Gene havoc, plant form

Reaching for the Sun - The Evolutionary Biology of Plants - The Nature of Disease in Plants

June 12, 1998

The Nature of Disease in Plants, dedicated to the memories of J. C. Walker and Armin Brown, may now serve as a memorial to Robert Scheffer himself, who died before his book was published. It is biased, appropriately, towards the roles of toxins, on which he became probably the world authority.

A section on the crown gall bacterium, Agrobacterium tumefaciens, and its mode of pathogenicity is valuable to lay people as well as biological students keen to appreciate progress in the discovery of genetic transformation and hence the gateway to genetic engineering. Scheffer's succinct ten-point history is helpful. What is remarkable is that his crown gall discussion is achieved without actually mentioning reverse transcription. This special genetic trick, not only of crown gall bacteria but also of HIV and similar viruses, enables them to cause the havoc that they do through possessing "infective genes" rather than being more conventional infective microbes. The many aspects of plant pathology covered in this book are well referenced, but there are perhaps rather too few literature citations before Scheffer's own teaching career begins. As an alumnus of Cambridge University, I regret, for example, the lack of acknowledgement of John Rishbeth's classic work in our Norfolk forests on Peniophora, which established the biological control of pine root rot.

There is a theme running all through this book relating severity of crop diseases to the methods of modern agriculture. "There are many more ... diseases that increased with changes in agricultural practice, but the ones discussed are sufficient to make the point." And Scheffer provides some well-documented case studies to inform those more inclined to blow whistles or trumpets in defence of planet earth than is considered proper for sound academics. I fear, though, that the book is a little too dry to be read in quarters where its points made might best be taken and put to work. I hope otherwise. If this book were to have a paperback edition and at an affordable price and perhaps a more engaging title it would be a valuable guide for students in agricultural and horticultural colleges. It may be useful too for the development consulting fraternity, who sometimes require a quick handy reference work of diseases.

Reminders about the dangers of long-distance travel by pathogens may also be timely for those inclined to "germplasmania" who suppose that they may advance global knowledge, save the world's heritage or rapidly improve their own or other country's agriculture or horticulture by shipping around seedlots which quite likely contain seedborne pathogens alien to the countries where they may be grown and studied. There has been too much of this in recent decades. It is, however, for "the crown gall story" that I would most commend this book to a wider readership.

Reaching for the Sun: How Plants Work is a very readable slim book about plant physiology. It exemplifies the idea that the popular can also be educational but also illustrates pitfalls of semi-scholarly publication. It should be an easy and quite exciting text to enthuse keen teenagers about plants. It could even become a bestseller for sixth forms and last for many years with a fair wind. I am reminded a little of MacGregor Skene's book The Biology of Flowering Plants, which was first published in 1924. If the subject matter of that book was included in the national curriculum we might start producing more enthusiastic professional plant biologists.

I expect each chapter originated as a lecture and, inevitably, this episodic format makes for some repetition. A little repetition is no bad thing. But what does irk a little is the book's tendency to acknowledge teleology as an acceptable form of scientific explanation, as if plants somehow had minds and purposes. This seems commonplace in biological writings now, though I still squirm when I meet it. For example: "What is doubtful is whether the production of any one of these chemicals (terpenes) is a deliberate strategy developed by a plant specifically for the purpose of preventing competition." Even to imply that any plant might have such a strategy and purpose seems patently absurd.

I do like the way in which John King goes back to the historical scientific roots, sometimes unearthing new ones too; to Lavoisier and Priestley and Stephen Hales for example. The quoting of secondary sources only is, however,a little odd. For example Hales, S. is referenced with the date as 1964, instead of 17 when Hales actually published his An Account of Some Statical Experiments on the Sap of Vegetables, and Priestley, J.'s Observations on Different Kinds of Air is given a 1955 date only via Gabriel and Fogel's Great Experiments in Biology. One wonders whether such possibly editorial treatment may risk obscuring the desirable long-term historical perspective of the book. The Pole, Theophil Ciesielski, whose work on root geotropism dated to "the 1870s" was news to me, gets no supporting reference at all, nor does Jacques d'Ortour de Mairan, who is credited with important observations and simple experiments on sleep movements of plant leaves. These omissions are irritating even in a popular book, let alone in a scholarly one.

I particularly liked the chapter titles, such as "Fragrance and flavour", "Chemical warfare" and "Not in my backyard", which are so much more encouraging to the reader than the "Effects of secondary plant metabolites" common to more conservative botany books. As the foreword and the back cover proclaim: "Plants live at a different pace from animals but they are just as fascinating." Thank you John King for encouraging more people to recognise this. I hope there will be further editions.

The Evolutionary Biology of Plants, the Cornell University botanist Karl Niklas's new tome, is a book about botany for botanists, aspiring botanists and serious evolutionists. It should become widely used by university students, particularly where pure botany is still taught as a major discipline. It reminds me of the almost total intellectual gulf at Cambridge University during the 1950s that existed between two adjacent buildings, respectively devoted to botany and agriculture - the houses, as it were, of the Montagues and Capulets. To write such a major book about plants with an almost studied disregard for the evolutionary biology that ordinary folk would regard as essential, suggests that the tradition of purity continues today in Ivy League America.

This is, however, an interesting and thought-promoting book in many ways, because it brings together aspects of descriptive palaeobotany and of modern developmental genetics that throw light on the possible major changes in form and function, ie macro-evolutionary leaps that might occur by "genomic parasitism". Referring to transposons, retro-transposons and mobile introns, Niklas writes: "A cautious analogy can be drawn between mobile elements and 'genetic parasites' because these elements replicate using the genomic machinery of their 'host' cell without conferring any immediate or obvious benefit ... it has been suggested that mobile elements can support speciation, and in some circumstances, trans-specific evolution."

The most recent reference here is dated 1996, which is a good indication of how well a fine scholar of plant evolution writing a text going back to descriptions of early plants can also keep pace with advances in genetic understanding.

The chapter "Tempos and patterns" is the most important in the book. Here Niklas brings a sharp critical faculty to bear on the problems of estimating and comparing rates of molecular evolution, especially that between plants and animals and in relation to supposed divergence times. He addresses the theoretical problems of Zuckerkandle and Pauling's molecular evolutionary clock. Niklas points out that the concept of the molecular clock hangs on an assumption that molecular change is steady over chronological time and is indifferent to the generation time of organisms as well as to the environment of each organism.

These assumptions, based on plant considerations he calls "somewhat doubtful" but, between the lines, I read his "somewhat" as a distinct understatement. I feel unqualified to act as an arbiter. However, if after careful thought by others, Niklas's doubts are as well founded as seems possible, then this book may be even more important than at first appears. I was less enthralled by Niklas's theoretical discussions of fitness by taking "walks" through different ecological landscapes.

The book, however, is full of gems. Niklas thinks, in relation to phylotaxy, that it is "purely utilitarian" that the Fibonacci series not only converges on the golden mean 0.38197, which is theoretically interesting, but that "the numerator of each fraction equals the number of circuits around a stem separating superimposed leaves while the denominator equals the number of leaves separating these superimposed leaves".

Colin Leakey is an applied biologist working mainly on agricultural, food and industrial feedstock aspects of pulses and leguminous oilseeds, and is an independent consultant.

Reaching for the Sun: How Plants Work

Author - John King
ISBN - 0 521 55148X and 58738 7
Publisher - Cambridge University Press
Price - £30.00 and £9.95
Pages - 232

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