A nodulation ( [formula omitted]) mutant of Phaseolus vulgaris L. has nodule-like structures lacking peripheral vascular bundles (Pvb −) and is resistant to mycorrhizal infection (Myc −)

Abstract

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.