title: Gravity-induced modification of auxin transport and distribution for peg formation in cucumber seedlings: possible roles for CS-AUX1 and CS-PIN1.
abstract: 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.
authors.affiliation: Graduate School of Life Sciences, Tohoku University, Katahira, Aoba-ku, 980-8577 Sendai, Japan.
authors.forename: Motoshi
authors.initials: M
authors.lastname: Kamada
authors.forename: Seiji
authors.initials: S
authors.lastname: Yamasaki
authors.forename: Nobuharu
authors.initials: N
authors.lastname: Fujii
authors.forename: Atsushi
authors.initials: A
authors.lastname: Higashitani
authors.forename: Hideyuki
authors.initials: H
authors.lastname: Takahashi
mesh_terms: D029681:Arabidopsis Proteins; D001483:Base Sequence; D001692:Biological Transport; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D018553:Cucumis sativus / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D017931:DNA Primers; D018076:DNA, Complementary; D018744:DNA, Plant / Q000235:genetics; D015723:Gene Library; D018522:Gravitropism / Q000502:physiology*; D007210:Indoleacetic Acids / Q000378:metabolism*; D008565:Membrane Proteins / Q000502:physiology*; D026901:Membrane Transport Proteins; D010940:Plant Proteins / Q000235:genetics / Q000502:physiology
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D017931:DNA Primers; D018076:DNA, Complementary; D018744:DNA, Plant; D007210:Indoleacetic Acids; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; C116363:PIN1 protein, Arabidopsis; D010940:Plant Proteins