Auxin polar transport of etiolated Ageotropum pea epicotyls is not affected by gravistimulation: Relevance to automorphosis-like growth and development

There appears to be a close relationship between automorphosis and changes in auxin polar transport due to the fact that microgravity conditions cause both changes in the activity of auxin polar transport and in automorphosis of etiolated Alaska pea epicotyls. In addition, the application of inhibitors of auxin polar transport results in automorphosis-like growth and development. To elucidate the role of auxin polar transport in gravimorphogenesis in etiolated pea seedlings, we have studied the effects of gravistimulation on growth and development, and auxin polar transport in epicotyls of an agravitropic pea mutant “Ageotropum” seedlings and the normal “Alaska” seedlings. When the embryo axes in seeds of Alaska pea were set in a vertical (parallel to the direction of gravity) or a horizontal (vertical to the direction of gravity) position, and allowed to germinate and grow under 1g conditions in the dark for 3 or 6.5 days, the epicotyls grew upward due to negative gravitropic responses regardless of gravistimulation during seed germination. On the other hand, epicotyls of etiolated Ageotropum pea seedlings showed automorphosis-like bending away from the cotyledons regardless of gravistimulation during seed germination. Automorphosis-like epicotyl bending of etiolated Ageotropum pea seedlings was also unaffected by clinorotation on a three-dimensional (3-D) clinostat. The activity of auxin polar transport in the 2nd internodes of 6.5-d-old etiolated Ageotropum pea seedlings was lower than those of Alaska pea seedlings, and was not affected by clinorotation on a 3-D clinostat or by changes in gravity conditions during seed germination. These findings strongly support our previous studies that showed that normal auxin polar transport is required for the normal graviresponse of epicotyls in etiolated pea seedlings.