Hypersensitive Reaction of Nodule Cells in the Glycine sp./Bradyrhizobium japonicum‐Symbiosis Occurs at the Genotype‐Specific Level*

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Abstract
Three Glycine genotypes, G. max cv. Williams, G. soja PI 468397, and G. soja PI 342434 in combination with the two rhizobial strains Bradyrhizobium japonicum USDA 123 and Rhizobium fredii USDA 193 were analysed for phytoalexin concentration in the nodules. In the nodules of PI 468397/B. japonicum USDA 123 a very strong glyceollin I accumulation occurred around 30 d.p.i. Ultrastructural analysis of these nodules revealed several symptoms of a severe plant defense response associated with plant cell death (hypersensitive reaction): The cytoplasm of the infected cells was degraded and organelles had vanished. The cell walls of the infected cells showed remarkable thickening. This plant defense response could only be observed in this strain/genotype interaction. The same strain did not elicit a phytoalexin accumulation in the other plant genotypes tested, indicating that this response occurs at the genotype‐specific level. This special character of G. soja PI 468397 is heritable as indicated by glyceollin I analysis of the nodules formed by F1 hybrids of PI 468397xWilliams inoculated with B. japonicum USDA 123. The genotype/strain specific occurrence of the hypersensitive response in root nodules resembles the race/cultivar specific incompatibility of several plant‐pathogen interactions. This specificity, together with the phenomenon of the HR itself, points out the close physiological relationship between the late stages of the root nodule symbiosis and a plant/pathogen interaction.