Introduction
The Green Revolution was driven by the development of novel cultivars along with government subsidised fertilizer inputs, mechanization and irrigation and has dramatically increased crop and farm productivity (Pingali 2012). These changes required massive and coordinated investment across public and private institutions. While most of the world enjoyed significant increase in agricultural productivity, Africa and in particular its sub-Saharan parts, maintained the same degree of productivity (Johnson, Hazell & Gulati 2003). Recent surveys on agriculture practices in six sub-Saharan countries (Niger, Nigeria, Ethiopia, Malawi, Tanzania and Uganda) covering over 62,000 plots estimated that only 1 to 3 % of the lands cultivated by smallholders are irrigated and no more than 10% of the households have water control on their field (Sheahan & Barrett 2017). For instance, 41% in Nigeria, 17% in Niger and 3.2% in Uganda of the cultivated plots used inorganic fertilizer, while 84% of the total studied area did not use agro-chemicals (pesticides, herbicides, fungicides and insecticides; Sheahan & Barrett 2017). Although policy makers and donors are supporting efforts to stimulate a Green Revolution in Africa, it is unlikely that it will take the same forms of the first one because of the limited irrigated land and fertilizer resources (phosphorous (P) in particular) exacerbated by climate change and soil degradation (Bailey-Serres, Parker, Ainsworth, Oldroyd & Schroeder 2019; Gionfriddo, De Gara & Loreto 2019).
In Africa, efforts to deal with low fertility soils are restricted by the challenges associated with transporting and applying massive amounts of material to dispersed and inaccessible farms. In contrast, improving agricultural practices and developing new cultivars of key food crops can have a substantial impact on food security, income production and agro-ecosystem dynamics while minimizing expenditure (Lynch 2007; Fess, Kotcon & Benedito 2011; Gemenet et al. 2016; Joshi et al.2016). For this, identification of useful phenes (“phene” is to phenotype and “gene” is to genotype; York, Nord & Lynch 2013) and phene combinations useful for crops grown in low-input agroecosystems and their integration into breeding programs is of major importance. Foremost among the challenges is developing and deploying phenotyping tools in these environments, understanding GxE interactions and generating truly integrative phenotyping and selection approaches that ultimately increase yield and small-holder incomes (Reynolds et al. 2021).
Root architectural and anatomical phenes that increase the efficient acquisition of soil resources, as defined by carbon investment per resource gained, are potentially valuable selection targets. These traits can improve crop tolerance to the main primary constraints in the low-input agroecosystems of Africa, namely water and phosphorus scarcity (extensively reviewed in Lynch 2018, 2019; Schneider & Lynch 2020). However, trade-offs for specific traits have been identified due to the contrasting spatial and temporal dynamics of these two resources. For example, shallow root growth promotes topsoil foraging and P acquisition, while deep root growth promotes subsoil foraging and water acquisition (Ho, Rosas, Brown & Lynch 2005; Lynch 2011). Root ideotypes (target root phenotypes) for agroecosystems in Africa also need to consider agricultural practices. The implementation of new sustainable approaches for water saving and promotion of soil fertility through the use of beneficial root-soil microorganisms interactions is another potentially fruitful option.
In this review, we will illustrate how breeding for root traits could improve crop adaptation and resilience in low-input African agroecosystems subject to climate change using three case studies. We then discuss how these traits and innovations could be validated, made available to breeders and agronomists and finally adopted by farmers.