The MSG1 and AXR1 genes of Arabidopsis are likely to act independently in growth-curvature responses of hypocotyls

Growth-curvature responses of hypocotyls of Arabidopsis thaliana (L.) Heynh . were measured in double mutants between msg1 and axr1, both of which are auxin-resistant and defective in hypocotyl growth curvature induced upon un ilateral application of auxin. The msg1 axr1 double mutants showed no auxin -induced growth curvature, that is, they exhibited the msg1 phenotype, thou gh the axr1 defects were partial. Hypocotyls of both the msg1 and axil muta nts were partially defective in second-positive phototropism, whereas the d ouble mutants lost the response completely. When grown on vertically held a gar plates, the axr1 mutant showed normal hypocotyl gravitropism and the mu tation did not affect the reduced hypocotyl gravitropism of msg1. Hypocotyl s of msg1 and axr1 mutants grew upward like wild-type ones when grown along an agar surface, while they grew more randomly when grown without an agar support, suggesting that axr1 hypocotyls are not completely normal in gravi tropism. The extent of defects in growth orientation increased in the order : msg1 axr1 double mutants > msg1 > axr1 > wild type. The hypocotyls of the se mutants showed auxin resistance in the order: msg1 axr1 > axr1 > msg1 > wild type. The msg1 mutant had epinastic leaves and axr1 had wrinkled leave s; leaves of the msg1 axr1 double mutants were epinastic and wrinkled. Thes e results suggest that MSG1 and AXR1 act independently in separate pathways of the reactions tested in the present study. In contrast, the phenotype o f the msg1 aux1 double mutants shows that AUX1 is not significantly involve d in these phenomena.