Plant Science 157 (2000) 265 – 266
Book review
www.elsevier.com/locate/plantsci
Metabolism of Agrochemicals in Plants, Edited by Terry Roberts, Wiley, Chichester, 2000, 300 pp. ISBN 0-471-80150-X; £.95.00 Hardback
The metabolism of pesticides in crops has for many years been primarily the interest of registra-tion regulatory affairs groups in industry. Their goal was to show that a pesticide or any toxic product had disappeared from a plant, or at least the parts of a plant harvested. They were totally disinterested in how a pesticide was metabolized to the final products of regulatory significance, as transient intermediates, long gone before harvest were of no regulatory interest. The metabolism of pesticides in weeds is not part of regulatory re-quirements, and was thus rarely studied. The typi-cal industry metabolism group would take its first time point weeks after pesticide application, long after crop/weed selectivity was manifested. This hampered obtaining any understanding of herbi-cide selectivity between crop and weed, or metabolic resistance in weeds. Such understanding can lead to development of synergists and exten-ders to keep herbicides active in killing weeds for longer periods, and for protectants (sometimes called antidotes or safeners) to confer selectivity to crops. Most government or academic researchers trying to study initial metabolic reactions have been exiled into studying toxicologically-insignifi-cant parts per billion residues in the environment. Some went to industry at the time when industry was awakening, desiring to understand initial metabolism, for commercial development reasons that had nothing to do with regulatory affairs. Thus, a considerable amount of basic metabolic information has been gathered by industry and is being shared with us in this thin volume. Even then, the title is somewhat of a misnomer. It might well have been ‘Pesticide Metabolism in Crops’ as metabolism in non-economic plants was still not recognized as important. That was before some
weeds evolved resistance to herbicides by metabo-lizing herbicides, and a need arose for synergists. The book is timely, it is a long time since the last edition of D.C. Kearney and D.D. Kaufman [Herbicides: Chemistry, Degradation and Mode of Action, Marcel Dekker, 1988], and that series dealt only with the major group of pesticides, i.e. herbicides. Indeed, prior to the advent of systemic fungicides and insecticides there was little metabolism to study, as many did not pass the plant cuticle.
The book is a mixed bag, some chapters sopho-moric, some catalogs and some superb critical essays. The book is somewhat dated, a perusal of the reference dates suggests that it languished somewhere since mid 1997 with some updating in proof stage. Still, the best chapters are reason enough to have the book.
The first four-page chapter deals with regulatory issues, an area where one should be very short or very long. Unfortunately, it bears no reference section so we cannot tell where to find the longer treatments.
The second chapter is about experimental ap-proaches. This chapter considers ‘metabolism’ as the study of residuals for regulatory affairs, which is not what most of the book is about, thankfully. The words ‘intermediates, kinetics, time course’ are not to be found. Metabolism refers to ra-dioisotopes, except in a small paragraph on stable isotope and whole tissue NMR. The chapter is a good historical review of ‘how it has been done’ for regulation. There is a short section about using hydroponics or tissue pieces (called in vivo, even though the greenhouse and field studies are also in vivo). The short section on in vitro studies does not state their most important feature; whereas in the field or greenhouse grown plant one cannot be sure whether a pesticide is metabolized by the plant or by adhering microorganisms, sterile tissue cultures are microbe free. This chapter is well documented.
Book re6iew 266
The next two chapters are titled ‘primary metabolism’ and ‘secondary metabolism’, respec-tively, meaning the initial product and subsequent products. This is confusing as many reactions discussed in the secondary chapter are primary (e.g. glutathione conjugation, N glycosylation). The ‘primary’ chapter should have been entitled ‘pesticide cleavage’ and the ‘secondary’ chapter entitled ‘conjugations of pesticides’. The ‘primary’ chapter is full of information but is in the eyes of this physiologist didactically unpleasing. Instead of discussing metabolism by reaction, the authors discuss each chemical group of pesticide sepa-rately. Didactically this makes little sense as the same reactions are discussed with each group, and especially in herbicides, where one group member can be degraded by P450s in wheat and an analog by a glutathione transferase in a legume. Still, the ‘primary’ chapter is a very useful catalog for look-ing items up, but not for learnlook-ing concepts.
The chapter on secondary metabolism is a clas-sic, worth the price of the book. The ‘secondary’ chapter is organized by reaction (sugar conjuga-tions, amino acid (including with tripeptides) con-jugations, and it clearly explains the reactions, their kinetics and the various enzymes involved. It is written for someone who wants to learn and understand (where the previous chapter is for one who just wants to know). It discusses whole metabolic pathways and gives sufficient examples to suffice one who wants to know what happens to specific pesticides. Despite the ‘plant’ in the book title, the authors sneak in a section on the toxico-logical consequences of the arrival of plant conju-gates to the mammalian gut. I would argue with the authors’ contention that ‘glutathione conjuga-tion catalyzes the majority of herbicide detoxifica-tions in plants’ (p. 134). The majority of herbicide used worldwide is on wheat and rice, where other detoxification mechanisms predominate. Despite this maize/soy orientation to the world, the chap-ter is excellent.
The chapter on bound residues justifiably deals heavily with definitions and technologies that illus-trate that a residue is bound. The definitions re-quire that the pesticide is not considered ‘bound’ if it is fully broken down and recycled. It does not deal with methodologies that distinguish this;
clearly one cannot ascertain this with isotope la-belling. As many of these residues are covalently incorporated into lignin (and bound), and many others degraded and recycled into cellulose (not bound) there are some intractable problems here. The following two chapters, on comparative metabolism between plants and animals, and metabolism as a basis of selectivity are among the ‘classic’ chapters that will be of value for many years in the future. Had the latter chapter been written more recently, it would also have a section on metabolism as a cause of evolved resistance in weeds. This type of evolved metabolic resistance has become a major problem in grass weeds of wheat and rice, where the weeds have become biochemical mimics of the crops, and thus are no longer controlled.
The chapter on safeners and synergists is quite outdated, though it contains an adequate discus-sion of the safener side. The use of synergists to overcome propanil resistance in Echinochloa spe-cies to selectively prevent the evolved resistance is not discussed, despite the spate of pre-1998 papers that cover this exciting research. There is also no discussion of the papers dealing with synergizing oxidant-generating herbicides, possibly academic, as none reached commercialization.
A book like this should have more than a six-page index. The index should have been proofed by a scientist understanding indices; possi-bly entries for ‘plant 450s’ (meaning cytochrome P450s) and separate entries to different pages for sulfation and sulphation, etc. should not have crept into this micro-index.
This book should be on the bookshelf of anyone interested in the mechanisms of pesticide metabolism in plants. All but the (literally) periph-eral chapters have the ‘meat’ that should be in such a book, and have it in a well-presented manner. Many books with chapters written by industrial scientists deal only with compounds of their own companies. Here the authors have been universal and balanced. Bravo.
Jonathan Gressel
Department of Plant Sciences,
Weizmann Institute of Science, 76100 Reho6ot,
Israel
