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

Abstract

Auxin affects the shape of root systems by influencing elongation and branching. Because multidrug resistance (MDR)-like ABC transporters participate in auxin transport, they may be expected to contribute to root system development. This reverse genetic study of Arabidopsis thaliana roots shows that MDR4-mediated basipetal auxin transport did not affect root elongation or branching. However, impaired acropetal auxin transport due to mutation of the MDR1 gene caused 21% of nascent lateral roots to arrest their growth and the remainder to elongate 50% more slowly than the wild type. Reporter gene analyses indicated a severe auxin deficit in the apex of mdr1 but not mdr4 lateral roots. The mdr1 deficit was explained by 40% less acropetal auxin transport within the mdr1 lateral roots. The slow elongation of mdr1 lateral roots was rescued by auxin and phenocopied in the wild type by an inhibitor of polar auxin transport. Confocal microscopy analysis of a functional green fluorescent protein–MDR1 translational fusion showed the protein to be auxin inducible and present in the tissues responsible for acropetal transport in the primary root. The protein also accumulated in lateral root primordia and later in the tissues responsible for acropetal transport within the lateral root, fully supporting the conclusion that auxin levels established by MDR1-dependent acropetal transport control lateral root growth rate to influence root system architecture.

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

“…One possibility is that extra auxin synthesis at the tip of mdr1 roots compensates for the reduced delivery from the acropetal stream so that the basipetal stream is adequately supplied. The auxin maximum at the apex of mdr1 roots (as visualized by ProDR5:GUS or ProDR5:GFP) was similar to the wild type ( Figure 3C; Wu et al, 2007), which indicates that the acropetal stream either does not contribute to this feature or that the hypothetical compensatory synthesis faithfully restores the strength and pattern of the signal. The standard fountain model of auxin flow in roots (Swarup and Bennett, 2003) has been elaborated to include reflux loops that cause auxin to recirculate from epidermal and cortical cells back into the stele, where it rejoins the acropetal stream (Blilou et al, 2005).…”

“…Disruption of the reflux process could be a proximal cause of the erratic changes in growth direction. Indeed, the MDR1 protein is present on the inner, but less so on the outer, periclinal cell membrane of cortical cells (Wu et al, 2007), consistent with it being responsible for refluxing auxin toward the central cylinder from the cortex.…”

“…These two mutants were used here as genetic tools to dissect the roles of the two antiparallel auxin streams in root growth and development, which was quantified with unprecedented spatiotemporal resolution by a novel morphometric platform based on computer vision techniques (Miller et al, 2007). An accompanying article (Wu et al, 2007) used the same mutants to investigate the role of the auxin transport streams in lateral root growth and development. RT-PCR analysis of MDR4 transcript levels in Col-0 and the two independent mdr4 T-DNA alleles used in this study.…”

“…Particularly relevant to this work is that the expression patterns of MDR4 and MDR1 in the root are more complementary than overlapping. MDR4 is expressed primarily in the outer cell layers of the root (Birnbaum et al, 2003;Terasaka et al, 2005;www.arexdb.org), whereas MDR1 is present primarily in cells of the central cylinder (Wu et al, 2007). These two mutants were used here as genetic tools to dissect the roles of the two antiparallel auxin streams in root growth and development, which was quantified with unprecedented spatiotemporal resolution by a novel morphometric platform based on computer vision techniques (Miller et al, 2007).…”

Block of ATP-Binding Cassette B19 Ion Channel Activity by 5-Nitro-2-(3-Phenylpropylamino)-Benzoic Acid Impairs Polar Auxin Transport and Root Gravitropism

“…The Columbia-0 ecotype of Arabidopsis (Arabidopsis thaliana) was the wild type used in this study, and the genetic backgrounds of the mutant and transgenic lines employed were as follows: abcb19-3, a null phenotype transfer DNA insertion allele ; ProABCB19:GFP-ABCB19, an N-terminal GFP translational fusion expressed under the control of the ABCB19 promoter in the abcb19 mutant background, which it rescues Wu et al, 2007); and ProABCB4:ABCB4-GFP, a C-terminal GFP translational fusion expressed under the control of the ABCB4 promoter (Cho et al, 2007). The abcb19-6 allele, a D1173N substitution caused by a G-to-A mutation at position +3,517, was isolated using the Targeting Induced Local Lesions in Genomes method (Till et al, 2006).…”