Requirement for the gravity-controlled transport of auxin for a negative gravitropic response of epicotyls in the early growth stage of etiolated pea seedlings

Gravity-controlled transport of auxin was studied for a negative gravitropic response in the early growth stage of etiolated pea (Pisum sativum L. cv. Alaska) seedlings, in which epicotyl bending was observed near the cotyledon nodes of the seedlings grown continuously from seeds germinated in a horizontal or an inclined position. Increased expression of an auxin-inducible gene, PsIAA4/5, was observed in the elongated side of epicotyls grown in a horizontal or an inclined position. Regardless of the conditions of seed germination, polar auxin transport in the proximal side of the first internodes of the seedlings was significantly higher than in the distal side. Polar auxin transport in the proximal side of epicotyls grown in an inclined position was significantly lower than in those grown in a horizontal position. In contrast, lateral auxin distribution from the proximal to distal sides in epicotyls grown in an inclined position was significantly higher than in epicotyls grown in a horizontal position. Accumulation of PsPIN1 mRNA encoding a putative auxin efflux facilitator, which was observed in vascular tissue, cortex and epidermis in the proximal and distal sides of epicotyls, was markedly influenced by gravistimulation. These results strongly suggest that gravistimulation induces changeable polar auxin transport and one-way lateral auxin distribution in epicotyls as well as asymmetric auxin accumulation in the proximal and distal sides of epicotyls, resulting in a negative gravitropic response of epicotyls in the early growth stage of pea seedlings.     

Expression of PIN and AUX1 genes encoding putative carrier proteins for auxin polar transport in etiolated pea epicotyls [correction of epicotyles] under simulated microgravity conditions on a three-dimensional clinostat.

Etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown under simulated microgravity conditions on a 3-dimensional clinostat showed automorphosis-like growth and development similar to that observed in true microgravity conditions in space. Application of inhibitors of auxin polar transport phenocopied automorphosis-like growth on 1 g conditions, suggesting that automorophosis is closely related to auxin polar transport. Strenuous efforts to know the relationships between automorphosis and auxin polar transport in pea seedlings at molecular bases resulted in successful identification of PsPIN2 and PsAUX1 encoding putative auxin efflux and influx carrier protein, respectively. Significantly high levels in homology were found on nucleotide and deduced amino acid sequences among PsPIN2, PsPIN1 and AtPINs, and between PsAUX1 and AtAUX1. Expression of PsPIN1 and PsAUX1 genes in etiolated pea seedlings grown on the clinostat were substantially affected, but that of PsPIN2 was not. Roles of these genes in auxin polar transport and automorphosis of etiolated pea seedlings are also described.