The Functional Analysis Of Arabidopsis HPA1 Gene

Histidine is one of the essential amino acids for protein synthesis and is required by all living organisms. Its biosynthetic pathway has been extensively characterized. It has been well established that histidine biosynthesis is vital for the survival of plants. However, very little is known about whether homeostasis of histidine plays any specific role in plant growth and development. This is due to the difficulty in experimentally separating the metabolic and regulatory functions of this essential amino acid and the lack of non-lethal (histidine) deficiency mutants. Here we characterize a novel Arabidopsis mutant, hpal, in the histidine biosynthetic pathway. By investigation of hpal mutant and AtHPAl gene, we conclude as following:1. The hpal mutant displayed a specific developmental defect in root meristem maintenance and was unable to sustain primary root growth.2. The hpal mutant carries a point mutation in a histidinol phosphate aminotransferase (HPA) gene. The hpal mutant phenotypes can be rescued by either constitutive expression of the WT AtHPAl or exogenous histidine supplementation.3. AtHPAl encodes a functional HPA protein. The complete knockout of AtHPAl is embryo lethal, indecating that AtHPAl makes a vital contribution to HPA activity and histidine biosynthesis in Arabidopsis and the mutated HPA protein in hpal is not completely functionally inactive. The residual HPA activities may be sufficient to maintain the vitality of the hpal embryo/plants.4. Biochemical analysis shows that the mutation in hpal resulted in a 30% reduction in free histidine content and had no significant impact on the total histidine content. It did not cause any known symptoms of histidine starvation.Our results therefore reveal an important role of histidine homeostasis in plant root development.