Inflorescence stems of the mdr1 mutant display altered gravitropism and phototropism

Auxins are a major group of growth regulators that are involved in all stages of plant growth and development, and these molecules play a key role in the response phase of both phototropism and gravitropism. Polar auxin transport within tissues is mediated by auxin influx and auxin efflux carriers that are asymmetrically distributed across cell membranes. The MDR (multidrug resistance) family of proteins has been identified in plants and is involved in the transport of auxin. In these studies, we examined tropistic responses in the mdr1 (=abcb19) mutant of Arabidopsis thaliana using conventional reorientation studies as well as a computer-based feedback system to investigate the kinetics of root gravitropism and phototropism. Time course of curvature studies show that tropisms were not affected in light-grown seedlings of mdr1 mutants, with the exception of an enhancement of gravitropism in hypocotyls. In mature plants, mdr1 inflorescence stems exhibit attenuated gravitropism but have enhanced phototropic curvature in response to unidirectional blue illumination. Thus, the decreased auxin flow in the mdr1 mutant has a more profound effect in inflorescence stems compared to hypocotyls of seedlings, possibly due to alterations in membrane trafficking pathways in gravity-perceiving endodermal cells of stems. Our data add support to the hypothesis that hypocotyls and stems differ in their cellular mechanisms of gravitropism.

Research highlights▶ Tropisms were not affected in seedlings of mdr1 with exception of gravitropism in hypocotyls. ▶ mdr1 inflorescences exhibit reduced gravitropism but have enhanced phototropic curvature. ▶ Decreased auxin flow in mdr1 mutant has a more profound effect in inflorescence stems. ▶ Hypocotyls and stems differ in their cellular mechanisms of gravitropism.