Anatomy, physiology and biochemistry of root nodules of Sprint-2 Fix−, a symbiotically defective mutant of pea (Pisum sativum L.)

A plant-determined pea mutant Sprint-2 Fix- and the parental line Sprint-2 were compared for selected physiological and biochemical parameters. The Fix - mutation prevented differentiation of Rhizobium leguminosarum bacteria into bacteroids and produced large, white, non-fixing nodules. These lacked nitrogenase-linked respiration, but had a background rate of CO 2 evolution similar to the normal Fix + nodules. The cortical structure of the ineffective nodules suggests the existence of an oxygen diffusion barrier and this was supported by a low oxygen concentration in the central region (0.5-3.0%), measured using an O 2 -sensitive micro-electrode. Sucrose and starch contents were similar in normal and ineffective nodules while ononitol content was about 15 times lower in the Fix - nodules. The distribution of starch was also different in the two nodule types. The activities of glutamine synthetase (GS), sucrose synthase (SS), phosphoenol-pyruvate carboxylase (PEPC) and alanine pyruvate aminotransferase (APAT) were markedly higher in Fix + nodules while the activities of pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH) and glutamate dehydrogenase (GDH) were higher in Fix - nodules. The data from immunodetection of host nodule proteins confirmed the reduced levels of sucrose synthase and the almost complete absence of glutamine synthetase and leghaemoglobin in mutant nodules. There was no significant difference in the amount of nitrogenase component 1 extracted from the microsymbiont of normal and ineffective nodules, but component 2 was hardly detectable in the Fix - mutant.