Abstract
The Arabidopsis (Arabidopsis thaliana) genome includes eight PIN-FORMED (PIN) members that are molecularly diverged. To comparatively examine their differences in auxin-transporting activity and subcellular behaviors, we expressed seven PIN proteins specifically in Arabidopsis root hairs and analyzed their activities in terms of the degree of PIN-mediated root hair inhibition or enhancement and determined their subcellular localization. Expression of six PINs (PIN1-PIN4, PIN7, and PIN8) in root hair cells greatly inhibited root hair growth, most likely by lowering auxin levels in the root hair cell by their auxin efflux activities. The auxin efflux activity of PIN8, which had not been previously demonstrated, was further confirmed using a tobacco (Nicotiana tabacum) cell assay system. In accordance with these results, those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast and formed internal compartments in response to brefeldin A. These six PINs conferred different degrees of root hair inhibition and sensitivities to auxin or auxin transport inhibitors. Conversely, PIN5 mostly localized to internal compartments, and its expression in root hair cells rather slightly stimulated hair growth, implying that PIN5 enhanced internal auxin availability. These results suggest that different PINs behave differentially in catalyzing auxin transport depending upon their molecular activity and subcellular localization in the root hair cell.
Highlights
The Arabidopsis (Arabidopsis thaliana) genome includes eight PIN-FORMED (PIN) members that are molecularly diverged
Most PIN1 proteins are internalized by brefeldin A (BFA) treatment, whereas considerable amounts of PIN2 remain in the plasma membrane in addition to internal accumulation after BFA treatment
To investigate whether the other PINs have a similar phenotype in the root hair, genomic fragments of PIN genes (PIN1, PIN2, PIN4, PIN5, PIN7, and PIN8) were fused to a reporter gene encoding GFP, and the fusion genes were expressed using the EXPA7 promoter (ProE7) in Arabidopsis
Summary
The Arabidopsis (Arabidopsis thaliana) genome includes eight PIN-FORMED (PIN) members that are molecularly diverged. The auxin efflux activity of PIN8, which had not been previously demonstrated, was further confirmed using a tobacco (Nicotiana tabacum) cell assay system In accordance with these results, those PINs were localized in the plasma membrane, where they likely export auxin to the apoplast and formed internal compartments in response to brefeldin A. These six PINs conferred different degrees of root hair inhibition and sensitivities to auxin or auxin transport inhibitors. PIN5 mostly localized to internal compartments, and its expression in root hair cells rather slightly stimulated hair growth, implying that PIN5 enhanced internal auxin availability These results suggest that different PINs behave differentially in catalyzing auxin transport depending upon their molecular activity and subcellular localization in the root hair cell. Apical targeting and recycling of PIN2 is independent of GNOM and controlled by BFAresistant ARF GEFs (Geldner et al, 2003; Kleine-Vehn and Friml, 2008)
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