Expression of SULTR2;2, encoding a low-affinity sulphur transporter, in the Arabidopsis bundle sheath and vein cells is mediated by a positive regulator

The bundle sheath provides a conduit linking veins and mesophyll cells. In the C-3 plant Arabidopsis thaliana, it also plays important roles in oxidative stress and sulphur metabolism. However, the mechanisms responsible for the patterns of gene expression that underpin these metabolic specializations are poorly understood. Here, we used the Arabidopsis SULTR2;2 gene as a model to better understand mechanisms that restrict expression to the bundle sheath. Deletion analysis indicated that the SULTR2;2 promoter contains a short region necessary for expression in the bundle sheath and veins. This sequence acts as a positive regulator and is tolerant to multiple consecutive deletions indicating considerable redundancy in the cis-elements involved. It is highly conserved in SULTR2;2 genes of the Brassicaceae and is functional in the distantly related C-4 species Flaveria bidentis that belongs to the Asteraceae. We conclude that expression of SULTR2;2 in the bundle sheath and veins is underpinned by a highly redundant sequence that likely represents an ancient and conserved mechanism found in families as diverse as the Asteraceae and Brassicaceae.