CSIRO Plant Industry

Linkage of Gdcp1 with the Rp1 locus
-- Ayliffe, M and Pryor, T

Glycine decarboxylase is a nuclear encoded mitochondrial enzyme involved in the metabolism of 2 carbon glycine into 3 carbon serine.

Overall reaction:
    2 Glycine + NAD+ ----> Serine + CO2 + NH3
The enzyme complex is formed from four subunits (Oliver, Ann. Rev. Plant Phys. Mol. Biol. 45:323-337, 1994) and the 100kD P subunit has been shown to function as the amino acid decarboxylase part of the complex.

The fungus Cochliobolus victoriae is the causal agent of a blight in oats due to the production of a host-specific toxin, victorin. The victorin binding protein in oats has been identified as the 100kD P subunit of glycine decarboxylase (Wolpert et al., Plant Cell 6:1145-1155, 1994). Sensitivity to victorin and consequent susceptibility to fungal isolates that produce the toxin is specified by a single dominant gene Vb in the host. The Vb gene is thought to be coincident with a gene Pc-2 which specifies resistance to the oats crown rust, Puccinia coronata. It is therefore of interest to determine whether or not the gene (Gdcp) coding for the P subunit of glycine decarboxylase maps at any of the 5-6 loci known to specify resistance against rust infection in maize.

A comparison of the amino acid sequence of the GDC-P subunit from oats, pea and Flaveria made it possible to design primers which amplified, from a maize cDNA library, a 400bp PCR product that showed high homology to the GDC-P coding region. The PCR product was cloned and the cloned insert has been used as a probe in RFLP analysis to map loci, which specify the P subunit of glycine decarboxylase. Three lines of evidence suggest that one of the loci observed maps to the short arm of chromosome 10 at or near the Rp1 complex specifying resistance against the maize rust P. sorghi.

i. Mapping in the Recombinant Inbred Family-1 (CM37 x T232) (Burr et al., TIG, 7:55-60,1991) revealed three Gdcp loci, called pic7A, 7B and 7C , that segregated in genomic DNAs digested with EcoRI or BglII. Two loci, pic7A and 7B, are linked (about 25%) and show loose linkage to chromosome 6. The pic7C locus mapped near npi285 (R=0.07) and bnl3.04 (R=0.18).

ii. Crosses involving the 10S terminal deletion -def(bnl3.04-Rp5-Rp1-M)- show that the pic7C band is covered within this deletion. The deletion does not cover the marker npi422, which is located several map units proximal to Rp1. These data then place this Gdcp gene on 10S in a distal segment that also contains the Rp1 locus.

iii. The Gdcp1 locus maps close to Rp1-D13 in a backcross family: A plant heterozygous for Rp1-D13/rp was backcrossed to the susceptible parent (rp/rp) and the resultant progeny segregated 15 resistant to 23 susceptible. No recombinant progeny were recovered when these progeny were scored for the RFLP markers bnl3.04 and npi285 that flank the Rp1 locus. One of the 23 susceptible seedlings was recombinant for the Gdcp marker. While this individual cannot at present be distinguished from a potential contaminant the data suggest that the Gdcp locus is distal to the bnl3.04 locus. A proximal location is ruled out by the deficiency data. These data require confirmation.

A gene (Gdcp1) coding for the P subunit of glycine decarboxylase maps near the Rp1-complex. The Gdcp1 gene is very different from the known structure of plant genes specifying resistance against fungi, bacteria and viruses (Staskawicz et al., Science 268:661-667, 1995). Consequently it is unknown if this map location has some functional significance in terms of rust resistance or whether the linkage of a Gdcp locus to a rust resistance gene is fortuitous, reflecting synteny between the maize and the oat genomes. 

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