Auxin Polar Transport and Automorphosis in Plants

Abstract

Etiolated plants show automorphosis (automorphogenesis), spontaneous growth responses under a stimulus-free environmental condition, under microgravity conditions in space as well as simulated microgravity (or weightlessness) ones on a 3-dimensional clinostat. Although the mechanism to regulate automorphosis has been unclear yet, in STS-95 space experiments a close relationship between automorphosis and reduced auxin polar transport in etiolated Alaska pea seedlings has been suggested. Induction of automorphosis-like epicotyl bending by auxin polar transport inhibitors strongly supports causal relationship of auxin polar transport to induce automorphosis. Genetic and molecular biological studies of Arabidopsis have demonstrated an importance of PIN-FORMED (PIN) proteins as carrier (or facilitator) subcellularly localized at the polar side of the plasma membrane for auxin polar transport. This review is describing automorphosis and auxin polar transport focused on structure and functions of PIN proteins together with hormonal regulation of automorphosis in etiolated Alaska pea seedlings.