Genetic mapping and functional analysis of a nodulation-defective mutant (sym19) of pea (Pisum sativum L.)

The pea mutant line P55 is defective in root nodule formation, and this phe notype is controlled by a single recessive gene. Complementation analysis r evealed that the mutation in P55 is allelic to sym19, which has previously been mapped to linkage group I. Detailed mapping revealed that the sym19 an d ENOD40 loci are separated by 2.7 cM. We identified four recombination eve nts, demonstrating that the nodulation defect caused by mutation of the sym 19 locus cannot be due to mutation of ENOD40. RT-PCR experiments showed tha t P55 expresses ENOD12A, but there was little or no increase in the level o f its transcript in response to Nod factor or infection with Rhizobium. To investigate this expression pattern further, transgenic peas carrying a pEN OD12A-GUS reporter construct were made. One transgenic line was crossed wit h line P55, to generate F2 progeny homozygous for sym19 and carrying pENOD1 2A-GUS, In both WT and sym19 mutant lines, ENOD12A-GUS expression was induc ed at sites of lateral root emergence in uninoculated plants. In Nod(+) pla nts pENOD12A-GUS was induced in response to Rhizobium leguminosarum by. vic iae, but no such induction was seen in the Nod(-) (sym19) mutants.