Molecular dissection of the role of histidine in nickel hyperaccumulation in Thlaspi goesingense (Halacsy)

To understand the role of free histidine (His) in Ni hyperaccumulation in T hlaspi goesingense, we investigated the regulation of His biosynthesis at b oth the molecular and biochemical levels. Three T. goesingense cDNAs encodi ng the following His biosynthetic enzymes, ATP phosphoribosyltransferase (T HG1, GenBank accession no. AF003347), imidazoleglycerol phosphate dehydrata se (THB1, GenBank accession no. AF023140), and histidinol dehydrogenase (TH D1, GenBank accession no. AF023141) were isolated by functional complementa tion of Escherichia coli His auxotrophs. Northern analysis of THG1, THD1, a nd THB1 gene expression revealed that each gene is expressed in both roots and shoots, but at the concentrations and dosage times of Ni treatment used in this study, these genes failed to show any regulation by Ni. We were al so unable to observe any increases in the concentration of free His in root , shoot, or xylem sap of T. goesingense in response to Ni exposure. X-ray a bsorption spectroscopy of root and shoot tissue from T. goesingense and the non-accumulator species Thlaspi arvense revealed no major differences in t he coordination of Ni by His in these tissues. We therefore conclude that t he Ni hyperaccumulation phenotype in T. goesingense is not determined by th e overproduction of His in response to Ni.