Photo: WUR Plant roots don’t simply grow in random directions. They respond to stimuli, such as light, humidity or nutrients. So, why wouldn’t they respond to stimuli from micro-organisms? The soil is teeming with bacteria, nematodes and fungi. And each of these organisms exudes, as do the plant roots themselves, its own mix of substances, both volatile and non-volatile.
Y-shaped tube
Scientists are only just beginning to unravel this subterranean communication. Kay Moisan recently defended her PhD thesis on these interactions. To prove that roots grow towards certain fungi, she designed an ingenious device. In a Y-shaped tube, the plant is offered the choice to allow the root to grow in one direction or the other.
The difference between the two options is the presence of a fungus that emits volatile substances. Moisan tested rapeseed’s response to four different fungi cultures. One of these four was a pathogenic fungus. And it was precisely this fungus that prompted a reaction from the rapeseed. The shoot of the young plants gravitated towards the tube containing this fungus significantly more frequently. The plant distinguishes between the fungi.
‘Fatal attraction’
This does raise questions. Why would a plant grow towards a pathogen? Moisan suspects that the plant is being tricked. The fungus tempts the plant with a pleasing scent. Moisan calls this “fatal attraction”. However, it must be stressed that this is a test under lab conditions, where the plant has only one option. Under natural circumstances, where the soil is full of all types of signalling substances, the plant may make different choices.
In any event, the Y-tube method works. According to PhD supervisor Marcel Dicke, this method will certainly provide new insights in plant biology. The technique was inspired by entomological research into how (plague) insects react to volatile substances.