Case study 1: improving phosphorus acquisition efficiency in
common bean in South East Africa
Common bean (Phaseolus vulgaris ) has critical food security,
economic and agroecosystem value throughout South East Africa
(Wortmann, Kirkby,
Eledu & Allen 1998; Beebe 2012). It is fundamental for food security at
the household level, as well as economically important at the household,
village and regional level. Furthermore, common bean has a key
agroecosystem role as a nitrogen (N) fixer. Bean in South East Africa is
grown either as a sole crop or intercropped with maize, in both cases
usually in rotation with the latter. The primary constraints to
production in typical low-input small-holder and low-external input
cropping systems in SE Africa include a) low phosphorus (P) availability
in highly N fixing soils, b) terminal and/or intermittent drought, and
c) limited root growth and elevated root mortality due to acid soil
complexes, pathogens and insects
(Lynch 2019).
These challenges are further complicated by irrigation systems,
fertilizers and other soil amendments being inaccessible due to cost and
transportation limitations.
In Mozambique, researchers from the Mozambican Agricultural Research
Institute (IIAM) along with collaborators at the International Centre
for Tropical Agriculture (CIAT) and The Pennsylvania State University
targeted root traits to develop novel varieties that substantially
improved small-holder crop production and benefits to local
agroecosystems. In this context, increasing P acquisition efficiency,
defined by the ratio of carbon investment to P recovered, was identified
as a key cross-cutting element because it impacts multiple aspects of
plant productivity and agroecosystem function
(Lynch & Ho
2005). For instance, shallow root growth has an advantage for the
acquisition of soil resources with greater availability in shallow soil
zones, particularly P. Greater P acquisition is in turn related to
increased shoot growth, biological nitrogen fixation and yield. With a
larger shoot biomass shielding the soil, erosion is also reduced
(Henry, Kleinman
& Lynch 2009; Henry, Chaves, Kleinman & Lynch 2010).
Towards breeding of novel common beans for P acquisition efficiency in
Mozambique, greater basal root whorl number and longer and denser root
hairs traits were specifically targeted. These traits affect the zone of
soil explored (Fig. 1a), they can be easily and rapidly phenotyped in
the field with resources readily available in less developed countries
(Burridge, Jochua,
Bucksch & Lynch 2016), modifications of these traits do not require any
change to the cropping system or farmer practices, and they have
multi-scale impact. Basal roots are a class of roots specific to dicots
with epigeal germination and emerge from the transition zone between
radicle and hypocotyl
(Burridge,
Rangarajan & Lynch 2020). In common bean, basal roots emerge in whorls
composed of four individual roots, due to its tetrarch vascular
patterning (Fig.
1b; Miguel, Widrig, Vieira, Brown & Lynch 2013). Basal root whorl
number (BRWN) is under genetic control and ranges from one to five, with
two being the mode
(Miguel et
al. 2013). It can be phenotyped using low cost root roll-ups five to
nine days after germination (Fig. 1b) and is associated with field
phenotypes and field performance
(Strock et
al. 2019; Jochua, Strock & Lynch 2020). Greater BRWN increases the
vertical zone of soil explored, enabling the efficient exploration of
both deep and shallow soil. It may also afford a degree of root
redundancy that ensures at least a few roots will survive pathogen or
insect attack to contribute to both deep and shallow resource
acquisition
(Strock et
al. 2019).
Root hairs are subcellular protrusions emerging from specialized
epidermal cells on all root classes
(Bhosale et
al. 2018). Root hair length and density are under genetic control and
have been related to greater P acquisition in multiple crops and
experimental systems
(Ma, Walk, Marcus
& Lynch 2001; Yan, Liao, Beebe, Blair & Lynch 2004; Hanlon et
al. 2018). Root hairs have minimal construction and maintenance costs
making them an extremely efficient means to increase root volume and
acquire immobile soil resources such as P
(Lynch 2019).
They are relatively easy to phenotype at the seedling stage using a
low-cost root roll-up protocol
(Vieira, Jochua
& Lynch 2007). Substantial synergy between greater root hairs and basal
root growth angle have been observed in common bean in which the
advantage of longer and denser root hairs is magnified when paired with
shallow basal root growth
(Miguel, Postma
& Lynch 2015). Taken together, the shallower basal root growth angle
and longer and increased density of root hairs traits makes ideal
selection targets for improving P acquisition efficiency (Fig 2A).
The selection strategy for improved P acquisition efficiency varieties
involved extensive phenotyping of varieties and breeding lines in order
to identify accessions possessing high BRWN as well as long and dense
root hairs
(Burridgeet al. 2019). These superior lines were then crossed with common
varieties either already grown in Mozambique or to varieties with
similar and desirable agronomic and market characteristics, such as seed
colour and size. Several early generations of these lines were grown at
the experimental stations with non-limiting conditions to select them
for adaptation, grain quality, shoot architecture, disease and pest
tolerance. Subsequently, seeds from F4 plants were evaluated for key
root hair phenes with those possessing longer and denser root hairs
being advanced. Targeted field-based selection of varieties took place
first on multiple stations across Mozambique with single constraints,
such as low soil phosphorus or terminal drought. Simultaneously,
selections were made on more remote stations and farmer’s fields with
multiple, interacting constraints. A final set of 40 varieties were
included in a round of on-farm participatory variety selection using a
tricot model in which each of many individual farmers received a random
pack of three varieties
(van Ettenet al. 2019). As a result, three new varieties of common bean,
named Kufuna, Tiyela and Matina, have been released in Mozambique.
Trials on station and on farmer’s field suggests these lines have yields
roughly double the yield of local varieties when grown with or without P
fertilizer (Table 1). This is a noteworthy achievement, in part because
no new varieties had been released in Mozambique for over 12 years, and
none of those were bred specifically for Mozambique.
To further promote and study how new varieties are shared and impact
yield, food security and local economies, a pilot promotional campaign
was organized in three different regions and compared to three regions
that received the improved seed but not the promotional campaign (Fig.
1c&d). These improved varieties of local adapted and accepted material
are available directly from IIAM, as well as from multiple private
companies. Stocks of breeder and pre-basic seed are maintained by IIAM
and are distributed to companies to produce basic seed. Village level
farmer organizations receive pre-basic seeds to produce certified seed
lots. In the 2019-2020 season, approximately 11 tons of seed was
produced, 6.1 tons of which went to seed companies and the rest to
farmers either directly or via various programs, including farmer
organizations producing certified seed. In the 2020-2021 season,
approximately 45 tons of seed is expected to be produced by IIAM. At
typical planting rate of 80 kg/ha, 45 tons of seed converts to 3,600
hectares of improved P-efficient seed that will be planted by farmers in
the 2021-2022 season, up from zero in the 2018-2019 season. This mixed
public, private and farmer organization model promotes the maximum
penetration of the new varieties and promotes long-term sustainability
for both companies and farmer organizations. Other varieties of common
bean are in various stages of the pipeline with some nearly ready for
release and others in on-farm trials.
The accomplishments in Mozambique demonstrate the utility of a selection
strategy involving trait-based selection of seedling root phenes, which
was followed by on-station trials targeting specific abiotic constraints
and soil types and then on-farm trials with multiple biotic and abiotic
constraints and the use of local practices. Similar systems have proved
successful and continue to offer great potential
(Humphrieset al. 2015; Ryan et al. 2018; van Etten et al.2019). Other key legumes, namely cowpea, groundnut and chickpea have
very similar food security, economic and agro-ecosystem roles across
broad swaths of Africa. Lessons from common bean can be in large applied
to these other legumes due to similar root system, phenology as well as
seed saving, sharing and buying systems.