Pectin structure and biosynthesis
Introduction
Pectin is the most structurally complex family of polysaccharides in nature, making up ∼35% of primary walls in dicots and non-graminaceous monocots, 2–10% of grass and other commelinoid primary walls, and up to 5% of walls in woody tissue [1, 2] (Department of Energy, Energy Efficiency and Renewable Energy, Biomass Program: http://www1.eere.energy.gov/biomass/feedstock_databases.html). Pectin is abundant in walls that surround growing and dividing cells, walls of cells in the soft parts of the plant, and in the middle lamella and cell corners. Pectin is also present in the junction zone between cells with secondary walls including xylem and fiber cells in woody tissue. Pectin is a component of all higher plant walls and of the walls of gymnosperms, pteridophytes, bryophytes and Chara, a charophycean algae believed to be the closest extant relative of land plants [3]. The correlation of increased amounts of the pectin RG-II in vascular plants, and its appearance as plants adapted to upright growth on land and developed lignified secondary walls [4], suggests that pectin has fundamental roles in both primary and secondary wall structure and function. An understanding of pectin structure and synthesis is crucial to understanding, and potentially modifying, wall structure so as to promote efficient production of biofuel from recalcitrant plant lignocellulosic biomass [5, 6].
Several reviews on pectin biosynthesis synthesis [2, 7, 8], plant wall biosynthesis [9, 10, 11, 12], and regulation of cell wall synthesis [13, 14] have recently been published. The goal of this paper, after a brief review of pectin function, is to summarize recent developments in our understanding of pectin structure and biosynthesis, with special attention to genes that encode pectin biosynthetic transferases. For a more detailed summary of the enzymology of pectin synthesis, readers are directed to four comprehensive reviews [6, 7, 8, 15].
Section snippets
Overview of pectin function
Multiple lines of evidence indicate a role for pectin in plant growth, development, morphogenesis, defense, cell–cell adhesion, wall structure, signaling, cell expansion, wall porosity, binding of ions, growth factors and enzymes, pollen tube growth, seed hydration, leaf abscission, and fruit development [2, 16]. Pectin is also used as a gelling and stabilizing agent in the food and cosmetic industries and has multiple positive effects on human health including lowering cholesterol and serum
Pectin structure
Pectins are a family of covalently linked galacturonic acid-rich plant cell wall polysaccharides [19]. Galacturonic acid comprises approximately 70% of pectin, and all the pectic polysaccharides contain galacturonic acid linked at the O-1 and the O-4 position.
Location of pectin synthesis
Autoradiographic pulse chase experiments, immunocytochemical localization studies with anti-pectin-specific antibodies, and pectin biosynthetic enzyme subcellullar fractionation and topology studies demonstrate that pectin is synthesized in the Golgi and transported to the wall in membrane vesicles. Pectin synthesis occurs simultaneously in numerous Golgi stacks in the cell in a process that appears to include a compartmentalization of specific biosynthetic enzymes to drive the construction of
Conclusions: challenges and future directions in studying pectin synthesis
The first step in understanding how the 67 or more pectin biosynthetic transferases work together to synthesize the pectic polysaccharides is to identify the genes encoding the biosynthetic enzymes. The identification of the four glycosyltransferases involved in the synthesis of HG, XGA, and RG-II is a start, but rapid progress in identifying the other pectin biosynthetic enzymes will require overcoming the following challenges: (1) remaining uncertainty regarding pectin structure and how and
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The following funding is gratefully acknowledged for supporting the writing of this review and the research that drives the author's work in this area: NRI, CSREES, USDA Award 2006-35318-17301; NSF MCB awards 0313509 and 0646109; and the BioEnergy Science Center, the U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research in the DOE Office of Science.
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