Nodulation in Legumes

  • Janet I. Sprent
Royal Botanical Gardens: 2001. 146 pp. £27

The formation of nitrogen-fixing nodules on the roots of legumes (peas, beans and their relatives) is the basis of crop rotation, a mainstay of sustainable agricultural practice. These nodules are the result of an association between the legumes and Rhizobium and allied bacteria. This symbiosis is a wonder of evolution, and has for years been a target for molecular, genetic, physiological and population-level studies.

Many questions have been addressed through the study of model systems of bacteria and legumes. For example, how do bacteria and hosts recognize each other? How do they coordinate action so that the plant produces a completely new organ, the root nodule, which the bacteria invade and inhabit? Why do symbiotic bacteria express their enzymatic apparatus for nitrogen fixation only after developing into specialized forms within the nodule? And why do they fail to assimilate their own fixed nitrogen, instead exporting it to be assimilated by the plant?

And the mystery remains: why legumes? This question refracts into a spectrum of issues related to specificity and adaptation. Why are different legume species or genera nodulated by different (sometimes multiple) bacteria? Why do some legumes appear not to support nodulation? We assume that the plant gains some physiological advantage from the presence of the symbiosis, but how robust is this assumption? Do we know enough about the physiological constraints that limit plant productivity for diverse legumes?

The scope of this second set of questions is as large as the legume family itself (it is the third-largest family of flowering plants) and as wide as the extent of green Earth, for legumes inhabit the tropics and the Arctic, deserts and rainforests, beaches and mountains. Janet Sprent's Nodulation in Legumes is especially valuable because it reminds us that model systems represent only a small part of a big story: how legumes and their rhizobial symbionts work together in the real world.

Early chapters present a concise overview of diversity, physiology and mechanism. Legumes exist in widely varied environments, and their growth may be subject to varied constraints. During nodule development, bacteria and plant communicate through a set of signals, and new structures and new gene expression characterize both early and late stages of nodulation. The physiology of the nitrogen-fixing state involves careful control of (and often regulation by) the level and the flows of oxygen, nitrogen, carbohydrates or organic acids. These chapters are readable and compact, and do not generally refer to the fast-expanding primary literature on mechanisms of nodulation. Because this research area is dynamic, with conclusions and generalizations constantly being re-examined, students interested in going further should consult more specialized reviews.

A series of further chapters survey nodulation in each of three legume subfamilies: Caesalpinoiodeae, Mimosoideae and Papilionideae. These are especially valuable given the direction in which laboratory science is heading: complete genome-sequencing analyses have recently shown surprising diversity at the whole-genome level of various rhizobia. In parallel, plant genetics and genomics will soon make it possible to look in diverse legumes for symbiosis genes encoding receptors and response pathways. For the next generation of legume researchers, Sprent's book will provide a platform for the expansion of study from a few model systems to a wide world of bacteria and legume hosts.

Questions of evolution and diversity are addressed in the final chapter, using data from the fossil record, morphological analysis and molecular taxonomy, for example. It is extremely valuable to have so much work summarized in one place. Sprent speculates that ancestral legumes had to acquire two features in order to support symbiosis: the suppression of host defences and the pathways to build the nodule structure — a structure that shows considerable variation among members of the family. To these I would suggest a variation: legumes must have the active cellular machinery to support infection, whether or not the plant sustains enough cell divisions to form a nodule. Which prompts me to wonder whether the ancestral events for modern-day nodulation were based in infection, rather than morphogenesis per se, and whether these events might be found in extant legumes that show no visibly evident formation of nodules.

A second printing of the book would provide the opportunity to correct some minor typographical errors that turn up in the table of genera (for example, Daniellia is spelled in two ways, so it looks like two genera in separate parts of the book). But this is a minor issue for a valuable book that Rhizobium–legume researchers and plant-evolution scholars alike should have and should study. As well as reminding us how broad and diverse is the world of symbiosis, and beyond the valuable summaries of physiological and ecological studies, the author conveys a lifetime perspective that places these elements into a framework of knowns and unknowns, of questions central and ancillary. And through it all, every reader of this book will come away feeling the author's love for her subject. The caption to Figure 1.2 begins “Nodule hunting!” That exclamation mark says it all.