Requirement for the Gravity-Controlled Transport of Auxin for a Negative Gravitropic Response of Epicotyls in the Early Growth Stage of Etiolated Pea Seedlings

Abstract

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.

Auxin polar transport of etiolated Ageotropum pea epicotyls is not affected by gravistimulation: Relevance to automorphosis-like growth and development

“…In addition, our kinetic analyses on a 3-D clinostat revealed that automorphosis of epicotyls was a consequence of suppression of negative gravitropic responses in the early growth stage of etiolated Alaska pea seedlings (Miyamoto et al, 2005a). Moreover, under 1-g conditions, the application of 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), but not anti-auxin, p-chlorophenoxyisobutylic acid (PCIB), were shown to induce automorphosis-like epicotyl bending in etiolated pea seedlings, supporting our previous hypothesis that auxin polar transport in etiolated pea epicotyls is closely related to automorphosis (Miyamoto et al, 2005a;Hoshino et al, 2006).…”

“…We have demonstrated a close relationship between automorphosis and alternation in auxin polar transport based on the facts that both true and simulated microgravity conditions resulted in alternation of activity of auxin polar transport and automorphosis in etiolated Alaska pea epicotyls (Ueda et al, 1999(Ueda et al, , 2000Shimazu et al, 2001), and that the application of inhibitors of auxin polar transport induced automorphosis-like growth and development in etiolated Alaska pea seedlings under 1g conditions (Miyamoto et al, 2005a,b;Hoshino et al, 2006). Since Ageotropum pea seems to be a mutant showing automorphosis-like growth and development as shown in Figs.…”

“…Recent works with analyses of mutants related to auxin transport or gravitropic responses, and identification of auxin gradients with radiolabeled auxin and auxin-inducible gene reporter systems support this hypothesis (Muday and DeLong, 2001;Blancaflor and Masson, 2003;Morris et al, 2004;Morita and Tasaka, 2004). Hoshino et al (2006) have recently demonstrated that accumulation of mRNA of an auxin-inducible gene, PsIAA4/5, was asymmetrical between the proximal and distal sides of epicotyls in the early growth stages of Alaska pea seedlings, and greater accumulation of PsIAA4/5 mRNA was observed in the greater elongating side of the epicotyls in response to gravistimulation. This fact suggests that asymmetrical distribution of endogenous auxin is required for the negative gravitropic responses in the early growth stages of pea epicotyls, preceding visible bending.…”

“…This fact together with our previous findings suggests that suppression of the negative gravitropic response is dependent on reduced auxin polar transport in epicotyls, resulting in automorphosis in pea seedlings. Hoshino et al (2006) have suggested that normal auxin polar transport is required for the graviresponses of epicotyls in the early growth stage of etiolated Alaska pea seedlings.…”

Auxin Polar Transport is Essential for the Early Growth Stage of Etiolated Maize (Zea maysL. cv. Honey Bantam) Seedlings

“…Regulation by these proteins involve intricate and co-ordinated cellular processes, including protein-protein interactions, vesicular trafficking, protein phosphorylation, ubiquitination, and stabilization of the transporter complexes on the plasma membrane (Titapiwatanakun and Murphy, 2009). The expression of the genes encoding PIN proteins was substantially affected by various gravistimulations (Hoshino et al, 2006). These results strongly suggest that auxin polar transport regulated by PIN proteins is important for the subsequent growth and development on plants.…”