Parasitic plants such as the whitchweeds (Striga spp) and broomrapes (Orobanche spp) are unusual plants with often very attractive flowers (see pictures). However, their beautiful appearance is misleading, as these plants can be real killers.
They grow on the roots of a host plant and use a special organ called haustorium to obtain water and nutrients from their host. Parasitic plants are completely dependent upon their host: they have specialised in parasitation and let the host do the work. This specialisation is also risky: if the seeds of a parasite germinate in the absence of a host root, they will die. Therefore the parasites have developed another specialisation: for germination they require a signalling molecule that is exuded by the roots of their host. The seeds of the parasite are extremely sensitive to this compound and can detect the presence in the soil of minute amounts of the chemical. The question why plants are producing these – apparently non-beneficial - signalling molecules was recently answered by a Japanese group who discovered that the same signalling molecules, called strigolactones, are used by the beneficial arbuscular mycorrhizal (AM) fungi to find their host. AM fungi grow on and in the roots of their host and help the host to obtain water and minerals from the soil. In return they receive assimilates from the plant. Just like the parasitic plants, AM fungi use the strigolactones to detect the presence of their host. Parasitic plants simply abuse the signal intended for the AM fungi to find their host, while the host cannot do without the signal because it needs the AM fungi. In accordance with their role in attracting AM fungi, the secretion of strigolactones into the rhizosphere is greatly enhanced by a shortage in phosphate availability. Finally, it was recently discovered that the strigolactones are also internal signals in plants, that regulate shoot branching and probably also root morphology, and hence should be called plant hormones.
We study the role of the strigolactones in this underground communication between host, parasitic plant and AM fungi and their role in regulating plant development. We work on processes in the transmitter (the host) as well as the receivers of the signals (parasitic plants) and their interaction and the effect of environmental factors, particularly phosphate. We use a combination of disciplines such as chemistry, biochemistry, plant physiology and molecular biology to elucidate the formation and regulation of the chemical signals in the host and their perception in the parasite.