I was reading an article about how raphides (needle-like calcium oxalate crystals), together with proteins from the plant, are used by some plants to deter herbivory. However, there are plenty of insects that thrive on several plants known to have a high amount of raphides. Which strategies may insects use to avoid the crystals or to protect their gut cells? May these insects have specialised cells that act as filters? Do enzymes act on the oxalate and dissolve it?
Thank you for your answers.
Dear Ricardo,
Plenty of insects (?) which ones, how do they feed? Sucking? Raphides are contained in the vacuoles of agave cells, for example. So they are bundled inside an outer membrane. Insects could just consume the cell liquids without touching the needles, or even the young cells before they have the needles deposited. I think you should be more precise in your question and follow the insects (mouth parts, stage in which they attack, part of the plant and cells attacked, size relationship between mouth parts and cells of the host plant). Microscopy would give you hints when analyzing the attacked plant parts. Biochemistry is a very interesting tool, but should come after the anatomic and histologic observations.
Kind regards,
Carola Meierrose
.
Thank you for your answer Dr. Meierrose.
Talking about agave, it is attacked by species of Cossidae, Hesperiidae, Megathymiidae, Curculionidae, and Cerambycidae, all of which are borers of the leaves and the roots during their larval stage. All those tissues contain a high number of raphides (I learnt it the hard way). Besides, I have observed adult cerambycids chewing the leaves of full-grown agave plants. I have also observed several orthopterans feeding on Monstrera sp., Spathiphyllum sp. and Zantedeschia aethiopica. I'm sure that in their native ranges, they have many more pests.
Now, I don't think it is a big surprise, after all, it can be easily explained by natural selection. What I find interesting is the mechanisms that they may be using to degrade the raphides and / or the toxins the plants produce.
They might just avoid to feed on tissues with a high number of idoblasts and similar cell types cointining the crystals. In a PLOS One paper ( DOI: 10.1371/journal.pone.0091341) the authors describe that cysteine protease is in some plant species needed for a defence effect. Specialised insects might deactivate this mechanism (just a guess?).
And finally the sclerotisised cuticle of the foregut might serve as a protection against needles and since here already digestion starts, the needles might be gone before they come to more susceptible sections.
Thank you Dr. Hondelmann. That's exactly the article that got me thinking about this topic. I think all this is very interesting.
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