Snapshot Issue 25 August 2006
When a living entity is challenged by another, subtle mechanisms are triggered off, resulting in molecular crosstalk which should help to ward off the offender. It is a question of recognizing the ‘self’ from the ‘non-self’. If a virus is not recognized as a transgressor, an organism will do nothing to hinder it and infection will flourish. Likewise, if a dog gnashing its teeth does not cause your heart to beat and your legs to run, you may well be in deep trouble.
Plants cannot decamp but they do have their strategies. Giving off scents is one. Directly reacting to damage brought about by a herbivore is another. But these are classic ‘self’ and ‘non-self’ reactions. Recently, a far more cunning approach was unveiled. Indeed, researchers have discovered that some plants have devised a way to get tiny protein fragments to be part of insects’ oral secretions. When such secretions are perceived by the same species of plants, defense mechanisms are prompted by the protein fragments, and insect larval growth is impeded. This is a case of the ‘self’ inducing a self defense mechanism – something which is rarely beneficial to an organism.
How do plants do this? So far, the observation has been made in cowpea and maize, and seems to be species-specific. The tiny protein fragment termed ‘inceptin’ is part of the gamma subunit of the large chloroplastic ATP synthase. When insect larvae feed on the plants, they ingest the ATP synthases along with hordes of other things. The synthases are then broken down by the insects’ proteases, thereby releasing inceptin, which subsequently becomes part of the insects’ secretions. When inceptin is secreted as the larvae feed on another plant, it elicits the production of phytohormones such as ethylene, salicylic acid or jasmonic acid, which are an integral part of complex pathways that ultimately lead to ways of fighting off herbivores.
Nature will never cease to surprise us. The notion of ‘self’ and ‘non-self’ seemed a sensible dogma – like the ‘one gene, one protein’ dogma did – and a rational way to explain the promotion of defense mechanisms. In a way it still stands though, for without the larvae and their digestion, inceptin would not exist in the first place.
ATP synthase subunit gamma, chloroplast precursor, Vigna unguiculata (Cowpea): Q2LGZ2
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