Gene pyramiding and omics approaches for stress tolerance in leguminous plants - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 266

converting nitrogen into a bioavailable form. Biological
nitrogen fixation is done both by free-living and symbi-
otic bacteria and it has been estimated that out of all
nitrogen fixed biologically, the contribution by legumes
is approximately 80% (Garg & Geetanjali, 2007). Hence,
this process makes legumes very important plants both
agronomically and economically as they fulfil the need
for food of humans and livestock and also enrich the soil
thereby reducing the need for nitrogenous fertilizers.
Although legumes provide a major source of nitrogen
and also have key roles in agriculture, ecology, animal/
human nutrition and health, their benefits are com-
monly masked by various types of stress affecting the
plants'growth (Graham & Vance, 2003). Multiple stresses,
both biotic and abiotic, are encountered by legumes dur-
ing their life cycle thereby significantly affecting crop
yields. For stable and improved crop yields, it becomes
imperative to develop varieties that are resistant to more
than one stress, and biotechnological approaches are
now key to solving this problem (Kumar et al., 2011).
Fungal diseases are major problems that affect
legumes, but insect pests, nematodes, viruses, bacteria
and parasitic weeds are also reported to adversely affect
legume production (Dita et al., 2006). Powdery mildew
caused by Erysiphe pisi and downy mildew caused by
Peronospora viciae are fungal diseases in legumes that
have been reported to badly affect the yield and quality
of the crop (Sillero et al., 2006). Apart from mildews, rust
diseases also cause significant damage to legume crops,
with species belonging to the genus Uromyces responsible
for major diseases in numerous leguminous plants (Dita
et al., 2006). Many rust species such as U. appendiculatus
on common bean, U. ciceris-arietini on chickpea, U. pisi
on pea, U. viciae-fabae on faba bean, lentil and common
vetch, U. vignae on cowpea, Phakopsora pachyrhizi (caus-
ing Asian rust) and P. meibomiae on soybean or Puccinia
arachidis on groundnut severely affect the growth and
yield of legume crops (Rubiales et al., 2002; Carmona
et al., 2005). The common fungal disease of chickpea,
ascochyta blight, caused by Ascochyta rabiei , results in
maximum yield losses by mostly affecting the above-
ground parts of the plants (Nene & Reddy, 1987). Most
of the common soil-borne diseases of legumes attack
the seedling stage of the crop. These are known as
damping-off diseases; for example, damping-off due to
Rhizoctonia solani or Pythium spp. causes death of the
plant in most cases (Denman et al., 1995; Wang et al.,
2003). Severe seedling loss in common bean occurs due
to root rot caused by Fusarium spp. (Hamwieh et al., 2005).

Viruses such as bean common mosaic virus (BCMV)
and bean common mosaic necrotic virus (BCMNV)
affecting common beans also cause significant yield
losses (Dita et al., 2006). Insects are also one of the biotic
stresses faced by legume plants. These include aphids
such as Aphis glycine , pod borers such as Helicoverpa
armigera and H. punctigera (Yoshida et al., 1997), and wee-
vils such as Apion godmani and Zabrotes subfasciatus (Garza
et al., 1996) are important insect pests in grain legumes.
They may act as vectors, or provide sites of infection to
various pathogens, or directly damage the grain legumes
by feeding on them (Dita et al., 2006).
Abiotic stresses also adversely affect the growth and
productivity of crops. They activate a chain of morpho-
logical, physiological, biochemical and molecular changes
in plants (Mathur et al., 2008). Abiotic stress includes
drought, salinity, waterlogging, high temperature and
chilling, which ultimately lead to crop loss thereby
reducing average yields of major crop plants. Crops fac-
ing abiotic stress are more prone to weeds, insects and
diseases, which further increases the chances of yield
losses (Reddy et al., 2004). Numerous abiotic stresses
related to legume crops also directly affect the symbiotic
interactions between the roots and soil bacteria, thereby
affecting the overall growth of the plant. Waterlogging
results in severe loss in legume yields by limiting dif-
fusion of oxygen through the soil, and consequent
replacement of nitrification by denitrification (Laanbroek,
1990). Salinity and other stresses such as alkalinity and
temperature affect seed germination in legumes (Guan
et al., 2009). Most legumes react to moderate salinity by
a reduction in their growth, which may be by inhibiting
plant cell expansion as a result of osmotic stress or due to
accumulation of Na + and Cl − ions resulting in cell injury
in transpiring leaves and other plant tissues (Manchada &
Garg, 2008). Salinity affects nodule formation and
nitrogen fixation (Verdoy et al., 2004). Salt stress leads to
a reduction in the number of rhizobia that inhabit the
roots (Tu, 1981). It also affects the number and shape of
root hairs and interferes with infection thread formation
(Zahran & Sprent, 1986). This is mainly because nodule
formation, structure, physiology and function are very
sensitive to salt stress, moreso than the host legume or
the rhizobia (Zahran, 1999). Another major abiotic stress
affecting crop yields worldwide is drought. A study of
five cultivars of mung bean ( Vigna radiata L.) subjected to
drought stress revealed that stress significantly affected
flowering phase, plant height, grain yield, number of
grains per pod, number of pods per plant, number of side