“Often there is not even a thin line between a food plant, a toxic plant and a medicine”
Marjorie Grant Whiting, 1962
“Its all a matter of dosage” they say.
Lets take an example from a common medicinal compound, like codeine. Codeine is one of the opioid alkaloids produced by the poppy plant and can be detected in the human blood after eating a poppy seed bagel. Its magical powers (activating opioid receptors) justify the extensive use of codeine as a painkiller, analgesic, antidepressive, sedative and cough relieve. However at very high doses codeine will kill you by respiratory depression. The importance of dosage can be observed in many relationships between humans and plants. While the dosage issue might come off as common knowledge for many, we drinkers like our plant brews and distillates in the rage of 4 – 40 % alcohol, the parallels to other plant-animal interactions might not be so obvious.
After studying a beetle that loves cycad toxins, we might have found parallels to the human relationship to medicinal/drug plants. The beetles seek out the cycad plant when the new foliage is being produced and voraciously feed and mate on the plant. They actually sequester the cycads toxins as means of defense and upon threat they will expose a drop of hemolymph from the leg-joints. The tiny drop of hemolymph contains high concentrations of the plant toxins. This surely shows that the beetle is a cycad specialist and can deal with the plant’s toxicity. But when we looked at the relationship a bit closer we found that the insect does not feed on the leaves with highest concentrations of toxins as we predicted. They rather choose a lower range of dosage from which they take their fix. Our data suggest that the high concentrations are still protecting the youngest leaves even from these cycad-loving beetles. Just as in the human-plant interactions, the beetle benefits from a particular chemical compound present in the plant but only at a certain dosage.
The dosage dependent manner of plant-animal relationships is probably based on the enzymatic capacity of the animal in question. Intoxication arises from saturation of the detox capacity of the herbivore.
So why doesn't the plant produce more of these toxins if they are so effective? Well, no one really knows, but it could be due to the cost of production or a matter of autotoxicity. The plant cells are also vulnerable to the deleterious effects of these toxins if they are not properly controlled.
And why doesn't the insect increase its enzymatic capacity to be able to feed on the most toxic leaves? Maybe there is no need, we have not observed any predator feasting on these aposematic beetles despite their local abundance.
Plants produce a plethora of secondary metabolites many with the ability to deter herbivores and pathogens. Sometimes the same compounds that effectively deter one set of organisms will be mediating the interactions with others. How do these plant-insect relationships arise? Why are some insect groups more prone to tolerating a specific type of plant chemistry?So many questions to explore, so many lessons to be learned.
Photo credits: Don Windsor and Guillaume Dury.