Induced lipoxygenases in soybean root nodules

Abstract

Lipoxygenase (LOX; EC 1.13.11.12) 1 and 2&3 isozymic activities were measured spectrophotometrically in soybean nodules and their adjoining root tissue during nodule ontogeny. In effective nodules initiated by Bradyrhizobium japonicum strains 2122 and 2143, LOX1 specific activity gradually increased with nodule age. In ineffective nodules induced by strain 3122, LOX1 activity was 2–3 times less than those occurring in the corresponding effective nodules. LOX2&3 activity in effective nodules, showed a transient pattern with a peak at 35 days post-inoculation (dpi) as compared to LOX1 activity. In ineffective nodules, LOX2&3 activity peaked at 42 dpi and was half the rate observed in the effective nodules. LOX2&3 activity in the root tissues adjacent to effective nodules was extremely low and insignificant. LOX specific activities increased following the decline in nitrogen fixation capacity in root nodules at 28 dpi. The activity pattern of LOX temporally coincided with that of superoxide anion generation in nodules. LOX activity was inhibited by sodium azide, esculin, mercuric chloride and quercetin with high to low effectiveness, respectively. LOX activity was also inhibited by antioxidants such as ascorbate and reduced glutathione. Other inhibitors included superoxide dismutase, NADH, KCN and dithioerythretol. ‘Western’ immunoblot analysis revealed the cross-reactivity of 97 kDa nodule LOX with anti-soybean seed LOX antiserum. Induced LOX activity in the root nodules may suggest an early incompatibility between soybean and B. japonicum leading to nodule senescence.