‘Witchweed’ – a parasitic plant from the Striga family, named as such by African farmers – appears as if by magic, without warning and attacks crops, including maize, millet, rice, sugarcane or legume crops, often wiping out the entire crop. The ‘curse’ of Striga dates back to Roman times when the mythical witch ‘Striga’ was thought to feed on the essence of life, especially on children, killing without remorse. But can this curse be better understood and used for good?
In a Q&A article in BMC Biology, Steven Smith from the University of Western Australia, Australia, argues that the signalling compound isolated from the roots of plants vulnerable to ‘witchweed’– strigolactones – can indeed further our understanding of how plants sense, use and adapt to nutrients in their environment.
What do strigolactones do? When exuded from plant roots, they trigger germination of seeds, which is why ‘witchweed’ is so parasitic and whose growth can be so rapidly stimulated. An association between symbiotic fungi and plant roots is also stimulated by strigolactones. In so doing, the fungi form structures (arbuscular mycorrhizae) on host roots, which enable greater mineral nutrient uptake, benefiting the plant. In plants themselves, one main function of strigolactones is to promote root growth over that of the shoot system to help plants scavenge for mineral nutrients in the soil.
How do strigolactones act at a molecular level? Smith discusses recent work on the identification of a possible receptor (D3, an F-box protein, and D14-type proteins in rice) and discusses the D53 protein, which was identified in rice. D53 usually stimulates lateral shoot growth but when strigolactones are present, these proteins are tagged with ubiquitination signals for destruction, preventing shoot growth. Strigolactone signalling has also been found to interact with and mediate other plant hormonal signals, for example, auxins (that can inhibit lateral bud growth) and brassinosteroids/gibberellic acid signalling, which in turn modulate seed germination and stem elongation.
Clearly there is more yet to understand as to how strigolactones act in plants but one thing is clear: what we currently understand about their mode of action could be exploited in the way we breed plants. As Professor Smith speculates: “The natural tendency of strigolactones is to dampen shoot growth at the expense of root growth when soil nutrients are low. The possibility of genetically ‘tweaking’ the strigolactone response to relieve that brake on shoot growth while maintaining vigorous root growth is an exciting possibility for future breeding of nutrient efficient crops.” This in turn could decrease the need for fertilisers and the associated environmental impact the use of such chemicals currently has.