MDR-like ABC transporters function in auxin transport required for plant development and the response to environmental cues

Auxin controls many aspects of plant development through its effects on growth. Its distribution is controlled by specific tissue and organ level polar transport streams. The responses to environmental cues such as gravity, light, and nutrient availability are largely controlled by coordinated regulation of distinct auxin transport streams. A reverse genetic analysis was used to determine if the MDR-like ABC transporters influence auxin distribution important for plant development and the response to environmental cues in Arabidopsis. Mutations in MDR1 (At3g28860) reduce acropetal auxin transport in the root. This is correlated with deviation from the vertical axis, however, this relationship may not be causal. mdr1 plants also have reduced proDR5:GUS expression in embryonic and post emergence cotyledons. In post-emergence cotyledons, MDR1 is necessary for wild type levels of auxin import, which is NPA sensitive and specific for IAA. mdr1 cotyledons enlarge 50% slower than wild type, correlated with a 50% decrease in auxin import. Reduced cell expansion alone cannot account for the entire growth defect. MDR1-GFP fusions were used to determine the protein's tissue and sub-cellular localization. MDR1 was found symmetrically on the plasma membrane in petiole and cotyledon tissue in the epidermal and mesophyll layers. Mutations in MDR4 (At2g47000) reduce basipetal auxin transport in the root. This is correlated with hypergravitropism. It was theorized that reduced transport whithin the elongation zone is responsible for the increased curvature. Flavanols were found to regulate gravitropism upstream of MDR4. The mdr1 mdr4 double mutant showed additive but not synergistic phenotypes, suggesting that the two auxin transport streams are more independent than interdependent. MDR proteins seem to enhance auxin transport in situations where PIN-type effux alone is insufficient. This postulation is supported by the developmental patterning phenotypes of pin mutants versus the predominately quantitative phenotypes of mdr mutants.