Regulation of high-affinity sulphate transporters in plants: towards systematic analysis of sulphur signalling and regulation

Plants require the function of plasma membrane-bound sulphate transporters for the initial uptake of inorganic sulphate. Part of this fundamental process is the energy-dependent proton/sulphate co-transport systems that are located in the surface cell layers of roots. During sulphur limitation, plants are able to activate the expression of sulphate transporters that facilitate the uptake of sulphate in roots. SULTR1;1 and SULTR1;2 are suggested to be the essential components of the sulphate uptake system in Arabidopsis roots. The physiological importance of SULTR1;1 and SULTR1;2 is supported by characteristics that can cope with sulphur deficiency: they were (i) functional high-affinity sulphate transporters; (ii) induced by sulphur limitation at the mRNA levels; and (iii) predominantly localized in the root hairs, epidermis, and cortex. The expression of high-affinity sulphate transporters was primarily regulated by sulphur in a promoter-dependent manner. Aside from the sulphur-specific regulation, the induction of SULTR1;1 and SULTR1;2 high-affinity sulphate transporters by sulphur limitation was dependent on the supply of carbon and nitrogen. In this review, the application of SULTR promoter-GFP systems for the analysis of regulatory pathways of sulphate acquisition in plants is described.