SEQUENTIAL FUNCTIONING OF SYM-13 AND SYM-31, 2 GENES AFFECTING SYMBIOSOME DEVELOPMENT IN ROOT-NODULES OF PEA (PISUM-SATIVUM L)

Two Fix(-) mutants of pea (Pisum sativum L.) which are unable to fix m olecular nitrogen, E135f (sym-13) and Sprint-2Fix(-) (sym-31), were cr ossed to create the doubly homozygous recessive line, named RBT (sym-1 3, sym-31). The ultrastructural organization of nodules of the RBT lin e was compared with that of each of the two parental mutant lines and with the original wild-type genotypes of the cultivars Sparkle and Spr int-2. It was shown that the RBT line is similar to the mutant line Sp rint-2Fix(-) in having abnormal symbiosome composition and bacteroids with relatively undifferentiated morphology. Because the phenotypic ma nifestation of the sym-31 mutant allele suppresses the phenotypic mani festation of the sym-13 mutant allele, it is concluded that the functi on of the gene Sym-31 (which is mutated in the Sprint-2Fix(-) line) is necessary at an earlier stage of symbiosome development than the gene Sym-13 (which is mutant in the E135f line).