IMMUNOCYTOLOGICAL EVIDENCE FOR ABNORMAL SYMBIOSOME DEVELOPMENT IN NODULES OF THE PEA MUTANT LINE SPRINT2FIX(-) (SYM31)

Using a series of antibody probes as markers of symbiosome development , we have investigated the impaired development of symbiosomes in nodu les formed by the plant mutant line Sprint2Fix(-) (sym31). In wild-typ e pea (Pisum sativum L.) nodules, bacteria differentiate into large pl eiomorphic, nitrogen-fixing bacteroids and are singly enclosed within a peribacteroid membrane. In the sym31 mutant, several small undiffere ntiated bacteroids were often enclosed within one peribacteroid membra ne, or were found within a vacuole-like compartment. In wild-type nodu les, the monoclonal antibody JIM18, which recognizes a plasmalemma gly colipid antigen, bound to the juvenile peribacteroid membrane, and did not recognize the mature peribacteroid membrane. However, in the muta nt, the antibody bound to all peribacteroid membranes within the nodul e, suggesting that differentiation of the peribacteroid membrane was a rrested. Another antibody, MAC266, recognized plant glycoproteins whic h normally accumulate in symbiosomes at a late stage of nodule develop ment. Binding of this antibody was much reduced within mutant nodules, labelling only a few mature cells. Similarly, MAC301, which normally recognizes a lipopolysaccharide epitope expressed on differentiated ba cteroids prior to the induction of nitrogenase, failed to react with r hizobial cell extracts isolated from nodules of the sym31 mutant. On t he basis of these developmental markers, the symbiosomes of sym31 nodu les appeared to be blocked at-an early stage of development. The distr ibution of infection structures was also found to be abnormal in the m utant nodules. Models of symbiosome development are presented and disc ussed in relation to the morphological and developmental lesions obser ved in the sym31 mutant.