Abstract
In the early growth stage of etiolated Alaska pea seedlings germinated and grown in a horizontal or an inclined position under 1 g conditions, an increased auxin level estimated by auxin-inducible gene (PsIAA4/5) expression was found in the elongating side of epicotyls, resulting in bending to the direction of negative gravity vector. In epicotyls showing automorphosis under simulated microgravity conditions on a 3D clinostat, expression of PsIAA4/5 was observed all around epicotyls. Polar auxin transport (PAT) in the proximal side of epicotyls was responsible for the determination of the growth direction of epicotyls both under 1 g and simulated microgravity conditions. In agravitropic mutant, ageotropum, which shows automorphosis-like epicotyl bending, PAT in the proximal side of epicotyls was not affected by simulated microgravity conditions. These results suggest that PAT in the proximal side of epicotyls regulates auxin distribution, resulting in graviresponse in the early growth stage of etiolated pea seedlings.
