O-acetylserine (OAS) is a central metabolite of sulfur assimilation, providing the carbon backbone for synthesis of cysteine. Sulfide generated through sulfate reduction is incorporated into OAS to form cysteine. OAS displays a signalling function leading to changes in transcript levels of a specific gene set irrespective of the sulfur status of the plant. Treatment with external OAS increases sulfate transporter and adenosine 5′-phosphosulfate reductase gene expression consistent with a model of transcriptional induction by OAS.