A nodulation (Nod(+)/Fix-) mutant of Phaseolus vulgaris L has nodule-like structures lacking peripheral vascular bundles (Pvb(-)) and is resistant to mycorrhizal infection (Myc(-))

Phenotypic characterization of two nodulation mutants (R69 and R99) of Phaseolus vulgaris L. was undertaken. The nodule anatomy of R69 (Nod(+)/Fix(-), was examined and compared to its wild-type parent, OAC Rico. The arbuscular mycorrhizal (AM) status of R69 and R99 (Nod(-)) was also compared to OAC Rico. Small whitish growths resembling nodules were produced on the roots of R69 in response to inoculation with Rhizobium leguminosarum biovar phaseoli. These nodule-like structures were variable in shape and occasionally had lateral roots growing from them. A layer of crystalline inclusions (possibly calcium oxalate) occurred between the periderm and parenchyma cells in the mutant nodules. A central infected zone was not present and the vascular system occupied the centre of the nodule instead of the periphery, as seen in wild-type nodules. Given the distinctive phenotype of the nodules of R69, they were designated Pvb(-) (peripheral vascular bundles, minus). The Nod(-) mutant, R99, could establish a symbiosis with AM (Myc(+)), however, R69 was resistant to infection by AM (Myc(-)). Appressoria would form on the roots and internal hyphae would start to develop, but these would fail to develop into a normal AM symbiosis. We suggest that specific anatomical characteristics of the Nod(+)/Fix(-) and Myc(-) phenotypes in R69 supports the hypothesis of a common regulation of the development of the two symbioses.