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.