The Cell Membrane of a Novel Rhizobium phaseoli Strain Is the Crucial Target for Aluminium Toxicity and Tolerance

Soils with low pH and high aluminium (Al) contamination restrict common bean production, mainly due to adverse effects on rhizobia. We isolated a novel rhizobium strain, B3, from Kenyan soil which is more tolerant to Al stress than the widely used commercial strain CIAT899. B3 was resistant to 50 µM Al and recovered from 100 µM Al stress, while CIAT899 did not. Calcein labeling showed that less Al binds to the B3 membranes and less ATP and mScarlet-1 protein, a cytoplasmic marker, leaked out of B3 than CIAT899 cells in Al-containing media. Expression profiles showed that the primary targets of Al are genes involved in membrane biogenesis, metal ions binding and transport, carbohydrate, and amino acid metabolism and transport. The identified differentially expressed genes suggested that the intracellular γ-aminobutyric acid (GABA), glutathione (GSH), and amino acid levels, as well as the amount of the extracellular exopolysaccharide (EPS), might change during Al stress. Altered EPS levels could also influence biofilm formation. Therefore, these parameters were investigated in more detail. The GABA levels, extracellular EPS production, and biofilm formation increased, while GSH and amino acid level decreased. In conclusion, our comparative analysis identified genes that respond to Al stress in R. phaseoli . It appears that a large portion of the identified genes code for proteins stabilizing the plasma membrane. These genes might be helpful for future studies investigating the molecular basis of Al tolerance and the characterization of candidate rhizobial isolates that perform better in Al-contaminated soils than commercial strains

The significance of Napier Grass Stunt Phytoplasma and Its transmission to cereals and sugarcane

Production of Napier grass, Pennisetum purpureum, the most important forage crop in Africa, is severely constrained by Napier grass stunt (NGS) disease. Wild grasses have been identified as alternative hosts for NGS, but there is no record regarding the ability of this phytoplasma to infect cultivated food crops. To assess this threat, six crop species were placed around phytoplasma-infected Napier grass with the vector Maiestas banda (Hemiptera: Cicadellidae) in transmission cages for a period of 30 days. The crop plants were observed for 3 months after removal from the cages for disease development. Polymerase chain reaction based on the conserved 16S gene, primed by P1/P6-NapF/NapR nested primer sets, was used to diagnose phytoplasma in test plants, before and after transmission experiments. Pennisetum purpureum showed the highest infection level (81.3%), followed by Saccharum officinarum (56.3%), Eleusine coracana (50%), Sorghum bicolor (43.8%), Oryza sativa (31.3%) and Zea mays (18.8%). All the test plants remained symptomless except sugarcane that exhibited mild to moderate symptoms comprising yellow leaves and bright white or yellow midribs. Napier grass, however, was stunted with small yellow leaves. The study showed that food crops could be infected, suggesting their possible role as sources of inoculum for NGS and their contribution in the spread of the disease in regions where these crops are cultivated. This implies an additional disease cycle could occur for NGS disease, highlighting the challenges to development and implementation of management strategies for the disease

Napier grass stunt disease in East Africa: farmers' perspectives on disease management

Napier or Elephant grass (Pennisetum purpureum), the most important livestock fodder crop in East Africa, is under threat from Napier grass stunt (NGS) disease. This disease is caused by a phytoplasma, which is transmitted by the leafhopper, Maiestas banda (Hemiptera: Cicadellidae). After inoculation, the disease rapidly infects the whole plant causing extensive damage to Napier grass plants. There is therefore a need to develop an integrated management approach for the disease. A survey was conducted in three East African countries (Kenya, Uganda and Tanzania) during which 198 farmers were interviewed using semi-structured questionnaires. The questionnaire addressed the prevalence of NGS, source of planting material and Napier grass cultivars grown. Farmers were also questioned on how they perceived the severity of the disease, management options and on the knowledge of the existence of wild grass hosts of the disease and its vector. Disease prevalence in survey areas was expressed as a percentage of the total 198 fields assessed. Chi-square (χ2), F tests and one-way analysis of variance (ANOVA) were conducted to asses any differences between districts, gender and education levels with regards to the knowledge and perceptions of NGS in the three countries. The prevalence of NGS ranged from 33% in Uganda to 95.7% in Kenya with an average of 55.1%. Among the farmers interviewed, 49.5% were able to recognize NGS symptoms. Most farmers did not have effective management approaches for the disease but cited a variety of measures including roguing and introduction of alternative fodder grasses that could potentially form part of an integrated management approach for the disease. Sedge grass (Cyperus sp.) and Star grass (Cynodon dactylon) were listed by the respondents as the likely hosts of stunt diseases caused by phytoplasma. It has been long suspected that weeds could play a role in the spread of phytoplasma by acting both as reservoirs from which healthy plants could be re-infected and also as hosts for the vectors. Since there is no well-established control method for NGS, the majority of farmers uproot infected plants and replant with new ones to lower the infection pressure by providing fewer infected plants for vectors to feed on. This pinpoints the need for farmers' awareness and the need to develop resistant Napier grass cultivars as a management option

Competitive Plant-Mediated and Intraguild Predation Interactions of the Invasive Spodoptera frugiperda and Resident Stemborers Busseola fusca and Chilo partellus in Maize Cropping Systems in Kenya

Following its recent invasion of African countries, fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera: Noctuidae), now co-exists with resident stemborers such as Busseola fusca (Lepidoptera: Noctuidae) and Chilo partellus (Lepidoptera: Crambidae) causing severe damage to maize crops. Due to niche overlap, interspecific interactions occur among the three species, but the mechanisms and degree remain unclear. In this study, we assessed plant-mediated intraspecific and interspecific interactions, predation in laboratory and semi-field settings, and larval field occurrence of S. frugiperda and the two stemborer species. Larval feeding assays to evaluate competitive plant-mediated interactions demonstrated that initial S. frugiperda feeding negatively affected subsequent stemborer larval feeding and survival, suggesting induction of herbivore-induced mechanisms by S. frugiperda, which deters establishment and survival of competing species. Predation assays showed that, at different developmental larval stages, second–sixth instars of S. frugiperda preyed on larvae of both B. fusca and C. partellus. Predation rates of S. frugiperda on stemborers was significantly higher than cannibalism of S. frugiperda and its conspecifics (p < 0.001). Cannibalism of S. frugiperda in the presence of stemborers was significantly lower than in the presence of conspecifics (p = 0.04). Field surveys showed a significantly higher number of S. frugiperda larvae than stemborers across three altitudinally different agroecological zones (p < 0.001). In conclusion, this study showed that the invasive S. frugiperda exhibited a clear competitive advantage over resident stemborers within maize cropping systems in Kenya. Our findings reveal some of the possible mechanisms employed by S. frugiperda to outcompete resident stemborers and provide crucial information for developing pest management strategies for these lepidopteran pests