Initiation of a legume nodule with an indeterminate meristem involves proliferating host cells that harbour infection threads.

A comparative analysis of nodule morphogenesis was carried out for three symbiotically defective pea (Pisum sativum) mutants that show abnormalities in nodule development.In the wild-type lines, resumption of cell proliferation in the pericycle and inner cortex results in the development of a nodule primordium, within which are found proliferating cells that harbour infection threads. However, this class of cell is not observed in the mutants RisFixA (sym41) and SGEFix­-2 (sym33) where nodule development is arrested at the point of formation of the apical nodule meristem. Itis proposed that the presence of proliferating cells harbouring infection threads is a prerequisite for normal formation of the nodule meristem.In mutant SGEFix­-1 (sym40), nodule development does not differ from that of wild-type plants in the early stages but is blocked at the stage after nodule meristem persistence. A scheme is proposed for the sequential functioning of pea symbiotic genes Sym33, Sym40 and Sym41 in the programme of nodule development.

The Sym35 gene required for root nodule development in pea is an ortholog of Nin from Lotus japonicus.

Comparative phenotypic analysis of pea (Pisum sativum) sym35 mutants and Lotus japonicus nin mutants suggested a similar function for the PsSym35 and LjNin genes in early stages of root nodule formation. Both the pea and L. japonicus mutants are non-nodulating but normal in their arbuscular mycorrhizal association. Both are characterized by excessive root hair curling in response to the bacterial microsymbiont, lack of infection thread initiation, and absence of cortical cell divisions. To investigate the molecular basis for the similarity, we cloned and sequenced the PsNin gene, taking advantage of sequence information from the previously cloned LjNin gene. An RFLP analysis on recombinant inbred lines mapped PsNin to the same chromosome arm as the PsSym35 locus and direct evidence demonstrating that PsNin is the PsSym35 gene was subsequently obtained by cosegregation analysis and sequencing of three independent Pssym35 mutant alleles. L. japonicus and pea root nodules develop through different organogenic pathways, so it was of interest to compare the expression of the two orthologous genes during nodule formation. Overall, a similar developmental regulation of the PsNin and LjNin genes was shown by the transcriptional activation in root nodules of L. japonicus and pea. In the indeterminate pea nodules, PsNin is highly expressed in the meristematic cells of zone I and in the cells of infection zone II, corroborating expression of LjNin in determinate nodule primordia. At the protein level, seven domains, including the putative DNA binding/dimerization RWP-RK motif and the PB1 heterodimerization domain, are conserved between the LjNIN and PsNIN proteins.