It is a known fact that plants are firmly planted in the soil – they cannot flee from enemies which want to eat them. They are not helpless, however; they oppose their enemies with a large number of sometimes highly poisonous substances. But the plant itself – how can it protect itself from these poisons? This is what the Bochum plant physiologists Dr. Markus Piotrowski and colleagues have examined together with Prof. Birger L. Møller from the Royal Veterany and Agricultural University (KVL) of Copenhagen.
They found that the plant is able to decompose poisonous cyanogenous glycosides without producing any toxic substances. The nitrogen stored in these substances, which is indispensable for the plant, is recovered in the form of ammonium. The main role in this process is played by the enzyme nitrilase.
Poisonous Only When Injured
Many poisonous substances in plants are stored in it as non-toxic pre-stage products; the toxic substance is only set free when the plant is injured. The same is true for cyanogenous clycosides which are available as sugar compositions in separate chambers of plant cells (vacuoles). When the cell is injured, sugar is split off, and the formation of unstable hydroxynitriles starts, setting free the strong respiratory toxin, prussic acid – 50 to 200 mg will suffice to kill a man. Such cyanogenous clycosides can be found in large quantities in bitter almonds, manioc, which is a main food plant in Africa, and young millet plants. Every year, many people in Africa are acutely and chronically poisoned with prussic acid from manioc which was insufficiently processed.
Only Two Will Work Together
Higher plants continuously produce small quantities of prussic acid as a waste product of their own metabolism. At first, the plant will couple the prussic acid to the amino acid cysteine, resulting in the formation of the amino acid Beta-cyanoalanine; the latter is still toxic and is only converted to the amino acids asparagine and aspartic acid, which the plant can use, by action of the enzyme nitrilase. "We knew about this process," Markus Piotrowski states, "but we have encountered problems when examining nitrilases in grasses. The nitrilases in barley, rice, corn and millet were inactive in our tests. We knew, however, that these plants can also convert cyanoalanine." The solution of the riddle: All of these grasses have two nitrilases which will have to from a hetero-complex in order to interact to become active. "This phenomenon has never been described by anyone else before us," Piotrowski reports.
New Way of Recycling
The scientists discovered something else, too: The millet contained a third nitrilase. Wherever this latter is available in the hetero-complex, it can convert other substances, including 4-hydroxyphenylacetonitrile. Piotrowski explains: "Young millet plants store a large quantity of the cyanogenous glycoside dhurrin. If an insect bites the plant, prussic acid is set free. The older the plants are the more they decompose dhurrin themselves – not in the same way as when they are injured, however."
The discovery that nitrilases of the millet can also convert 4-hydroxyphenylacetonitrile, which is a potential decomposition product of the dhurrin, opens another path during which no prussic acid will be eliminated at all. The proof that dhurrin can actually be converted to 4-hydroxyphenylacetonitrile was then provided in Copenhagen. "It is obvious that older plants no longer need dhurrin in such an urgent way to protect themselves against being eaten," Piotrowski concludes.
The dhurrin has valuable nitrogen for the plant which it needs for its metabolism. The newly discovered decomposition process offers a chance to recover this nitrogen in the form of ammonium without previously having to eliminate prussic acid. The next step planned by the Bochum and Copenhagen scientists is to identify the enzyme initiating the endogenous decomposition of the cyanogenous glycosides. This knowledge could lead to the control of the forming and decomposition of such vegetable toxins.
The results of this study have been published in the Proceedings of the National Academy of Sciences.
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