Rengel, Z. (ed.) Handbook of plant growth. pH as the master variable

Abstract

Protons are ubiquitous in the aqueous environment that provides the stage for life. Moreover, the proton is a key player in a multitude of essential processes; and while its signature, the pH of an aqueous solution, is readily measured in a wide variety of circumstances and locations, this in itself does not lead immediately to a full appreciation of its mechanistic importance. So the proton is a legitimate target for the investigations of scientists from many disciplines, and this book, which aims to provide a unified view of the role of pH in plant growth, draws on the knowledge that has accumulated in areas as diverse in scope and outlook as cell physiology, ecology and forestry. In an ideal world, one or two enthusiasts would take a synoptic view and write an authoritative account of the proton in biology. This would be a challenging assignment, but it is probably the only way to iron out the quirks of interpretation that accumulate as disciplines and sub‐disciplines evolve, and thus to produce a truly coherent account. In the absence of volunteers, the next best option is an edited volume, still written by enthusiasts but with only an editor to impose the coherence that characterizes a successful book. That contributors and editors can still be found for volumes of this kind is remarkable given the proliferation of review articles and special issues in mainstream research journals, but in this case the effort has been worthwhile and the result is an informative and accessible account of the multifaceted role of the proton in plant growth. The scope of the book is deliberately wide, ranging from chapters on plant organelles to chapters that describe the interactions of plants with their biotic and abiotic environments. The unvarying theme is the involvement of pH, and the fluxes of protons, in the processes that underpin plant growth, and the 16 chapters can be roughly divided into three groups. The first group focuses on transmembrane H+ fluxes, and includes detailed molecular descriptions of the processes that occur at the plasma membrane, the tonoplast, the thylakoid membrane and the inner mitochondrial membrane. The second section considers the impact of pH on a wide range of subcellular, cellular and whole plant processes, including such topics as pH regulation and the utilization of pH changes as signals. The third and final group of chapters describes the involvement of pH and H+ fluxes in the biotic and abiotic interactions that influence the growth of plants rooted in the soil, and includes chapters on the impact of soil pH on inorganic nutrient acquisition, plant–microbial symbioses and the distribution of plant species. The overall result is a comprehensive and wide‐ranging account of the involvement of the proton in the fundamental processes that permit plant growth. Most of the chapters are well written and informative, with substantial reference lists and an appropriate coverage of the chosen topic. It is reassuring to note from the preface that the chapters were ‘reviewed according to the standards of high impact international journals’, and certainly the authors and editor are to be congratulated on producing a book of high quality. Many of the authors have written on similar topics elsewhere, so excellence might have been expected, but there is still the question of whether the book succeeds in integrating the topics into a seamless account. Perhaps inevitably, because this could only really be achieved with a greatly reduced number of authors, the book is less successful in this respect. Thus there is a certain amount of overlap between the various chapters, and while this is by no means excessive, it does generate some slight inconsistencies in presentation. Of course, it is interesting to compare the interpretations that different authors put on the same piece of work, but for a number of recurring themes, for example pH regulation and pH signalling, it would have been useful if the material from different chapters could have been brought together more effectively. Perhaps a more fundamental difficulty is the concept of pH as a ‘master variable’. This term is never fully explained, and perhaps it is meant to convey no more than that the proton, and hence pH, is intimately involved in many of the processes that are crucial for plant growth. However, the term is rather disconcerting for two reasons. First, as mentioned by several authors, the pH of a solution is actually determined by the total composition of the solution, making pH a dependent variable. So while pH undoubtedly influences many processes, the pH itself is determined by a host of other events, and it is at least arguable that these other processes are more fundamental in determining the properties of the system. Secondly, the notion of pH as a master variable is at odds with the emerging view that biological processes are best analysed on a system‐wide basis, and without preconceptions as to a dominant role for one or more components. This is frustrating if a particular component, such as the proton, can be readily measured, and especially so when there are instances where pH seems to be the dominant determinant of the system. Yet caution is required in elevating pH to the status of a master variable and, as an example, it may be noted that there appear to have been no quantitative measurements of the response coefficients that would establish the role of pH in controlling the fluxes through the metabolic pathways that are generally implicated in cytoplasmic pH regulation. But back to the point. Handbook of plant growth: pH as the master variable is an excellent compilation of material that relates to the intricate relationship between the proton and plant growth. Few readers will have the time or inclination to read the whole book, but many would benefit from consulting it, and this is a compelling argument for placing it on the shelves of plant biology libraries.