Surber, S. M.; Hsieh, C.; Harding, S. A. A.; Tsai, C.-J.

Sulfate-proton co-transporters (SULTRs) mediate sulfate uptake, transport, storage, and assimilation within plants. The SULTR family has historically been categorized into four groups, SULTR1 to SULTR4. While the roles of SULTR groups 1, 2, and 4 are well characterized, there is much less consensus regarding the functions of the large SULTR3 group. Here, we present an updated SULTR phylogenetic analysis of angiosperms, including multiple early divergent lineages. Our results suggest that the enigmatic SULTR3 group consists of four distinct subfamilies which predate the emergence of angiosperms. This finding justifies reclassification of the SULTR family from four to seven subfamilies. Key evidence supporting this expansion includes subfamily-specific variations in gene structure and amino acid substitutions in the substrate-binding pocket. Gene expression analysis of Populus tremula x alba revealed divergent tissue expression preferences among SULTR subfamilies and between genome duplicates. We observed partitioned expression in vascular tissues among the four SULTR3 subfamilies, with PtaSULTR3.4a and PtaSULTR3.2a preferentially expressed in primary and secondary xylem, respectively. Gene coexpression analysis revealed coordinated expression of PtaSULTR3.4a with genes associated with phosphate starvation responses and various nutrient transport functions. In contrast, PtaSULTR3.2a showed strong coexpression with lignification genes and their upstream transcription regulators. The transcript abundance of PtaSULTR3.2a was down-regulated in response to genetic perturbation or tension wood formation which resulted in reduced lignin accrual. PtaSULTR3.2a belongs to a eudicot-specific branch of the SULTR3.1 subfamily found only in perennial species, supporting dedicated roles in lignifying tissues. Together, our findings provide a refined phylogenetic inference of the SULTR family and suggest potential neofunctionalization of the expanded SULTR3 subfamilies during the evolution of vascular and perennial plants.