Physiological and biochemical aspects of symbiotic nitrogen fixation in cowpea (Vigna unguiculata (L.) Walp. var. Tuy) Plants infected by Cowpea Mosaic Virus

Three-day-old cowpea seedlings were inoculated with the severe strain of Cowpea Mosaic Virus (CpMV) and 24 h later with Bradyrhizobium sp. cowpea, strain I-125, when virus translocation to roots still had not taken place. Plants were harvested at 22, 30, 45 and 59 d after germination. Active virus replication was associated with increased protein content, detected in the leaves of 22-d-old plants. CpMV infection reduced the total leaf area, dry weight of shoots and the chlorophyll content of the first trifoliolate leaf at all experimental times. Low sugar content was recorded in leaves of 22- and 30-d-old CpMV-infected plants and in nodules and roots of 30- and 45-d-old CpMV-infected plants. Up to 45 d, the nodule mass of CpMV-infected plants was lower than in controls, but reached control values at the 4th harvest. In CpMV-infected nodules, massive agglomeration of virus particles, crystalline virus inclusions and the proliferation of the endoplasmic reticulum were observed only in those cells containing bacteroids. In 30- and 45-d-old plants, CpMV infection decreased the content of ureides in nodules, roots, and petioles. Virus infection did not alter the α-amino-N content of roots and nodules but induced a transient 74% reduction in the level of α-amino-N in petioles of 45-d-old plants. At the 1st and 2nd harvests the activity of uricase (EC 1.7.3.3) in the nodules and of pyruvate kinase (EC 2.7.1.40) in the nodules and leaves were decreased severely by virus infection. CpMV did not hinder the allantoinase (EC 3.5.2.5) activity in the leaves but caused a 9% transitory decrease in the activity of this enzyme in nodules of 45-d-old plants. Measurements of NAD-malic enzyme (EC 1.1.1.38) in nodules also showed the non-effect of CpMV on this enzyme, except for a temporary 16% reduction at the 2nd harvest. As compared to controls, the relative abundance of ureides in 30-d-old CpMV-infected plants indicated a 15%, 10%, and 51% reduction in the nodules, roots, and petioles, respectively. Results indicate that at the time of the 4th harvest the symbiotic process, measured in terms of ureide content and enzymatic activities, was functioning at a near normal level despite nodule infection by CpMV.