Felix Dapare Dakora

Leguminous crops contribute approximately 40–60 million tonnes of biologically fixed nitrogen annually to agricultural soils through symbiotic associations with rhizobia, underpinning the sustainability of diverse cropping systems. However, rising herbicide use, driven by the adoption of herbicide-tolerant crops and intensifying weed resistance, poses significant threats to the integrity of the legume–rhizobia symbiosis, depending on the herbicide type, exposure level, soil conditions, and microbial variability. This review synthesizes possible herbicide-induced disruptions of symbiotic nitrogen fixation, including herbicide impairment of symbiosis through multiple interconnected pathways, such as cytotoxicity to rhizobia and soil microbial communities, inhibition of nod-gene-inducing flavonoid biosynthesis by the host plant, suppression of Nod factor production and signalling by rhizobial disruption of cytoskeletal functioning that enhances the progression of infection thread formation, oxidative stress induction, alterations of auxin–cytokinin homeostasis, and modification of soil physicochemical parameters that can affect rhizobial survival and competitiveness. Though still unclear, laboratory (in vitro), field and glasshouse studies suggest that symbiotic impairment is dose- and time-dependent, species-specific, and strongly influenced by the rhizobial strain and probably the host plant species variety. Mitigation strategies, such as the use of herbicide-tolerant rhizobial strains, organic soil amendments, optimized herbicide application, and integrated weed management frameworks, have shown considerable promise. However, many research gaps exist regarding tropical legume systems and the long-term consequences of herbicide use on the legume–rhizobia symbiosis. Little is known about the direct effects of herbicides on the bacterial symbionts and host plants, as well as how soil type, climate and management/agronomic practices influence herbicide exposure and the effect of herbicide degradation products on rhizobial survival, competitiveness, and symbiotic N₂-fixation. The existence of herbicide-tolerant soybean suggests that not all herbicides are lethally phytotoxic, an aspect that can be exploited for sustainable agriculture.

Herbicide use in legume-based cropping systems raises concerns about potential disruption of the rhizobium-legume symbiosis essential for biological nitrogen fixation (BNF). This study evaluated the effect of herbicide-treated soil inoculum on plant growth, nodulation, and symbiotic nitrogen fixation in cowpea (Vigna unguiculata L. Walp.) and soybean (Glycine max L. Merr.) under glasshouse conditions. Soil samples were collected from paired herbicide-treated (≥3 years of application) and untreated (≥6 years without herbicides) fields across fifteen locations in the Upper West Region of Ghana and used as inocula for cowpea (cv. Kirkhouse) and soybean (cv. Favour) grown in sterilised sand. Results showed that herbicide-treated soils had significantly higher available phosphorus, iron, calcium, zinc, and soil organic carbon than untreated soils (p < 0.01). However, no significant overall differences (p ≥ 0.05) were detected between herbicide-treated and untreated soil inocula for nodule number, N-fixed, or whole-plant dry matter in either legume. Location was the predominant determinant of nodulation and symbiotic effectiveness (p ≤ 0.001). A significant location × soil type interaction for soybean nodulation indicated site-specific herbicide sensitivity among Bradyrhizobium populations. Herbicide type × legume interactions revealed differences in effects, with specific formulations selectively reducing symbiotic parameters in either cowpea or soybean. Cowpea exhibited consistently superior nodulation and nitrogen fixation relative to soybean across all treatments. These findings demonstrate that indigenous rhizobial populations in the study area retain functional symbiotic capacity despite prolonged herbicide exposure, though herbicide-specific and location-dependent effects warrant consideration in integrated weed management strategies for grain legumes.

Leguminous crops contribute approximately 40–60 million tonnes of biologically fixed nitrogen annually to agricultural soils through symbiotic associations with rhizobia, underpinning the sustainability of diverse cropping systems. However, rising herbicide use, driven by the adoption of herbicide-tolerant crops and intensifying weed resistance, poses significant threats to the integrity of the legume-rhizobium symbiosis. This review synthesises current mechanistic and agronomic evidence on herbicide-induced disruption of symbiotic nitrogen fixation. We discuss evidence from studies demonstrating that herbicides impair symbiosis through multiple interconnected pathways, such as cytotoxicity to rhizobia and soil microbial communities, inhibition of flavonoid biosynthesis and Nod factor signalling, disruption of cytoskeletal dynamics that enhance the progression of thread infection, oxidative stress induction, alterations of auxin-cytokinin homeostasis, and modification of soil physicochemical parameters to the detriment of rhizobia spp. Furthermore, we synthesise recent advances from field and greenhouse studies demonstrating that symbiotic impairment is dose- and time-dependent, species-specific, and strongly influenced by rhizobial strain genotype. Additionally, we discuss mitigation strategies, such as herbicide-tolerant rhizobial strains, organic soil amendments, optimised herbicide application, and integrated weed management frameworks, which have shown considerable promise. Nevertheless, research gaps regarding tropical legume systems and long-term evolutionary consequences of herbicides on legume-rhizobia compatibility are highlighted.

The benefits of legume-nitrogen-fixing bacteria symbioses are vital in agricultural systems globally. Cross-infectivity studies are important for identifying rhizobial strains with potential for use as inoculants. The native rhizobial isolates inoculated on different legume species are the first step to determining host range and ecological adaptive traits. This study reports on the water-use efficiency and mineral nutrition of diverse legume species cross-inoculated by native rhizobial isolates from Eswatini, Ghana and South Africa under glasshouse conditions. A portable infrared red gas analyzer was used for water use efficiency. Data from a gas exchange study shows that rhizobial strains can significantly influence the photosynthetic functioning of their host plants. As a result, photosynthetic rates differed depending on bacterial compatibility with the host plant, as well as its symbiotic efficacy. Isolate TUTGmGH2 induced greater accumulation of P, K, Mg, Zn, Cu and Mn in soybean and Winged bean, clearly suggesting that rhizobia do have an influence on the mineral nutrition of their host plants. Therefore, these findings further show that native rhizobial isolates can be manipulated to enhance mineral nutrient uptake, promote growth and development and also produce nutrient-dense food with a low environmental impact globally since rhizobia do have an influence on the mineral nutrition of their host plants.

Legume crops, such as the common bean (Phaseolus vulgaris L.), are significant in many Sub-Saharan African (SSA) countries, including Eswatini, due to their numerous health benefits, including high protein, fiber, vitamins, and mineral content. Common beans are a staple food in many parts of the world and play a crucial role in nitrogen fixation, thereby improving soil fertility. A field experiment was conducted at Malkerns research station, Eswatini, using 27 common bean genotypes to assess their ability for N-fixation and water relations using the ¹⁵N and ¹³C natural abundance techniques. The data revealed significant differences among the common bean genotypes. Genotypes Cim-Rm-36 and Mwctz20a-Rm19 recorded an increase in plant growth by (6% and 5.74%), N content (5.69% and 5.97%) and greater C content (6.1% and 5.67%) while genotype Mwctz20a-Rm19 also showed an increase in N-fixation (155.73 kg.ha⁻¹). Genotype Mwctz20a-Rm-4 had the highest grain yield (1747.39 kg.ha⁻¹), while genotype Cim-Rm-14-Als61 had the highest N concentration (3.50%), indicating efficient N uptake. The genotypes with the lowest δ¹³C values (−27.38‰ to −28.06‰) suggested similar water use efficiency among the genotypes. The findings of this study revealed that common beans can make a significant contribution to N fertility under drought conditions. Genotypes Cim-Rm-36, Mwctz20a-Rm19, and Mwctz20a-Rm-4 showed desirable characteristics and can be good candidates for possible inclusion in breeding programs. These results have implications for improving common bean production in drought-prone areas and promoting sustainable agriculture practices.

Groundnut is an essential legume crop vital for global food security, especially in resource-limited areas. However, poor soil fertility and climate change often hinder crop growth in Africa, resulting in low yields, particularly on smallholder farms. This study systematically evaluates, for the first time in the North West Province of South Africa, the inter-annual variations in symbiotic nitrogen fixation and water use efficiency among four major groundnut varieties during the 2023 and 2024 cropping seasons, using ¹⁵N and ¹³C natural abundance techniques. Significant differences were observed in plant growth, nitrogen content, δ¹⁵N, %Ndfa, N-fixed, soil N uptake, and grain yield across both years. Groundnut varieties derived 35.51% to 85.77% of their nitrogen from symbiotic fixation, contributing 22.13 to 168.48 kg of nitrogen fixed per hectare. The grain yield was much higher (4305.45 to 7465.91 kg ha⁻¹) in 2024 cropping season compared to (514.73 to 1207.95 kg ha⁻¹) in the 2023 cropping season. The shoot ¹³C values for the groundnut varieties ranged from −28.57‰ to −28.19‰ in 2023 and −30.17‰ to −28.34‰ in 2024. Notably, varieties Kwarts (85.77%) and Shubert (82.95% demonstrated higher symbiotic dependency in 2023 cropping season, whiles varieties ARC K6 (106.86 kg ha⁻¹) and PC 474 (164.48 kg ha⁻¹) fixed the most N in the 2024 cropping season, along with better water-use efficiency. These varieties namely Kwarts, Shubert, and PC 474 show promise for inclusion in future breeding programs.

Selecting symbiotic rhizobia for use as inoculants in agriculture is a major challenge, though it is necessary for exploiting biological nitrogen fixation as an eco-friendly source of N in contrast to chemical N fertilizers which can pollute the environment. In addition to high symbiotic efficiency, bacterial strain ability to infect and effectively nodulate a wide range of host plants is also desired. Cross-infectivity studies are therefore important for identifying rhizobial strains that are highly effective with a broad host range. The legume/rhizobia symbiosis has the potential to contribute about 80% or more N to agricultural systems, thus providing a sustainable source of N in cropping systems. This study assessed the cross-nodulation, colony morphology, relative symbiotic effectiveness and N₂ fixation of native rhizobial isolates from Africa that nodulate diverse legume species. The results showed that the rhizobial isolates differed significantly in symbiotic performance and relative symbiotic effectiveness. As a result, they differed markedly in nodulation and shoot DM induced in their host plants.

Cassava (Manihot esculenta) is a vital tropical staple crop with expanding relevance beyond food security, particularly in developing functional beverages and nutraceutical products. This review discusses the implications of selected chemicals in cassava roots for beverage production, notably cyanogenic glycosides and phenolic compounds. We further highlight the role of cassava as a substrate for beverage production, the nutritional significance of cassava-based beverages, and the health benefits and functional potential of cassava as a key ingredient in beverage production. We also discuss the probiotic and prebiotic properties and the antioxidant activity of chemicals in cassava-based beverages for health benefits. Additionally, we review the challenges, opportunities, and innovations regarding commercialization.

The phenylpropanoid pathway remains a key target for most climate-resilient crop development, owing to it being a precursor to over 8000 metabolites, including flavonoids and lignin compounds, including their derivatives. These metabolites are involved in biotic and abiotic stress tolerance, inviting several studies into their roles in plant defense, drought, temperature, UV, and nutrient stress tolerance. Literature is currently inundated with cutting-edge reports on the phenylpropanoid pathways and their functions. Here, we provide a comprehensive update on the biosynthesis of phenylpropanoids, mainly lignin and flavonoids, their roles in biotic and abiotic interaction, and transcending topics, including pest and diseases, drought, temperature, and UV stress tolerance. We further reviewed the post-transcriptional, post-translational, and epigenetic modifications regulating phenylpropanoid metabolism and highlighted their applications and optimization strategies for large-scale production. This review provides an all-inclusive update on recent reports on the metabolism of phenylpropanoids in plants.