Abstract

Recently, we initiated a functional genomics project with the objective of characterizing the biological function of all the sulfotransferase (ST) coding genes of A. thaliana . Based on amino acid sequence alignment with previously characterized soluble STs, we have identified 17 genes coding for putative STs in the genome of this plant. Prior to this work, only one ST-coding gene ( AtST1 ) has been characterized from A. thaliana . AtST1 was shown to sulfonate brassinosteroids and was proposed to be involved in the modulation of their biological activity. This thesis presents the characterization of two more ST-coding genes from A. thaliana , namely AtST2a and AtST3a . The recombinant AtST3a protein was found to exhibit strict specificity for position 7 of flavonoids. In contrast with the previously characterized flavonol 7-ST from Flaveria bidentis that sulfonates only flavonol disulfates, AtST3a was found to accept a number of flavonols and flavone aglycones, as well as their monosulfate derivatives. The AtST3a is expressed only at the earlier stage of seedling development. In contrast, the expression pattern of the flavonol 3-ST from Flaveria species is detectable at all stages of plant development, with highest activities found in the terminal buds and first pair of leaves. The natural occurrence of a ST exhibiting high specificity for flavonoids in A. thaliana suggests that sulfated flavonoids may be of more common occurrence in the plant kingdom than once thought. In this study, we demonstrate that the AtST2a from A. thaliana encodes a sulfotransferase specific for 11- and 12-hydroxyjasmonic acid. Jasmonic acid and its derivatives, commonly named jasmonates, are of ubiquitous occurrence in the plant kingdom and they play an important role in the plant response to biotic and abiotic stresses. More recently, it has been demonstrated that jasmonates are also involved in the control of key developmental processes such as anther development. AtST2a is not expressed in plants growing in the light but is induced 8 hours after their transfer to the dark. Overexpression of AtST2a in transgenic Arabidopsis leads to a delayed flowering phenotype observed only when the plants are growing under long-days. In contrast, decreasing AtST2a expression by expressing antisense AtST2a RNA leads to an early flowering phenotype observed only in short day-grown transgenic plants. Our results suggest that the function of AtST2a is to sulfonate hydroxylated jasmonic acids under growth conditions that do not favor flowering. This data also suggest that hydroxylated jasmonic acids act as signals that promote the transition from vegetative to reproductive growth when A. thaliana is exposed to an inductive photoperiod.