Profiling Cultivars Development in Kersting's Groundnut [Macrotyloma geocarpum (Harms) Maréchal and Baudet] for Improved Yield, Higher Nutrient Content, and Adaptation to Current and Future Climates

Kersting's groundnut [Macrotyloma geocarpum (Harms.) Maréchal and Baudet], Fabaceae, is an important source of protein and essential amino acids. As a grain legume species, it also contributes to improving soil fertility through symbiotic nitrogen fixation. However, the crop is characterized by a relatively low yield (≤500 kg/ha), and limited progress has been made so far, toward the development of high-yielding cultivars that can enhance and sustain its productivity. Recently, there was an increased interest in alleviating the burdens related to Kersting's groundnut (KG) cultivation through the development of improved varieties. Preliminary investigations assembled germplasms from various producing countries. In-depth ethnobotanical studies and insightful investigation on the reproductive biology of the species were undertaken alongside morphological, biochemical, and molecular characterizations. Those studies revealed a narrow genetic base for KG. In addition, the self-pollinating nature of its flowers prevents cross-hybridization and represents a major barrier limiting the broadening of the genetic basis. Therefore, the development of a research pipeline to address the bottlenecks specific to KG is a prerequisite for the successful expansion of the crop. In this paper, we offer an overview of the current state of research on KG and pinpoint the knowledge gaps; we defined and discussed the main steps of breeding for KG' cultivars development; this included (i) developing an integrated genebank, inclusive germplasm, and seed system management; (ii) assessing end-users preferences and possibility for industrial exploitation of the crop; (iii) identifying biotic and abiotic stressors and the genetic control of responsive traits to those factors; (iv) overcoming the cross-pollination challenges in KG to propel the development of hybrids; (v) developing new approaches to create variability and setting adequate cultivars and breeding approaches; (vi) karyotyping and draft genome analysis to accelerate cultivars development and increase genetic gains; and (vii) evaluating the adaptability and stability of cultivars across various ecological regions.

Bradyrhizobium Inoculation of Field-Grown Kersting's Groundnut [Macrotyloma geocarpum (Harms) Marechal & Baudet] Increased Grain Yield and N2 Fixation, Measured Using the Ureide, and 15N Natural Abundance Techniques

Here, we report on the symbiotic N2 fixation and water use efficiency (δ13C) of Kersting's groundnut, an important but underutilized grain legume in Africa, in response to inoculation with Bradyrhizobium strains 3267 and CB756. The 15N natural abundance and xylem ureide techniques were used to quantify N2 fixation. The landraces in this study derived variable levels of their N requirements from symbiosis, which translated into marked differences in the amounts of N-fixed by the landrace–inoculant combinations across the test locations. In most instances, the landrace-strain combinations that elicited higher shoot biomass also recorded greater N-fixed and/or ureide-N in xylem sap as well as grain yield. Although some landraces coupled increased grain yield with higher water use efficiency (shoot δ13C), a trait that could be tapped for crop improvement, others recorded lower yields despite eliciting relatively high shoot δ13C values, indicating genotypic variations in adaptation to the different environments. Grain yield of the test landraces showed marked variation and ranged from 131 to 1349.8 kg ha−1 depending on the landrace–strain combination used and the planting location. The high symbiotic dependence by landraces in this study could explain why Kersting's groundnut thrives in the low nutrient soils that are prevalent in its cultivation areas. These results provide more insights into the literature regarding the Kersting's groundnut–rhizobia symbiosis as well as the crop's water use efficiency.

Morphological Variation and Discriminating Traits of Kersting’s Groundnut Accessions

Kersting’s groundnut [Macrotyloma geocarpum (Harms) Maréchal & Baudet] (KG) is a nutritious, subterranean grain legume in West and Central Africa. Only limited information is available on the morphological traits that can discriminate accessions; without such information, appropriate breeding strategies cannot be devised. This study aimed to identify discriminating traits and assess the diversity among accessions of Kersting’s groundnut. Eighty-one KG accessions from Benin and Burkina Faso were evaluated based on 29 qualitative and quantitative traits. An experiment was conducted using an Alpha lattice design with three replications. Standardized Shannon-Weaver index (H') and descriptive statistics were calculated for qualitative traits. Pearson correlation coefficients, stepwise discriminant analysis, principal component analysis, cluster analysis and canonical discriminant analysis were conducted. Results showed that accessions varied greatly based on growth habit (H'= 0.68), flower color (H' = 0.50), seed-eye shape (H' = 0.47), and stem pigmentation (H' = 0.41). Eight quantitative traits, viz., seed width, seed thickness, number of branches per plant, petiole length, days to 50% flowering, number of seeds per pod, pod width, and pod length, were found to significantly discriminate the accessions. Accessions were grouped into three clusters based on quantitative traits. Cluster 1 had accessions with late flowering and good vegetative growth, Cluster 2 contained accessions with high germination percentage and Cluster 3 had accessions with high yield performance. Seed length varied greatly among accessions, thus indicating the potential for improving yield via seed size.

Ecological-niche modeling of genetically distinct populations revealed Kersting’s groundnut [Macrotyloma geocarpum (Harms) Maréchal et Baudet] as a resilient orphan crop to present and future climates

Orphan legume crops play an important role in smallholder farmers’ food systems. Though less documented, they have the potential to contribute to adequate nutrition in vulnerable communities. Unfortunately, data are scarce about the potential of those crops to withstand current and future climate variations. Using Macrotyloma geocarpum as an example, we used genetically informed ecological niche models to explore the role of ecology on the current and future distributions of genetic populations of Kersting’s groundnut. Three main conclusions emerged: i) the models had good predictive power, indicating that M. geocarpum’s distribution was correlated with both climatic and soil layers; ii) identity and similarity tests revealed that the two genetic groups while overlapping, are each, locally adapted and display differences in climate suitability; iii) by integrating the genetic information in niche modeling, niches projections show divergence in the response of the species and genetic populations to ongoing climate change. This study highlights the importance of incorporating genetic data into ENM approaches to obtain a finer information of species’ future distribution, and explores the implications for agricultural adaptation, with a particular focus on identifying priority actions in orphan crops conservation and breeding.

Metabolite Fingerprinting of Kersting's Groundnut [Macrotyloma geocarpum (Harms) Maréchal & Baudet] Seeds Using UPLC-qTOF-MS Reveals the Nutraceutical and Antioxidant Potentials of the Orphan Legume

The identification and subsequent quantification of phenolic compounds in plants is the first step toward harnessing their associated nutritional and health benefits. Due to their diverse phenolic compound compositions, grain legumes are known for their high nutritional and health values. The aim of this study was to assess the inter-variations in chemical composition, phytochemical content, and antioxidant capacity of seed extracts from eight Kersting's groundnut [Macrotyloma geocarpum (Harms) Marechal & Baudet] landraces. The chemical profiles were evaluated using UPLC-qTOF-MS. Total phenolics and flavonoids content were determined by the Folin-Ciocalteu and aluminum chloride methods, respectively. The antioxidant capacities in the forms of DPPH and ABTS were evaluated using spectrophotometric methods. Principal component analysis was used to define similarities/differences between the landraces. Based on untargeted metabolomics analysis, 57 metabolites were identified, with phenolics, triterpenes, fatty acids, and sphingolipids being the most predominant. The results showed that the black seeded KG1 (Puffeun) had the highest total phenolic (9.44 mg GAE/g) and flavonoid (3.01 mg QE/g) contents, as well as antioxidant capacity (9.17 μg/mL and 18.44 μg/mL based on DDPH and ABTS assays, respectively). The concentrations of ferulic acid hexoside, procyanidin B2, eryodictyiol-7-rutinoside and quercetin pentoside ranged from 51.78–441.31, 1.86–18.25, 3.26–13.95 to 5.44–63.85 μg/mg, respectively. This study presents a useful report on the phytochemical characterization of Kersting's groundnuts and shows that the grains can be used as a source of nutraceuticals for human consumption.

Genetic Diversity and Population Structure in a Regional Collection of Kersting’s Groundnut (Macrotyloma geocarpum (Harms) Maréchal & Baudet)

Kersting’s groundnut is an important source of protein and essential nutrients that contribute to food security in West Africa. However, the crop is still underexploited by the populations and under-researched by the scientific community. This study aimed to investigate the genetic diversity and population structure of 217 Kersting’s groundnut accessions from five origins using 886 DArTseq markers. Gene diversity was low and ranged from 0.049 to 0.064. The number of private alleles greatly varied among populations (42–192) and morphotypes (40–339). Moderate to very high levels of selfing and inbreeding were observed among populations (s=56–85%, FIS=0.389–0.736) and morphotypes (s=57–82%, FIS=0.400–0.691). Moreover, little to very high genetic differentiations were observed among populations (0.006≤FIS≤0.371) and morphotypes (0.029≤FIS≤0.307). Analysis of molecular variance partitioned 38.5% of the genetic variation among and 48.7% within populations (P<0.001). Significant isolations by distance were detected between populations (R2=0.612, P=0.011) and accessions (R2=0.499, P<0.001). Discriminant analysis of principal components and neighbour joining consistently distinguished eight distinct clusters. These data provide a global picture of the existing genetic diversity for Kersting’s groundnut and will guide the choice of breeding strategies to increase production.