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16.45F: Strigolactones

Strigolactones are a group of closely-related molecules synthesized by most plants (possibly using carotenoids as the starting material).

   Fig. Strigolactones

Strigolactones are manufactured in, and secreted from, the roots and have been known for some time to

  • promote the germination of some plant seeds. Unfortunately these include seeds of the genus Striga (witchweed) whose developing root invades the root of the strigolactone-secreting host plant (e.g., rice, corn, and sorghum) stealing nutrients from it and causing serious crop losses.
  • signal mycorrhizal fungi to connect to the root system forming a mutualistic relationship.

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.

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.