Phytotropins: receptors and endogenous ligands.

After lengthy exploration of structure-activity correlations, phytotropins were defined as compounds that inhibit plant gravitropic and phototropic responses and the polar transport of the hormone auxin. Their common structural theme is that benzoic acid is ortho-linked to a second aromatic ring system. 1-N-naphthylphthalamic acid (NPA) is a widely-used phytotropin, but 1-pyrenoylbenzoic acid is the most active synthetic compound. Not all polar auxin transport inhibitors (PATIS) are classed as phytotropins: 2,3,5-triiodobenzoic acid and the morphactins such as 2-chloro-9-hydroxyfluorene-9-carboxylic acid are the best established exceptions. PATIS can bind to the NPA receptor which is located on the plasma membrane. This leads to inhibition of an auxin efflux carrier that is considered to play a fundamental role in polar transport. The molecular properties of the NPA receptor are not fully characterised, and the mechanism of its interaction with the auxin carrier is not understood. Conformational mapping has indicated that the two aromatic ring structures of phytotropins may have receptor binding domains oriented at an angle of about 110 degrees. The question arises of whether there are endogenous regulators of auxin transport that act via the NPA receptor. Recent evidence suggests that certain C-15 flavonoid phenolic compounds, such as the flavonol quercetin and the flavone apigenin, are candidates for such a role. They have NPA-like effects on transmembrane and polar auxin transport and can inhibit NPA binding to the receptor. Structure-activity studies showed that simple phenolics like hydroxybenzoic and hydroxycinnamic acids are not active; the active flavonoids appear to require hydroxyl groups on both A and B rings of the C15 structure, linked by a pyrone ring. The only exceptions so far found to a generally good correlation between binding and transport effects are the 2'-hydroxyflavonoid morin and the isoflavonoid genistein where the B ring is moved from C2 to C3 of the central pyran ring. Both these compounds are more active as NPA binding inhibitors. Only aglycones and not flavonoid glycosides are active. Flavonoids are widely distributed in plants and are present in tissues capable of polar auxin transport. Their synthesis is under environmental control, as is auxin transport.