What are strigolactones?
Strigolactones are signaling compounds made by plants. They have two main functions: first, as endogenous hormones to control plant development, and second as components of root exudates to promote symbiotic interactions between plants and soil microbes. Some plants that are parasitic on other plants have established a third function, which is to stimulate germination of their seeds when in close proximity to the roots of a suitable host plant. It is this third function that led to the original discovery and naming of strigolactones.
Where does the name come from?
Strigolactones were discovered in root exudates due to their ability to stimulate germination of seeds of the parasitic plant Striga, the ‘witchweed’ [1]. One example of this family of plants is Striga hermonthica, the purple or giant witchweed (Figure 1). So where did Striga get its name from? Witchweeds were so-named by subsistence farmers in Africa because they appeared without warning apparently from nowhere, and attacked their crops. The scientific (Latin) name for these witchweeds derives from Striga, a mythical witch apparently with origins in ancient Rome but known in several parts of southern and central Europe. The witch Striga was thought to be filled with hatred towards others, especially children, feeding on their life essence, or consuming them without remorse. Striga species are members of the broomrape family (Orobanchaceae), most members of which are parasitic on other plants.
Strigolactones have several functions – which came first?
Strigolactones can be traced back to some simple single-celled algae and primitive land plants such as mosses and liverworts [9]. Their original function was presumably in signaling between cells and in the control of growth and differentiation in early plants. For example, strigolactones are found in mosses, liverworts and in the alga Chara coralline, where they promote rhizoid growth. The filamentous moss Physcomitrella patens produces strigolactones that can regulate protonema branching and growth of filaments of a neighboring colony [10]. Thus, we see how growth and competition of neighbors can be coordinated by strigolactones – a principle that operates within higher plants to coordinate root and shoot growth. With colonization of the land several hundred million years ago, came fungal symbioses. Some liverworts enter into symbiotic relationships with mycorrhizal fungi, and although we do not yet know if this interaction depends on strigolactones, it is a hypothesis worthy of testing. With the evolution of vascular plants came complex patterns of shoot branching and the opportunity for long distance transport of strigolactones. It is in the flowering plants that the important functions of strigolactones are best known and best understood. The exploitation by witchweeds of strigolactones exuded by host plants is the latest invention in the evolutionary history of strigolactones.
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