Additionally, RG-II has been implicated in several other plant physiological processes, including defense against pathogens. Its conserved structure across plant species suggests that it may have a fundamental role in plant biology that is yet to be fully understood.

Biosynthesis

The biosynthesis of RG-II is a complex process involving numerous glycosyltransferases that assemble the polysaccharide's backbone and side chains. The precise mechanisms and the enzymes involved in the biosynthesis of RG-II are still under investigation. However, it is known that the process occurs in the Golgi apparatus of plant cells, where polysaccharides are synthesized before being transported to the cell wall.

Research and Applications

Research on RG-II has focused on understanding its structure-function relationships, biosynthetic pathways, and its role in plant growth and development. There is also interest in exploring RG-II for various applications, including its potential use in foods as a dietary fiber, in pharmaceuticals as a delivery vehicle for drugs, and in agriculture to enhance plant resistance to environmental stresses and pathogens.

See Also

• Pectin
• Cell wall
• Polysaccharide
• Glycosyltransferase
• Plant physiology