Which are major players, canonical or non-canonical strigolactones?

Strigolactones (SLs) can be classified into two structurally distinct groups: canonical and non-canonical SLs. Canonical SLs contain the ABCD ring system, and non-canonical SLs lack the A, B, or C ring but have the enol ether–D ring moiety which is essential for biological activities. The simplest non-canonical SL is the SL biosynthetic intermediate carlactone (CL). In plants, CL and its oxidized metabolites such as carlactonoic acid and methyl carlactonoate, are present in root and shoot tissues. In some plant species including black oat (Avena strigosa), sunflower (Helianthus annuus), and maize (Zea mays), non-canonical SLs are major germination stimulants in the root exudates. Various plant species such as tomato (Solanum lycopersicum) release carlactonoic acid, and poplar (Populus spp.) was found to exude methyl carlactonoate into the rhizosphere. These results suggest that both canonical and non-canonical SLs are active as host recognition signals in the rhizosphere. In contrast, limited distribution of canonical SLs in the plant kingdom and structure- and stereo-specific transportation of canonical SLs from roots to shoots suggest that plant hormones inhibiting shoot branching are not canonical SLs but are rather non-canonical SLs.