The genome of cowpea (Vigna unguiculata [L.] Walp.)

Cowpea (Vigna unguiculata [L.] Walp.) is a major crop for worldwide food and nutritional security, especially in sub‐Saharan Africa, that is resilient to hot and drought‐prone environments. An assembly of the single‐haplotype inbred genome of cowpea IT97K‐499‐35 was developed by exploiting the synergies between single‐molecule real‐time sequencing, optical and genetic mapping, and an assembly reconciliation algorithm. A total of 519 Mb is included in the assembled sequences. Nearly half of the assembled sequence is composed of repetitive elements, which are enriched within recombination‐poor pericentromeric regions. A comparative analysis of these elements suggests that genome size differences between Vigna species are mainly attributable to changes in the amount of Gypsy retrotransposons. Conversely, genes are more abundant in more distal, high‐recombination regions of the chromosomes; there appears to be more duplication of genes within the NBS‐LRR and the SAUR‐like auxin superfamilies compared with other warm‐season legumes that have been sequenced. A surprising outcome is the identification of an inversion of 4.2 Mb among landraces and cultivars, which includes a gene that has been associated in other plants with interactions with the parasitic weed Striga gesnerioides. The genome sequence facilitated the identification of a putative syntelog for multiple organ gigantism in legumes. A revised numbering system has been adopted for cowpea chromosomes based on synteny with common bean (Phaseolus vulgaris). An estimate of nuclear genome size of 640.6 Mbp based on cytometry is presented.     

Differences in calcium efficiency between cowpea (Vigna unguiculata (L.) Walp.) cultivars

The cowpea (Vigna unguiculata (L.) Walp.) cultivars TVu 354 and Solojo were grown in solution culture at 10 to 1000 M Ca supply. The Ca supply did not vary by more than 10% during the experiment. The pH value was kept constant within 0.1 units at 4.0 by automatic titration. The cultivar TVu 354 proved to be much more Ca-efficient than Solojo. At 10 M Ca supply Solojo died, whereas TVu 354 was hardly affected in dry matter production. The differences in Ca efficiency were independent of the P supply. They could not be explained by differences in Ca uptake or Ca concentrations in the plant tissue. Short-term studies using ⁴⁵Ca, both in the dark and in the light, indicated better transport of Ca from the roots to the shoots and within the shoots to the younger leaves in the Ca-efficient cultivar TVu 354. However, the main reason for the differences between the cultivars in sensitivity to low Ca supply were differences in the Ca requirement of the plant tissue to maintain tissue organization and function. Sequential fractionation of the freeze-dried leaf tissue with hot water, 0.5 M NaCl, 1 M CH₃COOH, and 2 M HCl did not reveal cultivar differences in Ca binding state. The results clearly show that considerable genetic potential in tolerance to low Ca supply exists in cowpea. However, a better understanding of the physiological/biochemical reasons for low internal Ca requirement is needed.     

Protein and cDNA sequences of Bowman-Birk protease inhibitors from the cowpea (Vigna unguiculata Walp.)

The protein and gene sequences of the cowpea Bowman-Birk type trypsin inhibitor which confers enhanced insect resistance to transgenic tobacco plants, and of cowpea trypsin/chymotrypsin inhibitors are presented. There are regions of high conservation and high divergence within the 5 leader, mature protein and 3 non-coding regions of the Bowman-Birk inhibitors and in the genes which encode them in different members of this family within the Leguminosae. The practical implications of this finding for studies on the evolution of plants and the utilization of these genes for enhancing insect resistance is discussed.     

Potassium and magnesium relationship in cowpea (Vigna unguiculata (L.) Walp.)

Summary The effects of K and Mg application on dry matter yield and uptake of K, Mg and Ca in cowpea were studied in greenhouse at Haryana Agricultural University, Hissar (india). Dry matter yields of leaves, stems and roots increased by 17, 30 and 27 per cent over control due to application of 150 ppm K and 17, 16 and 26 per cent by 40 ppm Mg respectively. Potassium application has antagonistic effect on Ca concentration of leaves, stems and roots and synergistic on root Mg concentrations upto 25 ppm K. However, Mg had a synergistic effect on concentration of K upto 20 ppm Mg and antagonistic at 40 ppm Mg in all plant parts. Uptake of K, Mg and Ca increased by Mg application, but K increased only K uptake.     

Fast-growing bacteria from nodules of cowpea (Vigna unguiculata (L.) Walp.)

D akora , F.D. & V incent , J.M. 1984. Fast-growing bacteria from nodules of cowpea ( Vigna unguiculata (L.) Walp.) Journal of Applied Bacteriology 56 , 327–330.
First plating from nodules of cowpea frequently yielded fast-growing large colonies, either apparently uniform or associated with small colony forms typical of the expected slow-grower ( Bradyrhizobium ). Most cultures from single large colonies nodulated both cowpea and siratro ( Macroptilium atropurpureum ), but all such nodules revealed Bradyrhizobium alone or associated with a fast-growing form. Six of nine plants inoculated with a mixed inoculum of slow and fast forms had nodules occupied by both although in no case was the fast-grower able to secure solo invasion. Most of the fast-growing forms shared some internal antigens with Rhizo-biurn meliloti and/or R. trifolii ; none reacted with antiserum to Bradyrhizobium CB 756.     

Allozyme diversity of cultivated cowpea Vigna unguiculata (L.) Walp.

A survey of allozyme variation in cultivar-groups of cowpea [Vigna unguiculata (L.) Walp.] was undertaken by examining 21 enzyme systems encoded by 36 loci in 271 accessions representing the five cultivar-groups. Very low levels of variation were found within accessions, which is typical of self-pollinating species. Little variation was also found among accessions. Compared with other legume crops, V. unguiculata is depauperate in allozyme variation. We found an average of 1.61 alleles per locus with 42% of the loci polymorphic and a total heterozygosity of 0.061. Of the variation present, 90% was found within cultivar-groups, while 10% was among cultivar-groups. Data analyses revealed continuous variation among cultivar-groups and geographic regions with the accessions failing to segregate into discrete morphophysiological or geographic clusters. However, evolved cultivar-groups (cv.-gr. Melanophthalmus and cv.-gr. Sesquipedalis) appear to be less diverse than their putative primitive cultivar-group progenitors. Due to the lack of availability of critical material, no clear center of origin can be established. However, the data presented suggest that Northeast Africa could be a possible center of domestication. Received: 18 February 1999 / Accepted: 4 November 1999     

Effect of light intensity on manganese toxicity symptoms and callose formation in cowpea (Vigna unguiculata (L.) Walp.)

In cowpea typical Mn toxicity symptoms are brown speckles on mature leaves representing depositions mainly in the cell walls and formation of non-constitutive callose. The histochemical charecterization of the brown speckles indicates the presence of oxidized Mn. However, the reducing agent hydroxylamine hydrochloride only slightly while thioglycolic acid almost completely decolorized the speckles. Brown boron-deficient roots treated with hydroxylamine hydrochloride and thioglycolic acid showed the same pattern of decoloration suggesting that the brown color of the Mn toxicity symptoms derives mainly from oxidized phenolics. To evaluate the effect of light on the formation of brown speckles by high Mn concentrations and non-constitutive callose in leaves, three approaches were used: (i) comparison of shaded and unshaded plants at different Mn supplies via the roots, (ii) local application of Mn to leaves in the light and in the dark, (iii) local application of Mn to leaves in the dark with subsequent light and dark treatments. Shading of whole plants (i) aggravated formation of both brown speckles and callose at similar Mn concentrations in the leaves. When the Mn application and the light treatments were locally confined (ii, iii), light had no effect on formation of either brown speckles or callose. The present results are in contradiction to the available reports in the literature showing aggravation of Mn toxicity by high light intensities.     

In vitro regeneration of Vigna unguiculata (L.) Walp. cv. Blackeye cowpea via shoot organogenesis

An in vitro regeneration system was developed in cowpea [Vigna unguiculata (L.) Walp.] Blackeye. Among several explants studied, shoot initiation response was observed from shoot apices of 3–5-day-old seedlings. The optimal medium for maximum shoot initiation comprised MS salts, B₅ vitamins, 8.88 μM N ⁶-benzylaminopurine, 1 gl^-1 casein hydrolysate, 342 μM L-glutamine, 3% sucrose, 0.3% phytagel, adjusted to pH 5.8. A shift in pH from 5.8 to 7.0 had no effect on shoot initiation and on number of shoots per explant. The highest shoot initiation frequency (77%) was obtained using this preferred medium, reaching a maximum of eight shoots per explant. For shoot elongation, 14 μM gibberellic acid was supplemented in the shoot initiation medium. Presence of indolebutyric acid in the rooting medium had no effect on root induction. The regenerated plants were fertile and developed normally.     

Alarming increase in the incidence of Cucumber mosaic virus in cowpea (Vigna unguiculata (L.) Walp.) in northern Nigeria

Cowpea plays a key nutritional role in the diet of the Nigerian people. Viral diseases are a major limitation to cowpea production worldwide, and thus, constant viral surveillance is crucial for monitoring and management purposes. In this study, cowpea leaf samples from fields in three northern Nigeria states, Kano, Kaduna and Niger, were tested to determine the status of six common viruses previously reported in these cowpea-producing states following the release of virus-resistant varieties. Cowpea aphid-borne mosaic virus (CABMV), Blackeye cowpea mosaic virus (BICMV), Cowpea mottle virus, Southern bean mosaic virus and Cucumber mosaic virus (CMV) were detected. Cowpea yellow mosaic virus, which was previously reported in all three states, was not detected in any of the samples tested, while CMV that was previously regarded as unimportant to cowpea production in Nigeria had the highest incidence in all three states, and the overall highest incidence of 58.8%, while CABMV had the lowest incidence (7.5%). CMV was also present in seven of the ten mixed infection combinations detected. Dual infection of CMV and BICMV, which often results in cowpea stunt, the most devastating cowpea disease in the USA, was the most frequently detected mixed infection (28.1%) and was detected in all three states. This observed elevation in CMV infection in cowpea must be closely monitored and swiftly managed to avert possible devastating crop yield losses.     

Recovery of nodulated cowpea plants (Vigna unguiculata (L.) Walp.) from waterlogging during vegetative growth

Summary Waterlogging pot-grown cowpea plants for eight days (24–32 from sowing) during the period of rapid vegetative growth when symbiotic nitrogen fixation rates are also increasing at maximum rate had effects on anatomical, morphological and physiological attributes of the legume system viz reduced nodule production and increased nodule cortication, stimulated adventitious root formation at, or near, the water table but killed older submerged roots and nodules, caused the main stem to become hypertrophic just above the water line, accelerated senescence of the lowermost leaves and delayed branch formation. After re-draining the rooting medium nodule growth and symbiotic fixation activity occurred at the expense of rapid recovery by the host plant. The physiological basis for these findings in relation to identifying cowpea cultivars more tolerant of waterlogged soils is discussed.One of a series of papers describing work undertaken in a collaborative project with the International Institute of Tropical Agriculture, Nigeria, sponsored by the U.K. Ministry of Overseas Development.     

Interactive effects of high temperature and elevated carbon dioxide concentration on cowpea [Vigna unguiculata (L.) Walp.]

Limitations in carbohydrate supplies have been implicated as a factor responsible for reproductive failure under heat stress. Heat stress affects two stages of reproductive development in cowpea [Vigna unguiculata (L.) Walp.], and genotypes are available with tolerance and sensitivity to heat during these different stages. The objectives of this study were to determine the responses of these cowpea lines to ambient and elevated [CO₂], under heat stress and optimal temperature, and test whether differences in carbohydrate supplies due to genotypes, CO₂ enrichment and heat stress are associated with differences in sensitivity to heat during reproductive development. Plants were grown in reach-in growth chambers and subjected to day/night temperatures of either 33/20 or 33/30°C, and [CO₂] levels of either 350 or 700 μmol mol^-1. Under intermediate night temperature (33/20°C), all lines set substantial numbers of pods. Under high night temperature (33/30°C) with either ambient or elevated [CO₂], one heat-sensitive line produced no flowers and the other set no pods, whereas the heat-tolerant line abundantly set pods. High night temperature reduced the overall carbohydrate content of the plants, especially peduncle sugars, and caused decreases in photosynthetic rates. The high pod set of the heat-tolerant line, under high night temperature, was associated with higher levels of sugars in peduncles compared with the heat-sensitive lines. The heat-tolerant line accumulated substantial shoot biomass, exhibited less accumulation of starch in leaves, and possibly had less down-regulation of photosynthesis in response to CO₂ enrichment and heat stress than the heat-sensitive lines. Elevated [CO₂] resulted in higher overall carbohydrate levels in heat-sensitive lines (starch in leaves, stems and peduncles), but it did not increase their heat tolerance with respect to flower production or pod set. Heat-induced damage to floral buds and anthers in the sensitive lines was associated with low sugars levels in peduncles, indicating that heat had greater effects on assimilate demand than on leaf assimilate supply. The heat-tolerant line was the most responsive genotype to elevated [CO₂] with respect to pod production under either high or intermediate temperatures.     

Nodulation and symbiotic nitrogen fixation of cowpea (Vigna unguiculata (L.) Walp)

Summary Rhizobium strains CIAT 301, CIAT 79 and SLM 602 were tested and found effective in the nodulation and nitrogen fixation of cowpea cv. MI-35 (Vigna unguiculata (L.) Walp) plants in growth chamber experiments. Fresh weight of nodules increased with plant age initially and stabilized in 20–30 days from planting, followed by a secondary flush of nodule growth after 30 days. Apparent nitrogen fixation per gram nodule fresh weight reached a maximum in 20–30 days after planting and then decreased, even though a flush of new nodules was produced.     

Callose deposition in leaves of cowpea (Vigna unguiculata [L.] Walp.) as a sensitive response to high Mn supply

First macroscopic visible symptoms of Mn toxicity in cowpea (Vegna unguiculata [L.] Walp.) plants grown in solution culture were dark brown spots on the older leaves. Close to these spots, large quantities of substances which fluorescence with aniline blue were deposited, indicating formation of (1,3)-β-glucan (callose). Callose formation in the leaf epidermis was a more sensitive indicator of Mn toxicity than the appearance of macroscopic symptoms, or the Mn concentration in the leaf.     

Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N₂ fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with ¹⁵NH₄ ¹⁵NO₃ to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.     

Pathogenic response of seed mycoflora associated with cowpea,Vigna unguiculata

Cowpea, Vigna unguiculata (L.) Walp. (Fabaceae), has always been an important grain legume in tropical countries and a veritable source of dietary protein for the teeming population of human and livestock. The crop is often attacked and deteriorated by a number of pathogens; many of these are seed-borne fungi, which produce an adverse effect on the growth and development of plants. In this study, the effects of fungi on germinability and seedling health were determined, and the pathogenicity was established. Eleven fungal species were isolated from cowpea seeds which were used as inocula on cowpea. All the tested fungi show a reduction in the germination rate of the cowpea plants, and different types of disease symptoms including reduction in shoot and root lengths were noted for different pathogenic fungi. The symptoms included seed rot, seedling rot, yellowing of leaves, deformation of leaves, etc. Many phytopathogenic fungi penetrate deep into the seeds. These fungi produce enzymes, toxins and metabolites which adversely affect the plant growth and produce characteristic symptoms and cause deterioration of this important protein-rich crop.     

Genetic transformation of cowpea (Vigna unguiculata L.) and stable transmission of the transgenes to progeny

Cowpeas are nutritious grains that provide the main source of protein, highly digestible energy and vitamins to some of the world's poorest people. The demand for cowpeas is high but yields remain critically low, largely because of insect pests. Cowpea germplasm contains little or no resistance to major insect pests and a gene technology approach to adding insect protection traits is now a high priority. We have adapted features of several legume and other transformation systems and reproducibly obtained transgenic cowpeas that obey Mendelian rules in transmitting the transgene to their progeny. Critical parameters in this transformation system include the choice of cotyledonary nodes from developing or mature seeds as explants and a tissue culture medium devoid of auxins in the early stages, but including the cytokinin BAP at low levels during shoot initiation and elongation. Addition of thiol-compounds during infection and co-culture with Agrobacterium and the choice of the bar gene for selection with phosphinothricin were also important. Transgenic cowpeas that transmit the transgenes to their progeny can be recovered at a rate of one fertile plant per thousand explants. These results pave the way for the introduction of new traits into cowpea and the first genes to be trialled will include those with potential to protect against insect pests.     

Maternal inheritance of plant variegation in cowpea, Vigna unguiculata (L.) Walp.

A chimeric plant was observed in the F₂ generation of a cross between a mutant cultivar, Ife BPC, and a germplasm line, TVu 2, in cowpea, Vigna unguiculata (L.) Walp. The chimeric plant had four lateral branches, one of which was intensely variegated, while the others were mostly green with few white sectors. F₃ progeny from the intensely variegated branch of this plant were all variegated, while seed derived from the mostly green branches produced only green progeny. In subsequent generations, the descendants of the variegated branch bred true for the variegated trait, while those of the mostly green branches were also true-breeding for green colour. No pure-green or pure-white shoots were observed in any of the variegated plants examined in this study. Consequently, no pure-green or pure-white seedlings were produced from seeds harvested from the variegated plants. The results of reciprocal crosses between variegated and normal green plants indicate that variegation is inherited in a strictly uniparental maternal fashion. This is the first report of a cytoplasmically inherited mutation affecting foliage colour in cowpea. Received: 10 March 2000 / Accepted: 16 May 2000     

In Vitro toxicity of oil extracted from neem seeds collected from different locations across savanna agro-ecological zones of Nigeria on seed and soil-borne pathogens of cowpea (Vigna unguiculata L. Walp)

Graded extracts of neem seed oil extracted across the savanna agro-ecological zones (AEZs) of Nigeria were tested on mycelial growth of Colletotrichum capsici and Macrophomina phaseolina of cowpea in Nigeria. Minimum inhibition concentrations (MIC) and effectiveness levels of oil from AEZs were determined. Data were subject to ANOVA. Biplot was employed to access variation and interactions among the AEZs. Across the AEZs, fungitoxicity, MIC and effectiveness level varies; concentrations of 0.1 and 1.0% reduced the growth of C. capsici, while M. phaseolina was reduced at 10% concentration. On M. phaseolina, neem oil extracted from Ilorin, Ogbomosho (derived savanna) and Mokwa (southern guinea savanna) performed similarly, while a strong positive association existed between Ogbomosho (derived savanna) and Hadejia (Sahel savanna) samples. On C. capsici, samples from Bida (southern guinea savanna) and Ilorin (Derived savanna) were similar. This result could be a possible link between ecology, biodiversity and toxic principles in plant materials.     

The potential for controlling Megalurothrips sjostedti (Thysanoptera: Thripidae) on cowpea,Vigna unguiculata (L.) Walp flowers using extracts of cashew (Anacardium occidentale L.) products

Abstract

The efficacy of aqueous extracts of cashew, Anacardium occidentale L. products was tested on Legume flower bud thrips on field cowpea in the 1999 and 2000 seasons using nutseed and nutshell at 10% w/v, bark and leaf at 20% w/v. Aqueous extract of cashew nutshell was superior to other cashew products in reducing thrip populations in cowpea flowers and compared favourably with cypermethrin + dimethoate. Pod density per plant was significantly enhanced on cowpea plants sprayed with aqueous extract of cashew nutshell compared to other extracts. Cashew extract contains cardole (a caustic substance) and anacardic acid, which may have been responsible for its insecticidal effect on thrips. This result indicates the potential of cashew products especially the nutshell for integrated management of thrips in legume crops.