|Campbell, Benjamin - University Of Minnesota|
|Mani, Dhananjay - University Of Minnesota|
|Curtin, Shaun - University Of Minnesota|
|Michno, Jean-michel - University Of Minnesota|
|Schaus, Philip - University Of Minnesota|
|Palmer, Reid - Iowa State University|
|Orf, James - University Of Minnesota|
|Stupar, Robert - University Of Minnesota|
Submitted to: Genes, Genomes, Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2014
Publication Date: 1/1/2015
Citation: Campbell, B.W., Mani, D., Curtin, S.J., Slattery, R.A., Michno, J., Ort, D.R., Schaus, P.J., Palmer, R.G., Orf, J.H., Stupar, R.M. 2015. Identical substitutions in magnesium chelatase paralogs result in chlorophyll deficient soybean mutants. Genes, Genomes, Genetics. 5(1):123-131.
Interpretive Summary: The recent development of genomic and mapping tools for soybean has facilitated map-based cloning efforts for specific traits in recent years. In this study, we have used modern genomic tools to rapidly map and fine-map a recently discovered spontaneous chlorophyll-deficient mutant line known as MinnGold. The reference soybean genome sequence was used to identify a Mg-chelatase subunit ChlI1a homolog as a candidate gene. The ChlI Mg-chelatase subunit is involved in catalyzing the insertion of Mg2+ into the protoporphyrin IX to form the first committed step in the chlorophyll biosynthesis pathway. Sequencing and functional analyses revealed that a specific nucleotide substitution in the coding region of this gene is responsible for the chlorophyll deficient phenotype of MinnGold. This study has identified genetic mutations of soybean ChlI1 alleles that confer chlorophyll-deficient foliage phenotypes. Identical missense substitutions at paralogous gene copies were found to confer nearly identical semi-dominant mutant phenotypes, whereas a similar missense mutation y11-2 conferred a completely recessive phenotype. We speculate that the soybean ChlIa paralogous proteins interact with one another, and the contrasting phenotypes observed from mutations a few base pairs apart may demonstrate the high level of specificity required for these interactions. The delicate nature of this interaction, along with conserved gene function, may contribute to the high sequence conservation of this duplicate gene family.
Technical Abstract: The soybean (Glycine max (L.) Merr.) chlorophyll deficient line MinnGold is a spontaneous mutant characterized by yellow foliage. Map-based cloning and transgenic complementation revealed that the mutant phenotype is caused by a non-synonymous nucleotide substitution in the third exon of a Mg-chelatase subunit gene (ChlI1a) on chromosome 13. This gene was selected as a candidate for a different yellow foliage mutant, T219H (Y11y11), that had been previously mapped to chromosome 13. Though the phenotypes of MinnGold and T219H are clearly distinct, sequencing of ChlI1a in T219H identified a different non-synonymous mutation in the third exon, only six base pairs from the MinnGold mutation. This information, along with previously published allelic tests, were used to identify and clone a third yellow foliage mutation, CD-5, which was previously mapped to chromosome 15. This mutation was identified in the ChlI1b gene, a paralog of ChlI1a. Sequencing of the ChlI1b allele in CD-5 identified a non-synonymous substitution in the third exon which confers an identical amino acid change as the T219H substitution at ChlI1a. Protein sequence alignments of the two Mg-chelatase subunits indicated that the sites of amino acid modification in MinnGold, T219H, and CD-5 are highly conserved among photosynthetic species. These results suggest that amino acid alterations in this critical domain may create competitive inhibitory interactions between the mutant and wild-type ChlI1a and ChlI1b proteins.