Abstract

The main objective of our laboratory is to characterize the function of the 18 sulfotransferase-coding genes of Arabidopsis thaliana . In this study, we describe the biochemical and biological characterization of the three members of the AtST4 subfamily ( AtST4a, b and c ). The analysis of published microarray data as well as transcript expression studies show that the three members of the AtST4 subfamily are expressed in roots and regulated by cytokinins. AtST4b is among the group of genes exhibiting the highest level of induction following treatment with the cytokinin trans -zeatin. In contrast, AtST4c is repressed under the same experimental conditions. To elucidate their biological function, we isolated AtST4a, AtST4b, and AtST4c loss of function mutants. Using metabolite profiling of the knockout mutant and mass spectrometry, we demonstrate that AtST4 b encodes a cadabicine (cyclic dihydroxycinnamoyl spermidine) sulfotransferase. Even though cadabicine has previously been reported to occur naturally in plants, this is the first report of its occurrence in A. thaliana and the first report of the accumulation of its sulfonated conjugate. Phenotypic analysis of the AtST4b -knockout mutant showed alternations in root, shoot and reproductive development as compared to wild type plants and a partial loss of sensitivity to cytokinins. Our results seem to indicate that AtST4b plays an important role in the cytokinin-mediated effects on growth. Unfortunately, we could not find the endogenous substrate of AtST4a and AtST4c, and the results of our biochemical and metabolic profiling experiments suggest that they might sulfonate the same substrate or substrates with very similar properties. However, the AtST4a and AtST4c loss of function mutants show differences in their growth behavior suggesting that they have different functionalities in A. thaliana