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Histidine (His) is an essential ingredient for protein synthesis and is required by all living organisms. In higher plants, although there is considerable evidence that His is essential for plant growth and survival, there is very little information as to whether it plays any specific role in plant development. Here, we present evidence for such a role of this amino acid in root development in Arabidopsis (Arabidopsis thaliana) from the characterization of a novel Arabidopsis mutant, hpa1, which has a very short root system and carries a mutation in one of the two Arabidopsis histidinol-phosphate aminotransferase (HPA) genes, AtHPA1. We have established that AtHPA1 encodes a functional HPA and that its complete knockout is embryo lethal. Biochemical analysis shows that the mutation in hpa1 only resulted in a 30% reduction in free His content and had no significant impact on the total His content. It did not cause any known symptoms of His starvation. However, the mutant displayed a specific developmental defect in root meristem maintenance and was unable to sustain primary root growth 2 d after germination. We have demonstrated that the root meristem failure in the mutant is tightly linked to the reduction in free His content and could be rescued by either exogenous His supplementation or AtHPA1 overexpression. Our results therefore reveal an important role of His homeostasis in plant development.
His is one of the essential amino acids for protein synthesis and is required by all living organisms. Its biosynthetic pathway has been extensively characterized in bacteria and lower eukaryotes and consists of 10 enzymatic steps catalyzed respectively by (in the order of reactions) ATP-phosphoribosyltransferase, phosphoribosyl-ATP pyrophosphohydrolase (PRA-PH), phosphoribosyl-AMP cyclohydrolase (PRA-CH), N'-(5''-phosphoribosyl)-4-imidazolecarboxamide isomerase (BBMII isomerase), Gln amidotransferase, cyclase, imidazoleglycerol-phosphate dehydratase (IGPD), histidinol-phosphate aminotransferase (HPA), histidinol-phosphate phosphatase, and histidinol dehydrogenase (which catalyzes the last two steps; Winkle, 1987; Carlomagno et al., 1988; Alifano et al., 1996). In Escherichia coli, these enzymes are encoded by eight genes (the PRA-PH and PRA-CH activities are encoded by a single gene) in a single operon (Carlomagno et al., 1988). However, in Saccharomyces cerevisiae, they are encoded by seven genes scattered throughout the genome. The difference in the number of genes between E. coli and S. cerevisiae is due to the multifunctional properties of two...