Cyclanilide: Mechanism of Action and Uses as a Plant Growth Regulator in Cotton

Cyclanilide [1-(2,4-dichlorophenylaminocarbonyl)-cyclopropane carboxylic acid] is a plant growth regulator that is registered for use in cotton at different stages of growth. It can be used to suppress vegetative growth when used in combination with mepiquat chloride; or it can enhance defoliation and boll opening when used in combination with ethephon. To understand how cyclanilide affects these different processes, the mechanism of action of cyclanilide was studied. We tested the hypothesis that cyclanilide acts by inhibiting auxin transport, or in some way alters auxin signaling. Auxin and ethylene are known to regulate abscission, so the interaction of cyclanilide and ethephon was studied on bean leaf defoliation. Ethephon induced bean leaf defoliation, but ethephon plus cyclanilide induced greater defoliation. The induction of defoliation by cyclanilide and ethephon was dose dependent. Combining auxin transport inhibitors (TIBA or NPA) or cyclanilide with a low, ineffective rate of ethephon induced 50% defoliation. In addition, movement of [H]IAA in etiolated corn coleoptiles was inhibited by 10 μM cyclanilide, NPA, and TIBA. These results demonstrated that cyclanilide affected polar auxin transport. However, NPA is a know inhibitor of auxin efflux, but cyclanilide did not effect efflux. Though NPA is a known inhibitor of the gravitropic response in tomato roots (85% at 1 μM) cyclanilide inhibition of gravitropism was 30% at 1 μM. Although NPA inhibited tomato root growth (30% at 1 μM), cyclanilide stimulated root growth (165% of control at 5 μM). Using plasma membrane fractions, cyclanilide inhibited the binding of [H]NPA and [H]IAA with an IC50 of 50 μM for both. NPA did not affect the binding of IAA, nor did IAA affect the binding of NPA. Cyclanilide is a noncompetitive inhibitor of both NPA and IAA binding, with inhibition constants (Ki) of 40 and 2.3 μM, respectively. These data demonstrated that cyclanilide interacts with auxin-regulated processes via a mechanism that is distinct from other auxin transport inhibitors, and identify a possible mechanism of action for cyclanilide when used as a plant growth regulator.