| Methionine is an essential amino acid for humans and non-ruminant animals in that it cannot be synthesized de novo and therefore must be obtained in the diet. In plants, the committing step towards methionine biosynthesis is catalyzed by the enzyme cystathionine γ-synthase (CGS). This study represents the first characterization of CGS from the plant species Cicer arietinum (chickpea), Pisum sativum (field pea), and Lens culinaris (lentil), in which methionine is a limiting nutrient from a human perspective, and a CGS-like locus in the Arabidopsis thaliana genome (At1g33320). The CGS coding sequences from chickpea, field pea, and lentil are 70.0-71.0% identical to the characterized A. thaliana CGS sequence (At3g01120) at the amino acid level, contain the plant-specific conserved MTO1 regulatory region within the first exon, and complement the methionine auxotrophy of an Escherichia coli strain lacking the metB gene, encoding CGS. The expression pattern of CGS in these species is similar to that of the A. thaliana At3g01120 locus. Investigation of published plant genomes suggests that CGS is likely single copy in these diploid pulse species, in contrast to the two to three copies identified in the genomes of some Brasicaceae family members, including A. thaliana. Phenotypic assessment of A. thaliana T-DNA insertion lines demonstrated that while homozygous insertions within the coding region for the At1g33320 locus do not result in an altered phenotype, a heterozygous T-DNA line with an insertion in the At3g01120 locus demonstrated retarded growth, delayed flowering, and fewer siliques. It is proposed that the At1g33320 locus accounts for little to none of the total CGS activity in A. thaliana. A more detailed understanding of CGS and its regulation in pulses will assist in increasing the level of the limiting nutrient methionine in these crop species. |
