Abstract
When cucumber seeds germinate in a horizontal position, resulting seedlings develop a protuberance, termed the peg, on the lower side of the root-hypocotyl transition zone (TR zone), due to gravistimulation. We previously showed that auxin induced peg formation and proposed that decrement of auxin in the upper side caused by auxin efflux induces peg suppression. To reveal the mechanism how gravistimulation induces this asymmetric auxin redistribution in the TR zone, we analyzed the localization of CsPIN1, an auxin efflux facilitator expressed in endodermis of the TR zone, by immunohistochemical method. The results showed that CsPIN1 localization pattern was symmetrical in the endodermis of the TR zone in vertically grown seedlings. In contrast, in cucumber seedlings grown in a horizontal position, prominent CsPIN1 signals were detected in the endodermis in the upper side of the TR zone, all of which were localized at the lower membrane. These results suggest that the change in localization pattern of CsPIN1 due to gravistimulation promotes auxin efflux from endodermal cells in the upper side so that induces decrement of auxin in the upper side of the TR zone in cucumber seedlings.
