For the effective recycling of nutrients, vascular plants transport pooled inorganic ions and metabolites through the sieve tube. A novel sulfate transporter gene, Sultr1;3, was identified as an essential member contributing to this process for redistribution of sulfur source in Arabidopsis. Sultr1;3 belonged to the family of high-affinity sulfate transporters, and was able to complement the yeast sulfate transporter mutant. The fusion protein of Sultr1;3 and green fluorescent protein was expressed by the Sultr1;3 promoter in transgenic plants, which revealed phloem-specific expression of Sultr1;3 in Arabidopsis. Sultr1;3-green fluorescent protein was found in the sieve element-companion cell complexes of the phloem in cotyledons and roots. Limitation of external sulfate caused accumulation of Sultr1;3 mRNA both in leaves and roots. Movement of 35S-labeled sulfate from cotyledons to the sink organs was restricted in the T-DNA insertion mutant of Sultr1;3. These results provide evidence that Sultr1;3 transporter plays an important role in loading of sulfate to the sieve tube, initiating the source-to-sink translocation of sulfur nutrient in Arabidopsis.
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