Conclusion
We propose two main root phene-based strategies to accelerate the
development of new cultivars better adapted to low-input environments in
Africa.
The first is to identify simple phenes that have broadest positive
influence on enhanced performance and that also minimize trade-offs.
Towards this, long and dense root hairs are likely selection criteria as
greater root hair length and density promotes exudation and have
positive effects on the beneficial microbial activity within the
rhizosphere
(Rongsawat,
Peltier, Boyer, Véry & Sentenac 2021). Additionally, root plasticity
phenes could be another potential selection criteria as for topsoil root
branching plasticity could be beneficial upon partial dry-down (for rice
in AWD agroecosystems) while subsoil root plasticity could be beneficial
during prolonged drought stress particularly in the reproduction and
grain filling stage (for sorghum and pearl millet in arid and semi-arid
agroecosystems).
The second major strategy is to understand and target phene synergisms
and integrated phenotypes. Synergisms between root phenes are defined as
interactions that have more than additive effect, as in the case of long
and dense root hairs paired with shallow root system architecture for P
acquisition
(Miguel et
al. 2015). Integrated phenotypes would clearly affect the utility of
selecting for a single component phene without selecting for their
complementary phenotypes. For example, the utility of high conductance
capacity xylem likely depends on root phenes that affect rooting depth
since deep roots can access and thus transport greater volumes of soil
water (Strock,
Burridge, Niemiec, Brown & Lynch 2020). The development of root
structural and functional models for crops such as sorghum or pearl
millet that can evaluate the effects of architectural and anatomical
phenes in changing soil environments and its effects on root functions
will be particularly useful
(Ndour, Pradal &
Lucas 2017a). Other less well characterized phene assemblages,
especially those involving transpiration, should be further investigated
and validated in particular stress scenarios
(Strock et
al. 2019; Klein, Schneider, Perkins, Brown & Lynch 2020). Considering
resource acquisition and use, especially that of water within the
context of phenology, leads to acknowledging the importance of
interactions among roots and shoots for timely water use across the crop
cycle (Vadez,
Kholova, Medina, Kakkera & Anderberg 2014). In that regard, combining
root models with crop models could potentially link above-ground phenes
to root phenes, the former serving as a proxy for root function
(Benes et
al. 2020).
In order to make these innovations readily available to breeders,
researchers and breeders need to work together to validate phene
utility, develop phenotyping protocols including field sites, the type
of genetic material to work with (RILs, NILs, tester lines, germplasm
deployment strategy) and ultimately identify marker or genes controlling
beneficial phenes. As in the case of common bean presented in case study
1, identifying a selection strategy, including the type of phene for the
appropriate stage, would be particularly useful. To maximize deployment
of improved cultivars and to then secure the adoption of those improved
cultivars, social scientists and farmers should be integrated in the
selection process
(Ameleworket al. 2016). The inclusion of useful root phenes in such
approaches may help to stimulate a sustainable Green Revolution in
Africa.