Separating the Roles of Acropetal and Basipetal Auxin Transport on Gravitropism with Mutations in Two Arabidopsis Multidrug Resistance-Like ABC Transporter Genes

Abstract

Two Arabidopsis thaliana ABC transporter genes linked to auxin transport by various previous results were studied in a reverse-genetic fashion. Mutations in Multidrug Resistance-Like1 (MDR1) reduced acropetal auxin transport in roots by 80% without affecting basipetal transport. Conversely, mutations in MDR4 blocked 50% of basipetal transport without affecting acropetal transport. Developmental and auxin distribution phenotypes associated with these altered auxin flows were studied with a high-resolution morphometric system and confocal microscopy, respectively. Vertically grown mdr1 roots produced positive and negative curvatures threefold greater than the wild type, possibly due to abnormal auxin distribution observed in the elongation zone. However, upon 90° reorientation, mdr1 gravitropism was inseparable from the wild type. Thus, acropetal auxin transport maintains straight growth but contributes surprisingly little to gravitropism. Conversely, vertically maintained mdr4 roots grew as straight as the wild type, but their gravitropism was enhanced. Upon reorientation, curvature in this mutant developed faster, was distributed more basally, and produced a greater total angle than the wild type. An amplified auxin asymmetry may explain the mdr4 hypertropism. Double mutant analysis indicated that the two auxin transport streams are more independent than interdependent. The hypothesis that flavanols regulate MDR-dependent auxin transport was supported by the epistatic relationship of mdr4 to the tt4 phenylpropanoid pathway mutation.

Mutations inArabidopsis Multidrug Resistance-LikeABC Transporters Separate the Roles of Acropetal and Basipetal Auxin Transport in Lateral Root Development

“…The large (80%) contribution of MDR1 to acropetal (but not basipetal) auxin transport (Lewis et al, 2007) implies that this ABC transporter functions in the central tissues of the root and not the epidermis. However, an expression profiling study that separately analyzed several root tissues at different stages of development indicated that MDR1 is expressed at reasonably high levels throughout the root apex (Birnbaum et al, 2003).…”

“…If this NPA-induced increase is due to blocked efflux of auxin synthesized at the root apex, then the opposite response of lateral root apices may be evidence that they produce less auxin than the primary root apex. An mdr4-1 line transformed with ProDR5:GFP was constructed for use in the Lewis et al (2007) studies on gravitropism. It was used here to examine the influence of MDR4 on auxin distribution in developing lateral roots.…”

ARL2, ARG1 and PIN3 define a gravity signal transduction pathway in root statocytes

“…The development of auxin asymmetry is thought to derive from redirection of auxin through the root tip toward the new lower flank of the root cap, followed by basipetal transport to the elongation zone (Blancaflor and Masson, 2003;Swarup et al, 2005). Basipetal transport of auxin from the cap to the elongation zone requires both auxin influx and efflux carriers, encoded by AUX1 and AGR1/PIN2/EIR1/WAV6, respectively (reviewed in Chen et al, 1999;Swarup et al, 2005), and members of the MDR/PGP ABC transporter families (Geisler and Murphy, 2006;Lewis et al, 2007;Lin and Wang, 2005;Terasaka et al, 2005). Basipetal auxin transport may also be regulated by molecules affecting the activity, expression and subcellular localization of auxin transporters in the lateral cell files of the root (as reviewed by Muday and Rahman, 2007).…”

Plasticity of Arabidopsis Root Gravitropism throughout a Multidimensional Condition Space Quantified by Automated Image Analysis

“…Upon reorientation, statoliths in the central columella cells of the root cap sediment to the new lower side of the cell (Blancaflor et al, 1998;Kiss, 2000). Rapid ionic fluxes are triggered within seconds, resulting in proton and possibly other ion concentration changes (Scott and Allen, 1999;Fasano et al, 2001;Hou et al, 2004) that result in redistribution of auxin efflux facilitator proteins, resulting in redistribution of auxin across the root (Friml et al, 2002;Lewis et al, 2007;, causing cells along the lower flank to expand more slowly than those on the upper flank (Mullen et al, 1998). Curvature is generated , resulting in downward bending of the tip.…”