Beans

The common bean ( Phaseolus vulgaris L.) is one
of the most important legume crops grown in all
continents of the world except Antarctica,
because of its high protein, fi ber, and complex
carbohydrate content. Beans ( Phaseolus spp.) are
extremely diverse crops in terms of cultivation
methods, in diverse environments and elevations
ranging from sea level to 3,000 m height,
morphological variability, and utilities (dry as
pulse and green as vegetable). The tremendous
variability for plant types and growth behavior
makes beans part of the most diverse production
systems of the world (Broughton et al. 2003 ).
They are cultivated in monoculture, in associations, or in rotations. Dry beans are consumed
both as pulse and vegetable (both leaves and
pods). Their genetic resources exist as a complex
array of major and minor gene pools, races, and
intermediate types, with occasional introgression
between wild and domesticated types. Beans are
thus a crop that is adapted to many niches, both in
terms of agronomic and consumer preference.
While its production is mainly centered on small
holdings, beans are grown in monoculture bush
beans to a complex association of indeterminate
or climbing beans with maize, sugarcane, and
other cereals or fruit crops. Short bush growth
habits offer minimal competition and permit
interplanting with other species, for example, in

reforestation projects or among fruit trees or
coffee plantations during the early years until the
main crop can be exploited. At the other extreme,
aggressive climbers are found at higher altitudes
on subsistence farms where a few plants are
maintained as a sort of insurance and are continually harvested for about 6 months. Over the past
20 years, beans have also been increasingly cultivated on a commercial scale and are now offered
in national, regional, and international markets.
The common bean regarded as a “grain of hope”
is an important component of subsistence agriculture grown worldwide over an area of about
28.78 million hectares with an annual production
of 23.14 million tonnes (FAO 2013 ), and feeds
about 300 million people in the tropics and 100
million people in Africa alone. Besides it is
emerging as an important income generation
especially in Central America where beans have
top ranking as income generators among fi eld
crops. In terms of global pulse production, the
common bean alone with 23 million tonnes
accounts for about half of the total pulse production (FAO 2012 ). However, this number is an overestimate as FAO does not report data for Phaseolus
and non -Phaseolus species separately. Many comprehensive reviews on the different aspects of the
common bean like origin and evolution (Gepts
1998 ), genetic diversity (Singh et al. 1991a , b , c ;
Blair et al. 2012a , b ), population structure and evolutionary dynamics (Zizumbo-Villarreal et al.

2005 ), biotic and abiotic stresses (Miklas et al. 2006b ;
Singh and Schwartz 2010 ), breeding for insect
pests (Singh and Schwartz 2011 ), genomics
(Gepts et al. 2008 ), genetic transformation
(Kwapata et al. 2012 ), etc., have been published
from time to time. In this chapter, an attempt has
been made to compile the information on various
aspects of the common bean from various sources
to provide an overview about this important
legume to the readers. The different sections consist of common bean origin, gene pool, evolutionary relationship and systematics, gene flow, gene
fl ow constraints, diversity in germplasm,
production- limiting factors, important traits for
broadening the genetic base, conventional and
contemporary approaches for gene transfer, and
the impact of molecular advances for the improvement of production and productivity of this
important legume crop.

Crop Gene Pool, Evolutionary Relationship,
and Systematics
The existence of archaeological, botanical, and
historical evidences indicated that the common
bean is a New World crop that originated
~7,000 years back in a long arc from the presentday northern Mexico (Chihuahua) through
Central America and the Andes mountains to
northwest Argentina (San Luis) (Broughton et al.
2003 ). Wild beans fi rst described in Argentina
(Burkart 1941 ; Burkart and Brucher 1953 ) and in
Guatemala (MacBryde 1947 ) are grown in a wide
arc stretching from north Mexico (approx. 30°N)
to northwestern Argentina (approx. 35°S) at an
altitude ranging from 500 to 2,000 m, experiencing annual rainfall of 500–1,800 mm. The two
taxonomic subdivisions of Phaseolus have been
described as P. vulgaris var. aborigineus and
P. vulgaris var. mexicanus (Delgado-Salinas
1985 ) based on morphological and molecular
differentiation. There is no accurate record of the
transition of wild beans to cultivated beans due to
gaps in the archaeological record, but this event is
estimated to occur somewhere between 7,000
and 5,000 years ago (Broughton et al. 2003 ).

The individuals of this important legume are
now well characterized on the basis of
morphological and molecular traits into two gene
pools: the Andean originating from the Andes
mountains of South America (southern Peru,
Bolivia, and northern Argentina) and
Mesoamerican from Central America and Mexico
with at least seven races (Gepts et al. 1986 ;
Koenig and Gepts 1989a , b ; Singh et al. 1991a , b , c ;
Becerra Velásquez and Gepts 1994 ; Tohme et al.
1996 ; Beebe et al. 2000 , 2001 ; Díaz and Blair
2006 ; Blair et al. 2007 ; Kwak and Gepts 2009 ).
Important parameters separating individuals into
different gene pools include seed size, phaseolin
(seed storage protein) patterns, plant morphology, isozymes, and RFLP, RAPD, AFLP, and
microsatellite markers (Evan 1976 ; Gepts et al.
1986 ; Koenig and Gepts 1989a , b ; Singh et al.
1991a ; Khairallah et al. 1992 ; Becerra Velásquez
and Gepts 1994 ; Blair et al. 2006a , 2007 , 2010 ,
2012a , b ). The morphological and agroecological
adaptation traits have been further used to distinguish the races among these gene pools (Singh
et al. 1991a , b ; Gepts 1988 ). Race structure
studies of Singh et al. ( 1991a , b , c ) demonstrated
that members of each race have distinctive and
specifi c physiological, agronomic, biochemical,
and molecular characteristics and diverge from
other races in the allelic frequencies at specifi c
loci. Domestication is an outcome of the selection process that leads to increased adaptation of
plants to cultivation and utilization by humans.
During domestication, plants and animals are
subjected to natural as well as human selection.
The traits that distinguished the domesticated
forms from wild forms are collectively called as
domestication syndrome (Hammer 1984 ). These
traits include lack of seed dispersal and dormancy, compact plant architecture, higher yield,
a synchronicity and early fl owering, photoperiod
sensitivity, harvest index and seed pigmentation
(Gepts and Debouck 1991 ; Koinange et al. 1996 ).
The domesticated common bean originated from
wild gene pools through independent domestication events (Gepts and Debouck 1991 ; Kwak and
Gepts 2009 ; Kwak et al. 2009 ) at two centers
of domestications, i.e., Mesoamerica (western
Mexico) and southern Andes of South America.