London (4.1.10) – It is very characteristic of biological systems that actions sooner or later tend to generate effective reactions.

Most people know about antibiotic-resistant pathogenic microorganisms which can be such a problem in medicine. The antibiotics themselves, or at least the original versions, were “natural products” in the sense that they have been made for aeons by soil microbes as defence and aids to competition in the microbial world, particularly the soil. Their concentrations there were low and, while some resistance developed, a balance was developed between resistance and susceptibility as between the organisms producing the antibiotics and those affected by them.

Once large quantities began to be used in medicine, the balance changed. Microbial populations in infections can be very large, with millions of individual organisms present. A few of them in the offending populations which by chance were already resistant, or which acquired such resistance by genetic exchange from other species and varieties, became more and more of a problem in medical practice particularly when antibiotics were over prescribed, the preparations themselves were suspect or of poor quality, or the patients failed to complete their courses of treatment. When the susceptible organisms were killed off, the resistants had the field to themselves and flourish accordingly. In addition to encouraging proper use of antibiotics, one response of the pharmaceutical industry has been to develop modifications of the antibiotics concerned in order to restore their effectiveness. But, with time, a new cycle of resistance tends to develop.

Something similar is likely to happen in agriculture where pesticides (weed-killers, insecticides, fungicides, etc) are widely used to control pests. It will probably not happen so rapidly as with pathogenic microbes because the numbers of individual plants is much lower and their rate of reproduction very much slower. But happen it probably will and the response of agriculturalists and their suppliers is to ring the changes on the crops for cultivation in particular fields so as to change the opportunities for particular weeds and pests to flourish, and endlessly to develop new pesticides to deal with new resistances.

One of the most popular and effective weed-killers in recent years has been glyphosate, relatively non-toxic to animals and non-persistent in the soil. So widespread has been its use that inevitably resistances have developed (1). The more glyphosate used, the more likely resistant plants will show up and, as the weed killer continues to be used, the resistant plant will multiply unencumbered by their susceptible relatives – just like resistant microbes in the presence of an antibiotic. The weed killer itself does not cause the resistance mutations; most likely in the wild the mutations occur spontaneously but rarely and then becomes more and more prevalent with the weed population increasingly resistant to the weed killer.

The molecular mechanism of resistance is the generation of a mutant version of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), the normally susceptible enzyme, which is now not affected by glyphosate.

However, a new study has uncovered another way in which resistance can arise. Amaranthus palmeri (Palmer's amaranth, Palmer amaranth, Palmer's pigweed, and carelessweed) is native to most of the southern half of North America. Studies of resistant plants in Georgia have shown that EPSPS remains sensitive to glyphosate but that resistance has arisen via gene multiplication: the genomes of the resistant plants carried 5 - 160 times more copies of the EPSPS gene than those of susceptible plants (2).

As the authors of the paper point out, this occurrence of gene amplification as a herbicide resistance mechanism in a naturally occurring weed population is particularly significant because it could threaten the sustainable use of glyphosate-resistant crop technology.

How serious a threat this will turn out to be remains to be seen. It certainly illustrates once more the versatility of biological systems in responding to constraints.

Sources:

C. Boerboom and M. Owen (December 2006). Facts about glyphosate-resistant weeds (http://www.ces.purdue.edu/extmedia/GWC/GWC-1.pdf)

2. Todd A. Gaines, Wenli Zhang, Dafu Wang, Bekir Bukun, Stephen T. Chisholm, Dale L. Shaner, Scott J.. Tranel, A. Stanley Culpepper, Timothy L. Grey, Theodore M. Webster, William K. Vencill, R. Douglas Sammons, Jiming Jiang, Christopher Preston, Jan E. Leach, and Philip West (29.10.09). Gene amplification confers glyphosate resistance in Amaranthus palmeri. Proceedings of the National Academy of Sciences of the US (http://www.pnas.org/content/early/2009/12/10/0906649107.abstract) The complete paper may be downloaded from http://www.pnas.org/content/early/2009/12/10/0906649107.full.pdf+html)

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