Anthony J. Gordon

N2 Fixation, Carbon Metabolism, and Oxidative Damage in Nodules of Dark-Stressed Common Bean Plants

Common beans (Phaseolus vulgaris L.) were exposed to continuous darkness to induce nodule senescence, and several nodule parameters were investigated to identify factors that may be involved in the initial loss of N2 fixation. After only 1 d of darkness, total root respiration decreased by 76% and in vivo nitrogenase (N2ase) activity decreased by 95%. This decline coincided with the almost complete depletion (97%) of sucrose and fructose in nodules. At this stage, the O2 concentration in the infected zone increased to 1%, which may be sufficient to inactivate N2ase; however, key enzymes of carbon and nitrogen metabolism were still active. After 2 d of dark stress there was a significant decrease in the level of N2ase proteins and in the activities of enzymes involved in carbon and nitrogen assimilation. However, the general collapse of nodule metabolism occurred only after 4 d of stress, with a large decline in leghemoglobin and antioxidants. At this final senescent stage, there was an accumulation of oxidatively modified proteins. This oxidative stress may have originated from the decrease in antioxidant defenses and from the Fe-catalyzed generation of activated oxygen due to the increased availability of catalytic Fe and O2 in the infected region.

Carbon and Nitrogen Metabolism in Plant-Derived Ineffective Nodules of Pea (Pisum sativum L.)

Organogenesis of legume root nodules and the construction of the nitrogen fixation system require exchanges of molecular signals between rhizobia and the host plant to activate the expression of all the necessary genes. Plant mutants with ineffective nodules are potentially useful for studies of the host-plant control and regulation of symbiotic nitrogen fixation. In this paper the nodules of three different Nod+ Fix- pea mutants (Sym 13, Sym 31 and FN1) as well as their parent lines (Sparkle, Sprint 2 and Rondo) formed on plants by the same rhizobial strain and grown in the same controlled environment cabinet, were compared for selected physiological and biochemical parameters. All mutants are monogenic, recessive and non-allelic. The Sprint 2 Fix- (Sym 31) mutant is characterized by a block in bacteroid differentiation and abnormal symbiosome structure (Borisov et al., 1993). By contrast, mutants El 35 (Sym 13) and FN1 were characterized by early senescence of the symbiosomes and nodules as a whole (Kneen et al., 1990; Postma et al., 1990).