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A Major Facilitator Super-family Transporter Plays a Dual Role in Polar Auxin Transport and Drought Stress Tolerance in Arabidopsis

A Major Facilitator Super-family Transporter Plays a Dual Role in Polar Auxin Transport and Drought Stress Tolerance in Arabidopsis

In an article entitled, “A Major Facilitator Super-family Transporter Plays a Dual Role in Polar Auxin Transport and Drought Stress Tolerance in Arabidopsis,” published in March 21, 2013, issue of The Plant Cell (doi: http:/​/​dx.​doi.​org/​10.​1105/​tpc.​113.​110353), the lead author Estelle Remy with seven other associates from four research groups present a functional analysis of an Arabidopsis ZIF1 (Zinc-Induced Facilitator 1) paralog, ZIF-LIKE1 (ZIFL1), and show that two isoforms produced through alternative splicing have distinct roles in polar auxin transport and drought stress tolerance. Arabidopsis ZIFL1 gene contains 17 exons and generates three distinct transcripts namely ZIFL1.1, ZIFL1.2 and ZIFL1.3. The first, ZIFL1.1 corresponds to the full-length transcript resulting from constitutive splicing of the precursor mRNA, while the second one (ZIFL1.2) likely arises from selection of an alternative transcription start site in the second intron. Finally, the ZIFL1.3 splice variant derives from selection of an alternative 3ʹ splice site in the fourteenth intron, with the second of two contiguous AGs being recognized. The ZIFL1.1 and ZIFL1.3 transcripts are therefore identical, except for the absence of two nucleotides in ZIFL1.3 that leads to the inclusion of a premature stop codon in the 15th exon in ZIFL1.3. However, the subcellular localization using yellow fluorescent protein (YFP) fusions showed that ZIFL1.1-YFP localizes to the tonoplast, whereas ZIFL1.3-YFP is targeted to the plasma membrane. These results were further confirmed in colocalization experiments using specific tonoplast and plasma membrane markers and in ZIFL1 mutant lines carrying ZIFL1.1 fused with green fluorescent protein under control of the native ZIFL1 promoter. The authors also examined ZILF1 function more specifically in Arabidopsis and yeast cells and found that ZIFL1.1 influences cellular auxin efflux and modulates polar auxin transport in roots, whereas ZIFL1.3 plays no role in auxin-related processes. They hypothesized that ZIFL1.1 might affect polar auxin transport by influencing the activity of the major auxin efflux carrier proteinase inhibitor II (PIN2). Immunofluoresence detection of PIN2 at the root tip plasma membrane indeed showed that PIN2 stability was significantly decreased by ZIFL1.1 loss of function and enhanced by its overexpression. Together with the specific ZIFL1 promoter activity, the drought stress hypersensitivity of the ZIFL1 mutants prompted them to investigate whether ZIFL1 disruption affects stomatal movements. Indeed, microscopy measurements of stomatal apertures revealed that the stomatal pore was significantly larger in the ZIFL1 mutants than in the wild type. Further experiments in yeast cells showed that both ZIFL1 isoforms catalyze K+- and proton-coupled transport, leading the authors to propose that ZIFL1 isoforms regulate stomatal movements (ZIFL1.3) and polar auxin transport (ZIFL1.1) by modulating K+ and proton fluxes. This interesting study provides an example of alternative splicing generating distinct biological functions for a transporter by altering tissue and subcellular localization. Future work will seek to uncover the mechanisms underlying tissue specificity of the ZIFL1 transcript

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