Gravity-induced Modification of Auxin Transport and Distribution for Peg Formation in Cucumber Seedlings: Possible Roles for CS-AUX1 and CS-PIN1

Cucurbit seedlings potentially develop a peg on each side of the transition zone between the hypocotyl and root. Seedlings grown in a horizontal position suppress the development of the peg on the upper side of the transition zone in response to gravity. It is suggested that this suppression occurs due to a reduction in auxin levels to below the threshold value. We show in this study that the free indole-3-acetic acid (IAA) content is low, while IAA conjugates are significantly more abundant in the upper side of the transition zone of gravistimulated seedlings, compared to the lower side. A transient increase in mRNA of the auxin-inducible gene, CS-IAA1, was observed in the excised transition zone. The result suggests that the transition zone is a source of auxin. Cucumber seedlings treated with auxin-transport inhibitors exhibited agravitropic growth and developed a peg on each side of the transition zone. Auxin-transport inhibitors additionally caused an increase in CS-IAA1 mRNA accumulation at the transition zone, indicating a rise in intracellular auxin concentrations due to a block of auxin efflux. To study the involvement of the auxin transport system in peg formation, we isolated the cDNAs of a putative auxin influx carrier, CS-AUX1, and putative efflux carrier, CS-PIN1, from cucumber (Cucumis sativus L.) plants. Both genes (CS-AUX1 in particular) were auxin-inducible. Accumulation of CS-AUX1 and CS-PIN1 mRNAs was observed in vascular tissue, cortex and epidermis of the transition zone. A reduced level of CS-AUX1 mRNA was observed in the upper side of the gravistimulated transition zone, compared with the lower side. It is therefore possible that a balance in the activities of auxin influx and efflux carriers controls intracellular auxin concentration at the transition zone, which results in lateral placement of a peg in cucumber seedlings.