Phototropism optimizes light capture during seedling development is a determinant of plant fitness. Initial phototropic events include perception of blue light by phototropin1, which directly interacts with and inhibits ATP-BINDING CASSETTE Subfamily B 19 (ABCB19), resulting in an inhibition of rootward auxin transport. Later events involve asymmetric auxin accumulations in the shaded side of the hypocotyl that result in differential elongation, presumably via the PIN-FORMED (PIN) auxin efflux carriers. To better identify the link between early perception and later auxin transport and elongation events, an open-air system was used to remove seedling hindrance and provide better spatio-temporal resolution. Interestingly, no auxin transport mutants tested showed defects in early phototropic responses, indicating their involvement during later stages of bending. Additionally, asymmetric auxin accumulations using auxin reporter lines were only detected after bending was already accomplished. Analysis of CONVERGENCE OF BLUE LIGHT (BL) AND CO21/2 (CBC1 and CBC2) mutants, which have been shown to act in downstream signaling during phot1-mediated regulation of stomatal conductance, show early defects in phot1-mediated phototropism. Further, CBC1 and CBC2 have been shown to regulate S-type anion channels. Analysis of S-type anion channel mutants also reveals defects in early bending responses. These results point to blue light-dependent regulation of anion channel activity having an important role during the earliest stages of phototropism.
