Plant cell walls. From chemistry to biology

Several lifetimes’ knowledge, insight and love of the subject have gone into the production of this excellent book, three of the five authors being emeritus. In terms of content, the simple main title, Plant cell walls, says it all. The broad and even-handed treatment of the subject matter will provide a very valuable introduction to this field, leading the reader gently but authoritatively into the subject. It will be extremely useful for postgraduate students and postdocs beginning a project on plant cell walls, and it will be superb for established scientists in other fields planning an excursion into this burgeoning area of science; it is also certain to gain a place on the shelves of all currently practising cell wall enthusiasts. And it is already in use, at least at Edinburgh University, as recommended reading for advanced undergraduate courses. The book’s subtitle, From chemistry to biology, highlights the welcome foundation that this book has in the realm of chemical composition, polysaccharide structure and enzymology. All too many accounts of plant cell walls nowadays adopt a heavily genomic approach in which the discovery of the genes that control biological processes takes precedence over any real attempt to provide a mechanistic explanation of their action. Such explanation is dependent on an understanding of the underlying biological chemistry. The present book offers us such an understanding including, if required, a simple introduction to concepts such as glycosidic linkage, mass spectrometry and hydrolysis. The book’s subsection titles (termed ‘Concepts’) are declarative: with a single exception (it’s 3F5), each is a whole sentence with a full-stop. This means that leafing through the five pages of ‘Detailed Contents’ will give the reader a concise summary of all the main facts contained in the book. The topics covered include cell walls as we see them under the microscope, emphasising the differences between cell-types; an introduction to polysaccharide chemistry and how to study it, though without bench-top detail; a thorough descriptive account of all the chemical inmates of the cell wall, including cellulose, pectin, the various hemicelluloses, glycoproteins, polyesters, waxes and minerals; a detailed account of the structure and dynamics of the membrane systems that manufacture and transport cell-wall components; everything you ever wanted to know (and many of the remaining mysteries) about the enzymes that manufacture wall polymers, including lignin; an excellent attempt to put together our knowledge of the cross-links and tethers that convert an otherwise simple mixture of polymers into a wonderful, functional fabric (I was pleased to see sugar-ester cross-links making a tentative appearance); and a wide-ranging discussion of the proposed roles of cell walls and wall components in growth, development and plant–microbe interactions. Inevitably, in the days when commerce drives science, and with an eye on research funding opportunities, there is an entertaining chapter on plant cell walls as a renewable material resource, ranging from biofuels to the pectic thickening agents used in Staehelin’s Dijon mustard. Curiously, the simplest and most effective application of cell walls as biofuel – firewood – is scarcely mentioned. Naturally, the book favours the authors’ preconceptions, which not all interested parties will share; but a strength of having five authors is that few wacky hobby-horses have made it into the final draft. Nevertheless, there are a number of quibbles that I suspect will not be unique to me. Some of these are expressed in the declarative subsection titles. For example: ‘Secondary walls are composed of primary walls plus additional layers of polymers’: most people would dispute this and say that the primary wall is distinct from, not part of, the secondary wall. ‘Lignin is a hydrophobic polymer of secondary cell walls’: this is the truth but not the whole truth, lignin being found also (and developmentally earlier) in the primary walls of xylem cells. ‘Homogalacturonan, rhamnogalacturonan-I and RG-II are three pectic polysaccharides’: this is arguably true but only in the sense that pectin can be dismembered into three polysaccharides artificially; it is widely thought (as acknowledged elsewhere by Albersheim et al.) that in muro they are part of the same polysaccharide, so they are pectic domains rather than pectic polysaccharides. ‘Our knowledge of lignin composition is based on the chemical hydrolysis of wood’: this slightly misleading heading introduces a perfectly clear account of lignin analysis, but the key methods described are not hydrolytic. In a wide-ranging and ambitious book like this, there are inevitably a few inaccuracies, which could potentially be rectified in a reprint even before any second edition. In these days of democratic science, maybe the publisher could even provide a website for readers to record these. A few that I noticed are listed below, which I hope will be useful: The overview of xyloglucans on page 69 points out the presence of acetylated glucose residues in the Solanaceae but doesn’t mention their even greater prevalence in the Poaceae. In addition, poacean xyloglucans do possess a few fucose residues, not ‘none’ as stated. Strontium, on page 79, should be Sr, not St. The diagram on page 81 fails to mention that the galactose residue in the interesting boron-binding side-chain A of rhamnogalacturonan-II is l-, not d-galactose; metabolically, these enantiomers are hugely different. And, in Table 5·3, the same l-galactose residue has illegally wandered from side-chain A to side-chain B. On page 86, monoamine oxidase is stated to be ‘in’ the intercellular spaces, when it is of course only in the walls facing these air-spaces (since enzymes are not volatile!). The list of sugar-nucleotides on page 163 wrongly identifies GDP-galactose as GDP-d-Gal (it is GDP-l-Gal). On page 170, it is incorrectly stated that ‘retaining glycosyltransferases’ produce β-glycosidic bonds (from NDP-α-d-sugars); in fact they produce α-bonds, thus ‘retaining’ the anomerism of the donor sugar-nucleotide. On page 185, the diagram incorrectly shows UDP-fucose as precursor of xyloglucan; it should be GDP-fucose. The ‘Concepts’ are referred to by codes e.g. 5D4. This practice must have facilitated the writing, but is unhelpful to readers because the Concepts are cross-referenced within the book only by these codes (e.g. ‘see Concept 5D4’), not by page numbers. It is difficult to locate a Concept because the page headers give only the chapter number (chapter 5 in this example). Leafing through chapter 5, looking for 5D4, you will find five different subsection 4s and you won’t easily know whether you’re looking at 5A4, 5B4, … or 5E4 until you turn back to the start of your randomly chosen section and discover whether, if you were lucky, it is 5D1 and not 5E1 or some other 5#1. The alternative is to leaf through the Detailed Contents (pages xiii–xvii), where you will discover that subsection 5D4 is on page 195. In one frustrating case, on page 71, we are referred to Concept 3B3 – which, after some searching, turns out to be just ten lines further down the same page (but labelled only ‘3’)! When the book is reprinted, which it deserves to be, I hope that each page will get a header that includes the Concept code. Unfortunately, the book does not have a general list of abbreviations, which would have been welcome. And it could be wished that xyloglucan was either written out in full or, if necessary, abbreviated to XyG – thus avoiding the ambiguous ‘XG’, which has a well-established usage in xyloglucan chemistry as the abbreviation for a specific trisaccharide. The book is dated in the front matter as ‘2011’, but I bought a copy in June 2010 so it is not quite as abreast of this fast-moving subject as it might seem. Nevertheless it is a classic and, notwithstanding the few errata and presentational imperfections, this is an outstandingly useful new book, which is certain to become the standard work on the subject at this level.