Plants Can Fix Bad Genes

Two of these threads, Bob, you and rosborne. Fascinating stuff.... can't wait to see what the spin-off looks like.

Hi littlek:

It's interesting that some plants can morph to protect itself.

As the late Gottfried S. Fraenkel suggested, it is not a plant's primary metabolites (the substances it synthesizes that are essential for its growth and reproduction) that make it suitable or unsuitable as a source of food. Rather, the plant's suitability depends to a large degree on its secondary metabolites: metabolic compounds that are not involved in the common processes of life and that vary from plant to plant, helping to determine each plant's unique characteristics.

In 1971 the late Robert H. Whittaker and Paul P. Feeny of Cornell University added a new level of precision to Fraenkel's concept. They suggested that secondary metabolites produced by an individual of one species and able to affect the growth, health, population biology or behavior of another species should be called allelochemics. (Chemical ecologists now use the terms allelochemic and allelochemical interchangeably; I much prefer the latter term.) Among the many types of allelochemicals are attractants, repellents, allergenics and toxins. In this presentation, I shall discuss the allelochemicals employed by certain plants to defend themselves from predation by insects and various other herbivores.

The customary way to determine the defensive capacity of a higher plant's allelochemicals is to demonstrate their toxicity toward one or more of a variety of insects that have come to be accepted as standard reference species in evaluating biological toxicity. The usual approach is to incorporate the natural allelochemicals into an artificial diet that would normally sustain the insect. J. M. Erickson, then a student of Feeny's, and Feeny, working on the black swallowtail butterfly, Papilio polyxenes, modified this method to provide a more natural approach. Instead of creating an artificial diet for their insects, they introduced a plant allelochemical into a plant that is part of the butterflies' natural diet.

Adult P. polyxenes avoid plants of the group Cruciferae (the mustards) which produce such allelochemicals as sinigrin, a compound that contains allylisothiocyanate, a toxic constituent. On the other hand, the butterflies forage avidly among the Umbelliferae, which include such plants as celery. Erickson and Feeny reared P. polyxenes larvae on a diet of celery leaves that had been induced to take up sinigrin. The larvae fed, and their growth was markedly inhibited. Celery containing a level of sinigrin equivalent to the level found in cruciferous vegetation was lethal to all the tested larvae. These experiments demonstrated that toxic allelochemicals could render an otherwise suitable host plant unacceptable to an insect pest.

David A. Jones and his colleagues at the University of Hull developed another experimental verification of the effectiveness of toxic allelochemicals. They studied bird's-foot trefoil (Lotus) and white clover (Trifolium), species that are capable of producing cyanogenic glycosides, compounds made of sugars bound to cyanide complexes, and storing them in their leaves. If two particular enzymes are present when the plant's leaves are damaged, the cyanogenic glycosides are broken down to release the cyanide complex, from which free cyanide is eventually liberated. Bird's-foot trefoil and white clover are "polymorphic" for cyanogenesis: only some individual plants can produce both the cyanogenic glycosides and the enzymes required to liberate cyanide. Hence not all plants of each species can defend themselves by means of cyanogenic glycosides. Jones exploited this peculiar property to determine how effective a stratagem cyanogenesis is for the plant population within his region of study. He examined a map, published in 1954 by Hunor Daday, then at the Welsh Plant Breeding Station in Aberystwyth, showing the geographic distribution of plants able to synthesize both cyanogenic glycosides and the appropriate enzymes and thus able to produce free cyanide.

http://www.uky.edu/~garose/link100.htm