Gravity-controlled asymmetrical transport of auxin regulates a gravitropic response in the early growth stage of etiolated pea (Pisum sativum) epicotyls: studies using simulated microgravity conditions on a three-dimensional clinostat and using an agravitropic mutant, ageotropum.

Increased expression of the auxin-inducible gene PsIAA4/5 was observed in the elongated side of epicotyls in early growth stages of etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown in a horizontal or an inclined position under 1 g conditions. Under simulated microgravity conditions on a 3D clinostat, accumulation of PsIAA4/5 mRNA was found throughout epicotyls showing automorphosis. Polar auxin transport in the proximal side of epicotyls changed when the seedlings were grown in a horizontal or an inclined position under 1 g conditions, but that under clinorotation did not, regardless of the direction of seed setting. Accumulation of PsPIN1 and PsPIN2 mRNAs in epicotyls was affected by gravistimulation, but not by clinorotation. Under 1 g conditions, auxin-transport inhibitors made epicotyls of seedlings grown in a horizontal or inclined position grow toward the proximal direction to cotyledons. These inhibitors led to epicotyl bending toward the cotyledons in seedlings grown in an inclined position under clinorotation. Polar auxin transport, as well as growth direction, of epicotyls of the agravitropic mutant ageotropum did not respond to various gravistimulation. These results suggest that alteration of polar auxin transport in the proximal side of epicotyls regulates the graviresponse of pea epicotyls.