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VI. Limitations to Development of Root Growth Models
MODELING CROP ROOT GROWTH AND FUNCTION
mental and endogenous factors, and genetics are only beginning to be
studied. Nevertheless, models serve the useful purpose of focusing clear
attention on deficiencies in our knowledge and, along with field and laboratory investigations, are an important part of the progress being made in
understanding root growth and function.
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ADVANCES IN AGRONOMY. VOL. 44
GENETIC MANIPULATION OF THE
COWPEA (Vigna unguiculata [L.]
Walp.) FOR ENHANCED
RESISTANCE TO FUNGAL
PATHOGENS AND INSECT PESTS
A. 0. Latunde-Dada
Department of Crop Production
College of Agricultural Sciences
Ogun State University
Ogun State, Nigeria
I . Introduction
Ill. Fungal Pathogens
IV. Tissue Culture Technology
A. Somaclonal Variation and Cellular Selection
B. Embryo Culture
C. Somatic Hybridization
D. Genetic Transformation
V. Conclusions and Epilogue
The cowpea, Vigna unguiculata (L.) Walp., is perhaps the most important papilionaceous grain legume in the Third World, particularly Africa.
Although conjectural claims to the contrary abound (Vavilov, 1951 ;
Steele, 1976; Zhukovskii, 1962, Piper, 1913; Sauer, 1952), the dominant
worldview is that the crop originated in West Africa, probably in the
subhumid savanna grasslands of Nigeria, the area of its greatest diversity
(Faris, 1965; Rawal, 1975; Lush and Evans, 1981; Ng and Marechal, 1985).
It is, however, agreed that the cowpea was domesticated about 4000 years
ago from the wild progenitors V . unguiculata ssp. dekindtiana vars. dekindriana (in sub-Sahelian West Africa) and mensensis (in the humid and
Copyright B 1990 by Academic Press. Inc.
All rights of reproduction in any form reserved.
A. 0. LATUNDE-DADA
subhumid zones) and distributed thereafter throughout sub-Saharan
Africa, reaching the Middle East by 2000 B.C. and India by 1500 b . c . (Lush
and Evans, 1981; Steele, 1976). The Indian cultigroups of V. unguiculata
ssp. unguiculata, namely, Biflora (the catjang bean) and Sesquipedalis (the
yardlong or asparagus bean), arose probably by selection from the early
cowpea domesticates (Faris, 1965). The crop is believed to have been
introduced into Mediterranean Europe before 300 B.C. and reached the
Americas only recently, during the Great Slave Trade of the sixteenth and
seventeenth centuries A.D. (Steele, 1976). The cowpea is today cultivated
throughout sub-Saharan Africa, Southeast Asia, Latin America, and
the United States (Fig. 1). Nigeria is the world's largest cowpea producer, with production levels ranging between 40% (Rachie, 1985) and
75%(Cobley and Steele, 1977) of the world's output.
The cowpea plant is a herbaceous annual with habits ranging from
determinate to completely indeterminate types. The stem is nonpubescent
and may be erect, semierect, prostrate, or twining. The inflorescence is a
simple raceme borne on an elongated axillary peduncle on which the
flower opening is acropetal. The pod, a legume, contains seeds that are
arranged in a single row within the pod wall. The taproot system develops
both secondary and tertiary roots along its length, all of which are capable
of nodulation, hence symbiotic nitrogen fixation. The mutualistic interaction between the cowpea root and soil bacteria of the genus Rhizobium
results annually in an estimated production level of 198 kg/ha of nitrogen,
a level higher than that of Arachis hypogaea (124 kg/ha) or Glycine max
+ dispersal rouln
FIG. 1. Major cowpea-growing areas of the world, with West Africa as the center of
GENETIC MANIPULATION OF THE COWPEA
(103 kg/ha) (Nutman, 1975). The cowpea plant by this feat not only provides for its own nitrogen requirements but also contributes to the replenishment of nitrogen in tropical soils, and assists in cutting the everincreasing capital outlay on nitrate fertilization when intercropped with
tropical cereals. The cowpea plant is drought tolerant.
Vigna unguiculata is diploid with a chromosome number of 2n = 22. It
belongs to the tribe Phaseoleae of the subfamily Papilionoideae. It is
classified further into the subtribe Phaseolinae and group Phaseolastrae.
The genus Vigna is pantropic, comprising 7 subgenera and about 170
species (Cobley and Steele, 1977). The taxonomy of the cowpea at levels of
taxon below the generic appears complex.
Marechal’s approach to the classification (Marechal e t al., 1978;
Marechal, 1982) is given in Table I . Sexual incompatibility barriers exist in
the Phaseolinae and so far interspecific crosses are yet to be reported with
Vigna unguiculata (Baudoin and Marechal, 1985). Nevertheless, these
authors regard V . neruosa (section Catiang) and V . frutescens (section
Liebrechtsia) to be close enough to introgress with the cowpea: no hybrid
has been reported between these close relatives and the cultivated cowpea. All members of the cultigroups of V . unguiculata ssp. unguiculata are
interfertile and also sexually compatible with the wild varieties of the
subspecies dekindtiana (Lush and Evans, 1981; Ng and Marechal, 1985).
In Nigeria, the cowpea is cultivated solely for its seed, which is harvested dry and eaten either boiled or processed into different dish forms.
The seed contains about 25% (w/w) protein (Bressani, 1985) and constitutes an important and cheap source of protein for most people in West
Africa, especially the poor. Moreover, the cowpea seed, compared with
some other pulses, i s relatively low in toxic substances and antimetabolites. These qualities have for long promoted its acceptability among the
peoples of Nigeria. The cowpea crop is well adapted to the savanna
grasslands of Nigeria. About 80% of the country’s output comes from this
region (Emechebe, 1981). In this region, it is cultivated either solely or in
mixtures with cassava and such cereal crops as maize, sorghum, and
millets. Sole crops of cowpea are more commonly cultivated in the southern humid Rain Forest Belt, where, in spite of an annual rainfall pattern
and distribution that can theoretically support three crops of cowpea, only
20% of the Nigerian output is produced. Figure 2 shows the attempts made
at the Ogun State University, Ago-Iwoye, Nigeria (about 50 km south of
Ibadan, and in the Rain Forest Belt of Nigeria), to produce three crops of
cowpea over two consecutive seasons (A. 0. Latunde-Dada, unpublished
Cowpea production, in the Rain Forest Belt of Nigeria and elsewhere in
Africa is limited by a number of constraints. The grain output from most
A. 0. LATUNDE-DADA
Taxonomy of the Cowpea, Vignu unguiculutu Walp."
V . unguiculata
Related taxonomic forms
( b ) Plectotropis
(a) V . neruosa
(b) V . frutescens (in section Liebrechtsia)
(a) V . unguiculata ssp. unguiculata spp. srenophylla
(b) V . unguiculata spp. tenuis
(c) V . unguiculata spp. dekindtianab
(a) BIFLORA = CYLINDRICA
(b) SESQUIPEDALIS (asparagus or yardlong bean)
Sources: MarCchal et a / . (1978); Markchal, (1982).
Vigna unguiculata ssp. dekindtiana comprises the following varieties: (a) dekindtiana,
(b) pubescens, (c) mensensis, and (d) protracta.
local varieties is between 100 and 300 kg/ha (IITA Research Briefs, 1984).
Apart from the obvious genetic inadequacies (such as extreme viny growth
habit, compulsive photoperiodism, low flowering and pod-setting abilities,
and low yields) of these local varieties, other problems abound. These
include susceptibility to insect pests, lack of resistance to viral, bacterial,
and fungal diseases, lack of tolerance of excessive moisture levels, weed
infestation, and inadequate soil nutrient supply. These problems have
been reviewed elsewhere (Summerfield et al., 1974; Thottappilly and
Rossel, 1985; Muleba and Ezumah, 1985; Poku and Akobundu, 1985).
GENETIC MANIPULATION OF T H E COWPEA
FIG.2. Annual rainfall distribution pattern for Ago-lwoye. Horizontal arrows represent
periods during which cowpea (vars. TV x 3236, Ife Brown, IT82E-60, and IT81D-994)
was cultivated. During period 1, plants were largely free from foliar pathogens and insect
pests; pods required additional artificial drying, however. During period 11, the incidence
of Ootheca mutabilis and Aphis cracciuoru was high and damage due to web blight, Pythium
stem rot, and anthracnose was severe in the susceptible varieties. Pods were picked compulsorily when light yellow and were dried artificially. Web blight was the greatest challenge,
especially in Ife Brown and IT83E-60, during period 111. Additional drying was unnecessary,
and yields were highest during this period.
Advances have been made in the various ramifications of cowpea research
during the past two decades to ameliorate the gloomy situation just presented. The achievements of two agricultural research institutes situated
in the Rain Forest Belt of Nigeria are worth considering.
The Institute of Agricultural Research and Training (IART) of the
Obafemi Awolowo University of Ibadan was responsible for earlier
research during the 1960s that led to the development and release of
the varieties Westbred and Prima, and later Ife Brown and Ife Bimpe
(Franckowiak et al., 1973; Ojomo, 1975). The varieties Ife Brown and Ife
Bimpe are short, determinate, and early, flowering within 35 days of
sowing. When given adequate care, Ife Brown yields about 1700 kg/ha
(Singh and Ntare, 1985) but lacks demonstrable resistance to insect pest
and disease problems.
Since its inception in 1967, the International Institute of Tropical Agriculture (IITA), also in Ibadan, has virtually taken the lead in Nigeria and
indeed international cowpea research. The VITA series of cowpea varieties released originally by IITA in the 1970s have since been supplanted in
A. 0. LATUNDE-DADA
Nigeria by such popular IITA varieties as TV X 3236, IT82E-60, IT82D716, IT84E-124, and lately IT84S-2246-4. IITA continues to draw on the
rich genetic diversity of the cowpeas and has succeeded in a seemingly
endless release of lines and varieties that combine high-yielding characteristics with multiple pest and disease resistance (Table 11), and without any
apparent impairment of the seed protein levels. The modern IITA varieties
of cowpea grown in the Rain Forest Belt are day-neutral, early, determinate, short, and erect and do not require staking. Moreover, they have
peduncles that elongate rapidly during the early reproductive phase to
display the maturing pods high up and above the leaf canopy. These pods
mature uniformly, thereby facilitating synchronous harvesting. In general,
the potential yield of these IITA varieties is 2 t/ha; nevertheless, there are
plans and projections afoot to develop varieties with yield potentials in the
Some IITA Cowpea Varieties Bred for Multiple
Resistance to Disease and Insect Pests”
Insect pest resistance‘
TV x 3236
a Sources: “Varietal Improvement of Cowpea.”
IITA Crop Production Training Series. Singh, S. R.
1986. Trop. Grain Legume Bull. 32, 10-24.
Varieties I , 2,3,4,5,7,8,9. 10, and 12 possess
multiple resistance to diseases. IT81D-I 137, for instance, is resistant to a total of 1 1 fungal, bacterial,
and viral diseases. TV x 3236 and IT84E-124 are
resistant to at least 7 such diseases.
A, B, C, and D represent, respectively, leaf
hoppers, aphids, thrips, and bruchids; + denotes
resistance; a blank space denotes lack of resistance
to the insect pest.
GENETIC MANIPULATION OF THE COWPEA
region of 6 tlha (IITA Research Briefs, 1984). However, it must be stated
that this projected target may be rather overambitious for a field crop such
as cowpea growing for 60-odd days.
A yield potential of between 3 and 4 t dry seed/ha may be more feasible.
The attainment of even this lower yield requires, on the one hand, a
reevaluation of the nagging problems of insect pests and fungal diseases
peculiar to the various ecological zones. Second, it also requires the
adoption of a technology that in consonance with conventional plant
breeding methods used so far at IITA seeks to both increase the genetic
diversity of the cowpea and improve the selection of desirable characteristics. These two considerations are explored in this chapter.
II. INSECT PESTS
Eight out of the 31 insect pest species identified in the cowpea by Singh
and Jackai (1985) constitute the major preharvest entomological pest problem in the Rain Forest Belt of Nigeria. These insects are:
1. The cowpea leaf beetle, Ootheca mutabilis (Shalberg), which, though
a sporadic pest, may cause total crop loss through foliage destruction. It is
also an important virus vector.
2. The cowpea bud thrip, Megalurothrips sjostedti (Trybom) = Taeniothrips sjostedti (Trybom), which may cause total crop loss through
interference with flowering.
3. The cowpea aphid, Aphis craccivora (Koch), which causes up to 35%
damage when pods are infested and also transmits a number of virus
4. The cowpea pod borer, Maruca testulalis (Geyer), the caterpillars of
which may cause 60% or more damage when coincident with those of
Lasperyresia ptychoru (Meyrick), a minor lepidopterous pest; and the
following cowpea coreids or pod suckers:
5. Acanthomia tomentosicollis (Stalberg) = Clavigralla tomentosicollis
6. Acanthomia horrida (Germar) = Clavigralla shadabi (Dolling)
7. Anocplocnemis curvipes (Fabricius)
8. Riptortus dentipes (Fabricius)
These coreid bugs feed massively on young developing pods and foliage of
cowpea and may cause up to 60% yield loss. Figure 3 illustrates the
activities of these pests in relation to the phenology of the crop.
Sources of resistance have been identified within the massive collection
A. 0. LATUNDE-DADA
FIG.3. Spans of activity of the major preharvest insect pests of cowpea, V. unguicduta,
in relation to the crop’s phenology. Shaded areas of bars denote span of peak activity.
Source: Singh, S. R. 1980. In “Biology of Breeding for Resistance to Arthropods and
Pathogens in Agriculture Plants” (M. K. Harris, ed.), Texas A & M Univ. Bulletin, MP-1451,
p. 398-42 1 .
of cowpea germplasm (about 12,000 accessions) at IITA for thrips
( M . sjostedti), aphids ( A . cracciuora),and the major harvest insect pest,
Callosobruchus maculatus (Fabricius), a curculionid beetle. These
resistance genes have been bred successfully in different combinations
into such cowpea lines as TVx3236, IT81D-1020, IT82D-716, IT81D-994,
and IT84E-124. The list of IITA varieties with insect pest resistance is
given in Table 11.
Sources of resistance to Ootheca mutabilis, Maruca testulalis, and the
coreid pod suckers are as yet unidentified. For these pests, chemical
control by insecticide applications remains the only pragmatic means of
protection. Given the medium-sized farms replete in an underdeveloped
country such as Nigeria, and the current depressingly low economic fortunes of the country, the rising costs of farm inputs continue to cut rather
deeply into the farmer’s profit margin. Moreover, the prices of such insecticides as Cymbush Sherpa, Decis, and Thiodan, which are prominent
items on the IITA’s production package for cowpeas, are high enough to
edge these all-important chemicals out of the reach of the small-time
farmer and thus depress his yield. In a virtually insect-pest-susceptible
variety like Ife-Brown, this could mean total crop loss, and even in IITA
advanced breeding lines a fourfold reduction in yield (IITA Research
Briefs, 1984). For this class of farmer, efforts that are aimed at introducing
GENETIC MANIPULATION OF THE COWPEA
into existing cowpea varieties and lines effective and durable resistance to
these destrutive insect pests remain the only panacea to the problem. Such
research must aim at introducing, for example, genes for antibiosis, nonpreference mechanisms as well as leaf pubescence for the control of
Ootheca mutabilis, genes for antibiosis for the control of Maruca testulalis, and genes for scabrous and pubescent pods for the control of the
coreid pod suckers. These may be achieved, theoretically, by attempting
crosses between the cowpea and other closely related Vigna species (such
as V . uexillata) as well as wide crosses with the more distantly related
genera of the Phaseoleae. The existence of incompatibility barriers has
been highlighted earlier.
Ill. FUNGAL PATHOGENS
Fungal diseases account for losses in the cowpea surpassed only by the
damage due to insect pests (Emechebe and Shoyinka, 1985). Out of these,
anthracnose and brown blotch (caused by Colletotrichum lindemuthianum
[Sacc. & Magn.] Bri. & Cav., and C . capsici [Syd.], respectively), leaf
spots (incited by Cercospora canescens Ell. & Mart., and Pseudocercospora cruenta [Deighton] Sacc.), web blight (caused by Rhizoctonia
s o l d [hill. & Delacr.] Bourd & Galz), and wilts (caused by Fusarium
oxysporum f.sp. tracheiphilum Schlect, ex Fr. and Sclerotium rolfsii
Sacch.) have been identified as the most damaging on cowpea in the Rain
Forest Belt of Nigeria (Emechebe and Shoyinka, 1985). Sources of resistance to some of these diseases have been identified in the IITA germplasm
accessions and varieties and lines such as TVx3236, IT82E-716, IT82D699, IT83D-326-Z, IT8 1 D- 1 137, IT835-1 1, and IT8 1D- 1020 have been bred
and selected from multiple resistance to various fungal diseases. More
significantly, these lines combine this characteristic with degrees of insect
pest resistance and high grain yield potentials also.
Resistance to web blight, a destructive foliar disease, remains largely
unresolved. This disease is caused by the imperfect fungus Rhizoctonia
solani (with Thanatephorus cucumeris [Frank] Donk. as its basidiomycete
teleomorph) and is a major problem in cowpea production in the humid
Rain Forest Belt. No cowpea variety is wholly resistant to this disease, to
this author’s knowledge. The development and deployment of an effective
and durable means of protection through breeding methods is therefore
Although descriptive studies abound on the biology and control of a
number of the fungal diseases mentioned (Williams 1975; Emechebe, 1981;