Natural variation in the ATPS1 isoform of ATP sulfurylase contributes to the control of sulfate levels in Arabidopsis.

Sulfur is an essential macronutrient for all living organisms. Plants take up inorganic sulfate from the soil, reduce it, and assimilate it into bioorganic compounds, but part of this sulfate is stored in the vacuoles. In our first attempt to identify genes involved in the control of sulfate content in the leaves, we reported that a quantitative trait locus (QTL) for sulfate content in Arabidopsis (Arabidopsis thaliana) was underlain by the APR2 isoform of the key enzyme of sulfate assimilation, adenosine 5'-phosphosulfate reductase. To increase the knowledge of the control of this trait, we cloned a second QTL from the same analysis. Surprisingly, the gene underlying this QTL encodes the ATPS1 isoform of the enzyme ATP sulfurylase, which precedes adenosine 5'-phosphosulfate reductase in the sulfate assimilation pathway. Plants with the Bay allele of ATPS1 accumulate lower steady-state levels of ATPS1 transcript than those with the Sha allele, which leads to lower enzyme activity and, ultimately, the accumulation of sulfate. Our results show that the transcript variation is controlled in cis. Examination of ATPS1 sequences of Bay-0 and Shahdara identified two deletions in the first intron and immediately downstream the gene in Bay-0 shared with multiple other Arabidopsis accessions. The average ATPS1 transcript levels are lower in these accessions than in those without the deletions, while sulfate levels are significantly higher. Thus, sulfate content in Arabidopsis is controlled by two genes encoding subsequent enzymes in the sulfate assimilation pathway but using different mechanisms, variation in amino acid sequence and variation in expression levels.