The pea (Pisum sativum L.) genes sym33 and sym40 control infection thread formation and root nodule function

Two novel non-allelic mutants that were unable to fix nitrogen (Fix−) were obtained after EMS (ethyl methyl sulfonate) mutagenesis of pea (Pisum sativum L.). Both mutants, SGEFix−–1 and SGEFix−–2, form two types of nodules: SGEFix−–1 forms numerous white and some pink nodules, while mutant SGEFix−–2 forms white nodules with a dark pit at the distal end and also some pinkish nodules. Both mutations are monogenic and recessive. In both lines the manifestation of the mutant phenotype is associated with the root genotype. White nodules of SGEFix−–1 are characterised by hypertrophied infection threads and infection droplets, mass endocytosis of bacteria, abnormal morphological differentiation of bacteroids, and premature degradation of nodule symbiotic structures. The structure of the pink nodules of SGEFix−–1 does not differ from that of the parental line, SGE. White nodules of SGEFix−–2 are characterised by “locked” infection threads surrounded with abnormally thick plant cell walls. In these nodules there is no endocytosis of bacteria into host-cell cytoplasm. The pinkish nodules of SGEFix−–2 are characterised by virtually undifferentiated bacteroids and premature degradation of nodule tissues. Thus, the novel pea symbiotic genes, sym40 and sym33, identified after complementation analysis in SGEFix−–1 and SGEFix−–2 lines, respectively, control early nodule developmental stages connected with infection thread formation and function.