Disruption of adenosine-5'-phosphosulfate kinase in Arabidopsis reduces levels of sulfated secondary metabolites

Plant sulfate assimilation genes: redundancy versus specialization

Sulfur is an essential nutrient present in the amino acids cysteine and methionine, co-enzymes and vitamins. Plants and many microorganisms are able to utilize inorganic sulfate and assimilate it into these compounds. Sulfate assimilation in plants has been extensively studied because of the many functions of sulfur in plant metabolism and stress defense. The pathway is highly regulated in a demand-driven manner. A characteristic feature of this pathway is that most of its components are encoded by small multigene families. This may not be surprising, as several steps of sulfate assimilation occur in multiple cellular compartments, but the composition of the gene families is more complex than simply organellar versus cytosolic forms. Recently, several of these gene families have been investigated in a systematic manner utilizing Arabidopsis reverse genetics tools. In this review, we will assess how far the individual isoforms of sulfate assimilation enzymes possess specific functions and what level of genetic redundancy is retained. We will also compare the genomic organization of sulfate assimilation in the model plant Arabidopsis thaliana with other plant species to find common and species-specific features of the pathway.

The plastidic bile acid transporter 5 is required for the biosynthesis of methionine-derived glucosinolates in Arabidopsis thaliana

Aliphatic glucosinolate biosynthesis is highly compartmentalized, requiring import of 2-keto acids or amino acids into chloroplasts for side chain elongation and export of the resulting compounds into the cytosol for conversion into glucosinolate. Aliphatic glucosinolate biosynthesis in Arabidopsis thaliana is regulated by three R2R3-MYB transcription factors, the major player being High Aliphatic Glucosinolate 1 (HAG1/MYB28). Here, we show that BAT5, which belongs to the putative bile acid transporter family, is the only member of this family that is transactivated by HAG1/MYB28, HAG2/MYB76, and HAG3/MYB29. Furthermore, two isopropylmalate isomerases genes, IPMI1 and IPMI2, and the isopropylmalate dehydrogenase gene, IPMDH1, were identified as targets of HAG1/MYB28 and the corresponding proteins localized to plastids, suggesting a role in plastidic chain elongation reactions. The BAT proteins also localized to plastids; however, only mutants defective in BAT5 function contained strongly reduced levels of aliphatic glucosinolates. The bat5 mutant chemotype was rescued by induced overexpression of BAT5. Feeding experiments using 2-keto acids and amino acids of different chain length suggest that BAT5 is a plastidic transporter of (chain-elongated) 2-keto acids. Mechanical stimuli and methyl jasmonate transiently induced BAT5 expression in inflorescences and leaves. Thus, BAT5 was identified as the first transporter component of the aliphatic glucosinolate biosynthetic pathway.