Strigolactones are a group of closely-related molecules synthesized by most plants (possibly using carotenoids as the starting material).
This is the molecular structure of one of them.
However, these activities do not qualify them as plant hormones (both activities take place in the soil surrounding the roots). Only if it can be demonstrated that strigolactones are translocated in the plant from the place of manufacture (roots) to another part of the plant where they exert an effect, can they be called hormones.
Two reports in the 11 September 2008 issue of Nature come close to proving the case.
Strigolactones (or possibly molecules derived from them) suppress the development of lateral buds and thus inhibit branching of the plant. Mutations in genes needed for the synthesis of strigolactones stimulate the development of lateral buds producing a more highly-branched plant than normal. Application of a synthetic strigolactone near the base of these mutant plants inhibits development of lateral buds above and thus restores normal branching (apical dominance).
Auxin and, in certain circumstances, abscisic acid also inhibit branching, that is, they promote apical dominance. But both auxin and abscisic acid participate in a number of different plant functions while the effect of strigolactones on branching seems quite specific.
|Other plant hormones|
|Abscisic acid (ABA)||Auxin||Brassinosteroids||Cytokinins||Ethylene||Gibberellins||Jasmonates|