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

Abstract

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.

Auxin Polar Transport and Automorphosis in Plants

“…50 % of that in a normal type of Alaska pea Hoshino et al, 2007). Furthermore, auxin polar transport in epicotyls of ageotropum pea was not affected by changes in gravity condition during seed germination, and by clinorotation on a 3-D clinostat Hoshino et al, 2007). These suggest that involvement of auxin polar transport in regulating automorphosis under microgravity conditions or in graviresponse under 1 G conditions.…”

“…1) and a reduced-auxin polar transport to ca. 50 % of that in a normal type of Alaska pea Hoshino et al, 2007). Furthermore, auxin polar transport in epicotyls of ageotropum pea was not affected by changes in gravity condition during seed germination, and by clinorotation on a 3-D clinostat Hoshino et al, 2007).…”

“…Weibull's Weitor (Blixt et al, 1958), support the suggestion. Etiolated ageotropum pea epicotyls showed automorphosislike shape (Schöldéen and Burström, 1960;Olsen and Iversen, 1980a, b;Hoshino et al, 2007;Miyamoto et al, 2005bMiyamoto et al, , 2007 (Fig. 1) and a reduced-auxin polar transport to ca.…”

“…Hormonal regulation to induce or to regulate automorphological epicotyl bending has been unclear yet. However, a close relationship between automorphosis and auxin polar transport is suggested, since auxin polar transport in etiolated Alaska pea epicotyls grown in space (Ueda et al, , 2000 as well as grown on a 3-D clinostat (Miyamoto et al, 2005bHoshino et al, 2006Hoshino et al, , 2007 was extremely reduced to ca. 50% of that in 1 G control.…”

“…Another approach to clarifying the importance o f g r a v i t y -c o n t r o l l e d a u x i n p o l a r t r a n s p o r t i n gravimorphogenesis has been examined by using chemicals. Inhibitors of auxin polar transport such as 2,3,5-triiodobenzoic acid (TIBA), N-(1-naphthyl) phthalamic acid (NPA) and 9-hydroxyfluorene-9-carboxylic acid (HFCA) phenocopied automorphological epicotyl bending under 1 G conditions (Miyamoto et al, 2005a;Hoshino et al, 2006;Hoshino et al, 2007). However, p-chlorophenoxyisobutylic acid (PCIB), antiauxin, had little effect.…”