A method for generating virus-free cassava plants to combat viral disease epidemics in Africa

Effect of sterilants and plant growth regulators in regenerating commonly used cassava cultivars at the Kenyan coast

Cassava is an important root crop whose seed system is undeveloped. Micropropagation of explants in vitro has the potential of addressing the challenge of the unavailability of healthy cassava planting materials. Therefore, the study determined the effect of sterilization and plant growth regulators on cassava explants to produce certified disease-free plants of commonly used cultivars at the coast of Kenya. The apical nodes drawn from the three cultivars of cassava, Tajirika and Kibandameno and Taita, were used as explants. The sterilant sodium hypochlorite (NaOCl) at 5, 10 and 15% and 70% ethanol for 1 and 5 min and sprayed for 20 s were tested for the effect on the explant. Similarly, the effect of BAP (6-Benzyl amino purine) and NAA (1-Naphthalene acetic acid) Plant Growth Regulators (PGRs) each at 0.5, 1 and 5 mg/L under optimal conditions of sterilization was determined. Surface sterilization using 10% NaOCl followed by spraying 70% ethanol for 20 s had 85% initiation on Tajirika whereas 5% NaOCl followed by spraying 70% ethanol for 20 s had 87% and 91% initiation in Kibandameno and Taita cultivars, respectively. In Tajirika, significantly (p < 0.05) high shooting of 68% was from 5 mg/L BAP in MS media whereas approximately 50% rooting was from either 0.5 mg/L BAP or 5 mg/L NAA in MS media. Kibandameno and Taita cultivars had approximately 50% shooting from MS media without PGRs. Kibandameno had >37% rooting from 0.5 to 5 mg/L BAP or NAA in MS media whereas Taita had approximately 50% rooting from 0 to 5 mg/L NAA in MS media. This protocol showed at least 50% success rate of initiation, shooting and rooting as a rapid multiplication regeneration of Tajirika and Kibandameno and Taita cultivar plantlets with little modification of humidity and temperatures in the growth chambers. This protocol requires validation for use in large-scale production of cassava plantlets to alleviate the inadequacy of cassava planting materials among farmers.

CRISPR/Cas technology: Opportunities for phytopathogenic viruses detection

Detection and monitoring of viruses are essential for healthy plants and prosperity. Recent development in CRISPR/Cas system in diagnosis has open an avenue well suited for pathogen detection. Variety of CRISPR associated proteins are being discovered, suggesting array of application and detection strategies in diagnosis. Phytopathogenic viruses are diverse with respect to their nucleic acid compositions, which presents a challenge in developing a single device applicable for almost all viruses. The review describes about the efficient use of CRISPR/Cas Technology in diagnosis, such as SHERLOCK, DETECTR and SATORI. These methods are different in their characteristic to identify specific nucleic acids and processing the detectable signals. These technologies are in their infancy and lot of scope is there to develop commercial kits. Plant tissue culture-based industries, climate control green houses, indoor cultivation facilities etc. has been considered as few examples. This review will be beneficial for researchers seeking to develop detection mechanism based on CRISPR/Cas technology. The outcome in the form of cost-effective detection of viruses will be boon for agro-based industries, which are facing challenges through virus contamination.

Production of Virus-Free Chrysanthemum (Chrysanthemum morifolium Ramat) by Tissue Culture Techniques

Almost all plants in their natural environment are commonly infected by viruses. These viral infections can cause devastating diseases and result in severe yield and economic losses, making viral diseases an important limiting factor for agricultural production and sustainable development. However, these losses can be effectively reduced through the productions and applications of virus-free plantlets. In vitro culture techniques are the most successful approaches for efficient eradication of various viruses from almost all the most economically important crops. Techniques for producing virus-free plantlets include meristem tip culture, somatic embryogenesis, chemotherapy, thermotherapy, electrotherapy, shoot tip cryotherapy, and micrografting. Among them, meristem tip culture is currently the most widely used. Here, we describe a detailed protocol for producing virus-free plantlets of Chrysanthemum morifolium Ramat using tissue culture techniques.

Why has permanent control of cassava brown streak disease in Sub-Saharan Africa remained a dream since the 1930s?

Effective control of ipomoviruses that cause cassava brown streak disease (CBSD) in Africa has remained problematic despite eight remarkable decades (1930-2021) of research efforts. Molecular mechanisms underlying resistance breakdown in genetically improved cassava are still unknown. The vast genetic diversity of cassava brown streak viruses, which is crucial for the improvement of routine reverse transcription polymerase chain reaction (RT-qPCR) assays in CBSD-endemic regions of Africa, is controversial and underrepresented. From a molecular epidemiology viewpoint, this review discusses the reasons for why permanent control of CBSD is difficult in the modern era, even with the presence of diverse in silico and omics tools, recombinant DNA, and high throughput next-generation sequencing technologies. Following an extensive nucleotide data search in the National Centre for Biotechnology Information (NCBI) database and a literature review in PubMed and Scopus, we report that genomic data of 87.62% (474/541) strains of cassava brown streak virus are missing due to poor sequencing capacity in Africa. The evolution dynamics of viral virulence and pathogenicity has not yet been fully explored from the available 67 (12.38%) genomic sequences, owing to poor bioinformatics capacity. Tanzania and Zambia have the highest and lowest disease inoculum pressure, correspondingly. Knowledge gaps in molecular biology and the overall molecular pathogenesis of CBSD viruses impede effective disease control in Africa. Recommendations for possible solutions to the research questions, controversies, and hypotheses raised in this study serve as a roadmap for the invention of more effective CBSD control methods.

The metabotyping of an East African cassava diversity panel: A core collection for developing biotic stress tolerance in cassava

Cassava will have a vital role to play, if food security is to be achieved in Sub-Saharan Africa, especially Central and East Africa. The whitefly Bemisia tabaci poses a major threat to cassava production by small holder farmers in part due to their role as a vector of cassava mosaic begomoviruses (CMBs) and cassava brown streak ipomoviruses (CBSIs). In the present study untargeted metabolomics has been used as a tool to assess natural variation, similarities and attempts to identify trait differentiators among an East African cassava diversity panel that displayed tolerance/resistance to the effects of Bemisia tabaci infestation. The metabolome captured, was represented by 1529 unique chemical features per accession. Principal component analysis (PCA) identified a 23% variation across the panel, with geographical origin/adaption the most influential classification factors. Separation based on resistance and susceptible traits to Bemisia tabaci could also be observed within the data and was corroborated by genotyping data. Thus the metabolomics pipeline represented an effective metabotyping approach. Agglomerative Hierarchical Clustering Analysis (HCA) of both the metabolomics and genotyping data was performed and revealed a high level of similarity between accessions. Specific differentiating features/metabolites were identified, including those potentially conferring vigour to whitefly tolerance on a constitutive manner. The implications of using these cassava varieties as parental breeding material and the future potential of incorporating more exotic donor material is discussed.

In vitro propagation of three mosaic disease resistant cassava cultivars

Background

Cassava is a staple food for over 800 million people globally providing a cheap source of carbohydrate. However, the cultivation of cassava in the country is facing to viral diseases, particularly cassava mosaic disease (CMD) which can cause up to 95% yield losses. With aim to supply farmers demand for clean planting materials, there is need to accelerate the production of the elite cultivars by use of tissue culture in order to cope with the demand.

Methods

Nodal explants harvested from the greenhouse grown plants were sterilised using different concentrations of a commercial bleach JIK (3.85% NaOCl) and varying time intervals. Microshoots induction was evaluated using thidiazuron (TDZ), benzyl amino purine (BAP), and kinetin. Rooting was evaluated using different auxins (Naphthalene acetic acid NAA and Indole-3-butyricacid IBA). PCR-based SSR and SCAR markers were used to verify the presence of CMD2 gene in the regenerated plantlets.

Results

The highest level of sterility in explants (90%) was obtained when 20% Jik was used for 15 min. The best cytokinin for microshoots regeneration was found to be kinetin with optimum concentrations of 5, 10 and 20 μM for Agric-rouge, Atinwewe, and Agblehoundo respectively. Medium without growth regulators was the best for rooting the three cultivars. A survival rate of 100, 98, and 98% was recorded in the greenhouse for Agric-rouge, Atinwewe, and Agblehoundo respectively and the plantlets appeared to be morphologically normal. The SSR and SCAR analysis of micropropagated plants showed a profile similar to that of the mother plants indicating that the regenerated plantlets retained the CMD2 gene after passing through in vitro culture, as expected with micropropagation.

Conclusion

The nodal explants was established to be 20% of Jik (3.85% NaOCl) with an exposure time of 15 min. Kinetin was proved to be the best cytokinins for microshoot formation with the optimum concentration of 5, 10 and 20 μM for Agric-rouge, Atinwewe, and Agblehoundo respectively. The protocol developed during this study will be useful for mass propagation of the elite cassava cultivars.