Global food security requires a major re-focusing of plant sciences, crop improvement
and production agronomy towards grain legumes (pulse crops) over coming decades, with
intensive research and development to identify climate-resilient species and cultivars
with improved grain characteristics. Labs contributing to this special issue have
undertaken research and breeding to improve pulse crops, together with innovative
production agronomy which contributes to the sustainability of cropping systems. The
reviews and research together form an invaluable resource for the research community
and policymakers.
The value of pulses in food cultures around the world is well known, and calling them
‘little marvels’ is apt (BBC Radio 4 Food Programme, broadcast in the UK, July 2016),
not least because of their significant health benefits (Foyer et al., 2016) (Box 1).
However, the use of legumes in agriculture and the genetic improvement of important
grain legumes have lagged behind cereal crops. The Food and Agriculture Organization
of the United Nations (FAO) facilitated the International Year of Pulses in 2016,
focusing on the contribution of pulses to production and dietary diversity to eradicate
hunger and malnutrition. This initiative was introduced with objectives to (i) promote
the value and utilization of pulses throughout the food system, (ii) raise awareness
of their benefits, (iii) foster enhanced research, (iv) advocate for better utilization
of pulses in crop rotations, and (v) address challenges in trade. The FAO initiative
was also linked to a growing recognition of the contribution of pulses to critical
targets under Sustainable Development Goal 2, particularly regarding food access,
malnutrition and smallholder incomes, as well as sustainable and resilient agriculture.
Box 1.Diverse market classes within each grain legume (pulse) species
Image: Pixabay, CC0 Public Domain.
Recognizing that increasing the global production of grain legumes has the potential
to provide a sustainable solution to food and protein security, significant efforts
are currently being made to increase genomic resources and apply innovative breeding
techniques to improve the yield and nutritional quality of legume crops, together
with enhanced resilience to climate change. Production agronomy and crop rotation
approaches could also be intensified to address the associated economic and environmental
challenges. The papers presented in this special issue bear testimony to the urgent
need for the intensification of basic and applied research into grain legumes, which
will form a cornerstone of future food and nutritional security and a global web of
biodiversity.
Improved genomic resources and breeding tools
Several papers in the issue highlight the development of genetic resources that will
unleash significant untapped potential for genetic improvement. The development of
effective phenotyping and breeding approaches is a challenge for the less-well studied
grain legumes in particular. Modern breeding efforts to improve yield, disease resistance
and quality are constrained by a low level of genetic diversity in breeding programmes.
Large genetic diversity exists in seeds of grain legumes held in gene banks, but these
are not fully used in active breeding programmes. Cowling et al. (2017) explore the
concept of evolving gene banks, applying optimal contribution selection to manage
long‐term genetic gain and genetic diversity in pre-breeding populations. They simulated
pre-breeding using a founder population based on crosses between elite crop varieties
and exotic lines of field pea, subjecting the population to 30 cycles of recurrent
selection for an index comprising four economically important traits. They conclude
that optimal contribution selection provides the control necessary to actively improve
evolving gene banks for economic traits, while maintaining high levels of genetic
diversity. This revolutionary plant breeding system will allow breeders to access
valuable genes that have been lost through modern breeding programmes. The plant breeding
method described by Cowling et al. captures valuable genes from wild relatives and
moves them into the breeding programme by crossing the genetically diverse exotic
lines with elite lines, creating evolving gene banks. The new rapid-cycle plant breeding
method will have long-term benefits for all plant breeders, and could help to adapt
and develop climate-ready crops, but the immediate challenge is to validate the results
in commercial pulse crops.
As one of the five major crops, soybean is the most widely planted and highest-yielding
grain legume (Foyer et al., 2016). The comprehensive overview of current genomic resources
from functional sequences to epigenomics provided by Li et al. (2017) discusses the
value of improving the soybean resilience to different climate change scenarios. High-throughput
genomic technologies including genome sequencing, genome re-sequencing (DNA-seq) and
transcriptome sequencing (RNA-seq) are being applied to a range of legumes. New insights
into the giant faba bean (Vicia faba) genome are provided by Cooper et al. (2017)
who used a combination of DNA-seq and RNA-seq to improve genomic resources in soybean.
Using RNA-seq analysis, Du et al. (2017) present interesting new data on the regulatory
networks that control seed set and seed size in soybean and identify hub genes that
control these processes.
Adapting to climate change through crop resilience
The genetic and biotechnology resources that are currently being applied to drought
and water-logging are described by Valliyodan et al. (2017), within the context of
existing QTLs and breeding approaches. The effects of terminal drought leaf parameters,
seed set and pod abscisic acid concentrations are reported in chickpea (Pang et al.,
2017). Moreover, the crucial importance of root trait variability and its role in
facilitating stress tolerance in chickpea is reported (Chen et al., 2017). The depletion
of soil water often brings the added burden of salt stress to limit plant productivity.
The beneficial effects of sucrose infusion at the reproductive stage of chickpea production
are presented by Khan et al. (2017), together with evidence that salt-stressed chickpea
is carbon-limited. Hence the provision of sucrose improves vegetative and reproductive
growth in plants exposed to high salt (Khan et al., 2017). Another study in this issue
presents novel findings showing that a drought-responsive legume, miR1514a, triggers
phasiRNA formation through modulation of a NAC transcription factor (Sosa-Valencia
et al., 2017).
The mechanisms that underpin drought tolerance in legumes are further elaborated by
an innovative analysis of root xylem plasticity and its role in improving water use
efficiency in soybean plants subjected to water stress (Prince et al., 2017). These
and other papers in this special issue not only highlight the importance of the availability
of water to legume agriculture, but also demonstrate that both drought and flooding
pose some of the greatest challenges to the current and future production of soybean
and forage legumes (Striker and Colmer, 2017). Their comprehensive analysis of the
diversity in forage legumes for flooding tolerance will be of particular interest
to those engaged in gaining a deeper understanding of the physiology of stress tolerance
in legumes. It is also of practical use to researchers and agronomists engaged in
forage plantings in flood-prone areas. Data for some key species are provided, in
which current eco-physiological understanding is limited. Suggestions for future areas
of priority in this important group of plants include the central importance of understanding
anoxia tolerance in roots, the ability to maintain symbiotic nitrogen fixation during
waterlogging in the field, and identification of traits conferring the ability to
recover after water levels subside (Striker and Colmer, 2017).
Different aspects of reproductive physiology are described in well-considered and
thought-provoking reviews by Cao et al. (2017) and Ozga et al. (2017). Cao et al.
examine the dependence of soybean flowering and stem growth habits on day length,
highlighting the interplay between photoperiod and miRNA-mediated flowering modules
in soybean. Meanwhile, Ozga et al. cast the net beyond soybean to explore how hormones
integrate high-temperature stress during reproductive development in grain legumes,
from meiosis to flowering, fruit set and seed maturation. Moreover, a gene expression
atlas is described for pigeon pea (Pazhamala et al., 2017), together with the application
of this knowledge to deduce novel information regarding the genes associated with
pollen fertility and seed formation.
Some grain legume species such as faba bean and pigeon pea (Cajanus cajan) have outcrossing
characteristics, and rely to some extent on pollination by animal vectors. Climate
change and associated extreme weather events such as sudden episodes of high temperature
during flowering can affect reproductive success in grain legumes, directly through
physiological damage and indirectly by affecting plant–pollinator interaction. Bishop
et al. (2017) report a substantial increase in the level of outcrossing in faba bean
by insect pollinators following heat stress both in a controlled environment and under
field conditions. Stoddard, in his Insight article, discusses the ability of faba
bean to self-pollinate in the absence of bee activity, known as ‘autofertility’ (Stoddard,
2017). It is argued that reliance on wild pollinators is a risky strategy when also
affected by climate change. Thus the provision of honey bees may be increasingly required
for adequate pollination of faba bean crops in the future.
Neglected orphan crops
As a protein staple in the diet of many of the world’s poorest, pulses are nature’s
gemstones because they are protein packed and nutritious. However, in many cases relatively
little is known about the biology of the plethora of ‘orphan’ legume crops that contribute
to human and animal diets. Cullis and Kunert (2017) address the issue of under-utilized
grain legumes directly, highlighting the considerable but neglected opportunities
to advance grain legume agriculture by developing the existing germplasm. They recognize
the investments of organizations such as the International Crops Research Institute
for the Semi-Arid Tropics (ICRISAT), International Centre for Agriculture Research
in the Dry Areas (ICARDA) and the Kirkhouse Trust, as well as initiatives to develop
genomic data for these underused crops, but emphasize the need for crop physiology
and production agronomy to support future crop development. Moreover, Kim and Cullis
(2017) describe the chloroplast genome of marama (Tylosema esculentum) for the first
time.
Interactions with the soil microbiome
Many unique functions of grain legumes take place beneath the soil. These plants offer
the promise of more sustainable use of nitrogen fertilizers in crop systems through
their ability to fix atmospheric nitrogen in symbiotic root nodules. The process of
bacteroid infection in legume roots that culminates in the formation of symbiotic
nodules is summarized by Ibañez et al. (2017). Highlighting the differences between
root-hair entry and intercellular invasion, these authors explore the evolution of
this process. Nodulation, however, is suppressed in soils replete with nitrogen, for
example in farms with a long history of nitrogen fertilization. This phenomenon is
explored by Murray et al. (2017), who provide a comprehensive overview of emerging
knowledge on nitrogen sensing in legumes and explore the complexity of physiological
and molecular signalling and responses. With a key focus on the role of nitrate and
other transporters in sensing of nitrogen availability, Murray et al. (2017) consider
how the signalling activities of such transporters might influence nodulation.
Crops of ancient origin come of age
The papers in this special issue describe, consider and discuss the state of our developing
knowledge and understanding of important topics within legume biology, as well as
identifying key areas where more knowledge is urgently needed such as an absence of
comprehensive genomic information and intensive breeding efforts. In the past 50 years,
global cereal production has almost tripled while pulse production has only increased
by about 60% (Foyer et al., 2016). The relatively low rate of yield improvement in
grain legumes relative to cereals can be explained, at least in part, by the low genetic
diversity in grain legume breeding programmes (Cowling et al., 2017). Legume crops
are also currently under-used compared to cereals in cropping systems, and yet intercropping
and rotation of grain legumes with cereals or other crops have many benefits, such
as enhanced crop yield, increased nitrogen-use efficiency, and reduced occurrence
of plant disease. Intercropping with grain legumes is now seen as a vital mechanism
for vertical intensification, as well as facilitating an increase in biodiversity
and creating a more diverse landscape for animals and insects.
The importance of legumes in current and future agriculture cannot be overemphasized.
Moreover, grain legumes form a minor part of current human diets, yet they are a vital
source of plant-based protein and amino acids for people around the world, a versatile
ingredient in human diets with a long shelf-life. The FAO recommends that they are
eaten daily as part of a healthy diet to prevent and manage chronic disease, and to
address growing global obesity issues (fao.org/pulses-2016). Prepare for more from
these little marvels in the future.