Annals of Botany 94/3, © Annals of Botany Company 2004; all rights reserved
Advances in pectin and pectinase research.
Voragen F, Schols H and Visser R. eds. 2003. The Netherlands: Kluwer Academic Publishers. 
145 (hardback). 491 pp.
Two indisputable features distinguish plants from animals – one is the chloroplast, the other the cell wall. This volume is directed to the newest information we have on the cell wall, the major polysaccharide component, pectin, and the ways that pectins can be changed.
A summary of 36 papers presented at the Second International Symposium on Pectins and Pectinases (2001), this book describes aspects of synthesis, chemical constitution, properties and immunological identification of pectins with the molecular genetics, structure and function of the pectinases that modify or degrade the pectins. It provides the reader with comprehensive state-of-the-art views of what is without doubt one of the fastest developing fields in plant developmental regulation; one that embraces the cohesion and dissociation of plant cells and the dynamics of the ever-changing molecular organization of the wall itself.
We learn the molecular requirements for successful pollen adherence and growth upon the stigma and within the style, with the interaction of both specific protein and polysaccharide components for eventual fertilization. Several papers describe pectin ‘hairy regions’, the highly branched rhamnogalacturans that carry galactan and arabinan side chains. Their in vitro degradation by fungal pectinases or chemical treatment has led to new findings on the likely covalent linkages and wall pore sizes that exist in vivo. The partial purification of a membrane-bound enzyme that catalyses transfer of galacturonic acid to the wall pectin polymer has been achieved successfully in mung bean hypocotyls and a role for borate availability is clearly demonstrated for the cross-linking of primary cell wall pectin, rhamnogalacturanan II. The biological effects of modifying these wall polymers by transformation, mutagenesis, or by the altered expression of pectinases are very clearly reviewed.
The level and positions of methyl esterification add another dimension to the complexity and properties of pectins. Not only is texture of fruits and vegetables so determined, so also is water holding capacity of the walls and the enzymic fragmentation patterns that can occur in vivo and in vitro. As with many other studies in plant biology, we really need to know precise details for specific cell types to see pectin relationships in proper context, but we are still a way from achieving this goal.
Such is the case in the comparisons of lime and orange pectin polymers extracted from fruit albedo. The hemicellosic comparisons of unripe and ripe fruit and the mechanical properties of young and mature pea cotyledons tell a complicated story. Here, pectin side chain changes taking place during organ maturation require further study. Already, however, much has been achieved by the use of monoclonal antibodies in elucidating the deposition of the many oligosaccharide domains within cell walls, and this work too is very comprehensively reviewed. Other analyses show us that the pectic components from parenchyma cell walls from one species differ significantly from that of others. The pectins of any cell are therefore highly individual and diagnostic of both the physiological and the genetic state.
In the molecular genetics and biosynthesis of the different pectin degrading enzymes of plant pathogens we have useful chapters devoted to their action. Aspergillus niger has an arsenal of over 21 genes encoding different pectinases. We have a summary of the difficulties and successes in using galacturonic acid oligomers for the in vitro synthesis of pectic fragments. We have kinetics for the absolute requirement for Ca2+ for lyases in vivo and a pinpointing of the arginine subsite for active endo-polygalacturonase. The anti-sensing of pectin methylesterase in flax and records of the expression in the different parts of transgenic plants, including callus cultures, is an intriguing picture of developmental and tissue-specific responses.
Pectins are a problem for all protein and secondary product extractions, so the upregulation of a xylogalacturonan-degrading enzyme is a valuable approach to solving incomplete pectin removal. Chapters on polygalacturonases, pectin methylesterases and their protein inhibitors, the conserved sequence similarities of rhamnogalacturonan acetyl-esterases and the single and synergistic action of bacterial and fungal esterases upon pectins in plant extracts provide a critical overview of the extreme complexities inherent in the saccharide components of the protective plant cell wall and the many endogenous or pathogenic enzymes to which it is constantly subjected.
Not only are these studies important for understanding the molecular control of survival of the growing plant, they are equally applicable to best storage of plant material and the commercial derivatization of pectins into other products from harvested plant parts. Chapters showing how molecular composition of pectins can determine the texture and acceptability of a final product bring home the tremendous importance of the investment that any plant makes in the distribution of fixed carbon to its cell wall pectins.
This volume is a gem. Contributors and editors should be congratulated on the careful and uniform presentation, the excellent texts and their wealth of up-to-date information and references. My only disappointment is the lack of an index – a great pity!
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