Dehydrocostus lactone, a naturally occurring polar auxin transport inhibitor, inhibits epicotyl growth by interacting with auxin in etiolated Pisum sativum seedlings

Yuta Toda,Kazuho Okada,Kensuke Miyamoto,Junichi Ueda

doi:10.5586/aa.1779

Abstract

We have isolated germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety, dehydrocostus lactone (DHCL), costunolide, santamarine, and a novel compound denoted artabolide [3-hydroxy-4,6,7(H)-germacra-1(10),11(13)-dien-6,12-olide] from oriental medicinal Asteraceae plants as novel naturally occurring inhibitors of polar auxin transport detected by the radish hypocotyl bioassay. To investigate the mode of action of natural sesquiterpene lactones on the inhibition of polar auxin transport as well as its relation to the growth of seedlings, the function of DHCL on growth and auxin dynamics in etiolated pea seedlings was studied intensively. DHCL reduced polar auxin transport in a dose-dependent manner together with the inhibition of the accumulation of mRNA of PsAUX1 and PsPIN1 genes encoding influx and efflux carrier proteins of auxin, respectively. DHCL applied to the apical hook region as a lanolin paste substantially inhibited elongation growth in the subapical region of epicotyls in intact etiolated pea seedlings, coupled with a significant reduction of endogenous levels of indole-3-acetic acid (IAA). DHCL also revealed the inhibition of IAA-induced cell elongation in etiolated pea epicotyl segments by affecting IAA-induced changes in the mechanical properties of cell walls. These facts suggest that germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety affect the expression of PsAUX1 and PsPINs genes, and then inhibit polar auxin transport and reduce endogenous levels of IAA necessary for stem growth in etiolated pea seedlings. These compounds are also suggested to show the inhibitory effects on auxin action in pea stem growth.

Highlights

The plant hormone group, auxins, of which indole-3-acetic acid (IAA) is the most common, are mainly synthesized in the shoot apex and actively transported between cells along physiological apical–basal direction in shoots
As a representative of germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety, we investigated the effect of dehydrocostus lactone (DHCL) on the epicotyl growth in etiolated ‘Alaska’ pea epicotyls with respect to auxin dynamics, including endogenous IAA levels and polar auxin transport, since etiolated pea seedlings have long been used in the study of mode of action of auxin
The structure/activity relationships suggest that germacranolide-type sesquiterpene lactones with an α-methylene-γ-lactone moiety are potent inhibitors of polar auxin transport, since ketopelenolide a and b, and hydroxypelenolide had little effect on this transport [24]

Summary

Introduction

The plant hormone group, auxins, of which indole-3-acetic acid (IAA) is the most common, are mainly synthesized in the shoot apex and actively transported between cells along physiological apical–basal direction in shoots. Directional auxin fluxes are important for reproductive tissues such as in floral formation and its development [9,10,11]. This polar-directional transport designated as polar auxin transport has been regulated by the function of the plasma membrane proteins of AUXIN RESISTANT1/LIKE AUXIN RESISTANT1 (AUX1/LAX) proteins as influx carriers and PIN-FORMED (PIN) proteins as efflux carriers in polar auxin transport [1,2,3,4,11,12,13,14]

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Conclusion