Cowpea Rhizobia Producing Dark Nodules: Use in Competition Studies

Abstract

During a program of screening rhizobia from West Africa, it was found that some strains produced nodules of unusually dark appearance on cowpeas, but not on peanuts, soybeans, pigeon peas, or mung beans. The dark pigmentation was in the bacteroid zone, was not correlated with nodule effectiveness, and was additional to the leghemoglobin pigment. Only rhizobial strains with a nongummy ("dry") colony morphology produced dark nodules. Visually distinguishable pink and dark nodules formed on the same root when a mixture of pink and dark strains was applied as inoculum. The dark-nodule phenotype was therefore appraised as a marker and found to be useful for studying nodulation competition with strains of the orthodox pink-nodule type. The competitiveness of 10 pink-nodule strains was examined relative to a black-nodule strain, IRc 256; a range of competitiveness was obtained of less competitive than, equally competitive to, or more competitive than IRc 256. Patterns of primary (early) nodulation were generally the same as patterns of secondary (later) nodulation. Mixed infections by dark and pink strains produced piebald nodules, the frequency of occurrence of which was much greater among primary than among secondary nodules. The internal pigmentation in the root nodules of legumes is indicative of their N2-fixing effectiveness. Actively fixing nodules are pink to redbrown due to the presence of leghemoglobin, and nonfixing nodules are normally white or green. When the nodule cortex of a species, e.g., cowpea, is translucent, the external nodule color is pale pink, white, or pale green. While screening rhizobial isolates from three West African locations for effectiveness with cowpea, we found that a significant proportion produced dark-colored nodules. Instances of unusually dark-colored nodules have been reported for several tropical legumes: Vigna marina (2), Dolichos lablab (7), Centrosema pubescens and Phaseolus atropurpureus (17), Mimosa spp. and Leucaena glauca (6), and Vigna sinensis (16). Although it has been suggested that the dark-nodule phenotype could be a very convenient marker for ecological studies (17), the phenomenon has been regarded merely as an oddity, and its utility has received little atten

Competitive interaction between non-nodulating and nodulating strains for nodulation of cowpea (Vigna unguiculata)

“…In a second experiment all treatments were similar to that of the first except that in one of the treatments Nod + was used as first inoculum and then after 24 h Nod-was added. In another competition experiment we used a dark brown forming nodule strain IRC256 [1] as Nod + instead of JRW3. Pouches were marked with a water proof marking pen to indicate the position of the root tip (RT) at the time of inoculation [9].…”

“…Most studies on the competitive interaction for nodulation have been limited to rhizobial Correspondence to: I. Singh, Biotechnology Centre, University of the West Indies, Mona, Kingston 7, Jamaica. strains capable of forming nodules [1][2][3]. Recently it has been suggested that ineffective strains of rhizobia are generally more competitive for nodulation than effective strains [2,4].…”

Localized Increase in Nodule Activity but No Competitive Interaction of Cowpea Rhizobia Due to Pre‐establishment of Vesicular‐arbuscular Mycorrhiza

“…Thirty randomly selected nodules from each pot (each half of all root systems) were assessed for the strains of Rhizobium they contained. The presence of IRC256 could be determined by the formation of very dark or piebald (part dark, part pink) nodules (Eaglesham et al, 1982). All 30 nodules from each pot were checked serologically (Somasegaran & Hoben, 1985) for isolates of Rhizobium using antisera labelled with fluorescein isothiocyanate (FITC).…”

The microbiome of Lotus nodules varies with plant health in a species-specific manner

“…However, the most striking difference was that in L. burttii 61.3 % of the starved-nodulated plants harboured white nodules, while in the other species most of the few nodules were pink. The pink colouration indicates the presence of leghemoglobin, a prerequisite for nitrogen fixation (41). Thus, the lack of leghemoglobin in white nodules indicates an environment in which rhizobia cannot fix nitrogen.…”