Sulfate assimilation in basal land plants - what does genomic sequencing tell us?

Sulfate assimilation is a pathway providing reduced sulfur for the synthesis of cysteine, methionine, co-enzymes such as iron-sulfur centres, thiamine, lipoic acid, or Coenzyme A, and many secondary metabolites, e.g., glucosinolates or alliins. The pathway is relatively well understood in flowering plants, but very little information exists on sulfate assimilation in basal land plants. Since the finding of a putative 3'-phosphoadenosine 5'-phosphosulfate reductase in PHYSCOMITRELLA PATENS, an enigmatic enzyme thought to exist in fungi and some bacteria only, it has been evident that sulfur metabolism in lower plants may substantially differ from seed plant models. The genomic sequencing of two basal plant species, the Bryophyte PHYSCOMITRELLA PATENS, and the Lycophyte SELAGINELLA MOELLENDORFFII, opens up the possibility to search for differences between lower and higher plants at the genomic level. Here we describe the similarities and differences in the organisation of the sulfate assimilation pathway between basal and advanced land plants derived from genome comparisons of these two species with ARABIDOPSIS THALIANA and ORYZA SATIVA, two seed plants with sequenced genomes. We found differences in the number of genes encoding sulfate transporters, adenosine 5'-phosphosulfate reductase, and sulfite reductase between the lower and higher plants. The consequences for regulation of the pathway and evolution of sulfate assimilation in plants are discussed.