Inducible resistance to maize streak virus

Maize yields have stagnated in sub-Sahara Africa: a possible transgenic solution to weed, pathogen and insect constraints

Despite major breeding efforts by various national and international agencies, yields for the ~40 million hectares of maize, the major food crop in sub-Saharan Africa, have stagnated at <2 tons/ha/year for the past decade, one-third the global average. Breeders have succeeded in breeding increased yield with a modicum of tolerance to some single-weed or pathogen stresses. There has been minimal adoption of these varieties because introgressing polygenic yield and tolerance traits into locally adapted material is very challenging. Multiple traits to deal with pests (weeds, pathogens, and insects) are needed for farmer acceptance, because African fields typically encounter multiple pest constraints. Also, maize has no inherent resistance to some of these pest constraints, rendering them intractable to traditional breeding. The proposed solution is to simultaneously engineer multiple traits into one genetic locus. The dominantly inherited multi-pest resistance trait single locus can be bred simply into locally adapted, elite high-yielding material, and would be valuable for farmers, vastly increasing maize yields, and allowing for more than regional maize sufficiency. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Maize streak virus research in Africa: an end or a crossroad

The economic importance of the maize streak virus disease to the African maize production dynamic is to be appreciated now more than ever due to the preponderant influence of a changing climate. Continued dependence on a single major-effect quantitative trait locus (QTL) called Msv1 on Chromosome 1 of Maize (Zea mays L.) is not guaranteed to ensure durable resistance to the causal pathogen. With over ten decades of research on the disease and its associated host plant resistance mechanisms, it is pertinent to consider future approaches to attaining durability by looking to the synergistic roles of moderate- and minor-effect QTLs located on other chromosomes so as to facilitate a secure farming system for sub-Saharan Africa. For this review, more than 40 publications relating to maize streak disease research were methodically analysed with about 30% making specific reference to conventional, molecular and transgenic approaches employed in introgressing, maintaining and improving streak resistance in maize. A meta-analysis of mapped QTLs conferring streak resistance was conducted in a bid to reveal any inter-dependence or co-localization of resistant loci and to aid decision-making for marker-assisted breeding. With the changing climatic conditions around the globe, man's preparedness in the event of an epidemic following any evolutionary process in the streak viral genome was determined as insufficient. Modern breeding approaches including gene pyramiding that could be considered in maize breeding programmes to ensure durability for streak resistance were proposed while improving maize for other abiotic stress tolerance, particularly drought.

Localization of Tobacco Yellow Dwarf Virus Replication Using the In Plant Activation (INPACT) Expression Platform

Geminiviruses and their diseases are a considerable economic threat to a vast number of crops worldwide. Investigating how and where these viruses replicate and accumulate in their hosts may lead to novel molecular resistance strategies. In this study, we used the Rep-inducible In Plant Activation (INPACT) expression platform, based on the genome of tobacco yellow dwarf virus (TYDV), to determine where this model mastrevirus replicates in its host tobacco. By developing an infectious clone of TYDV and optimizing its delivery by agroinfiltration, we first established an efficient artificial infection process. When delivered into transgenic tobacco plants containing a TYDV-based INPACT cassette encoding the β-glucuronidase (GUS) reporter, we showed the virus activates GUS expression. Histology revealed that reporter gene expression was limited to phloem-associated cell types suggesting TYDV replication has a restricted tissue tropism.

Maize Lethal Necrosis: An Emerging, Synergistic Viral Disease

Maize lethal necrosis (MLN) is a disease of maize caused by coinfection of maize with maize chlorotic mottle virus (MCMV) and one of several viruses from the Potyviridae, such as sugarcane mosaic virus, maize dwarf mosaic virus, Johnsongrass mosaic virus or wheat streak mosaic virus. The coinfecting viruses act synergistically to result in frequent plant death or severely reduce or negligible yield. Over the past eight years, MLN has emerged in sub-Saharan East Africa, Southeast Asia, and South America, with large impacts on smallholder farmers. Factors associated with MLN emergence include multiple maize crops per year, the presence of maize thrips ( Frankliniella williamsi), and highly susceptible maize crops. Soil and seed transmission of MCMV may also play significant roles in development and perpetuation of MLN epidemics. Containment and control of MLN will likely require a multipronged approach, and more research is needed to identify and develop the best measures.

The pros and cons of GM crops

The year 2015 marked the 20th year of the commercialisation of genetically modified (GM) crops. During the period from 1996 to 2014, the global hectarage of these crops increased 100-fold, making it the fastest adopted crop technology in recent times. The overall economic gains from these crops have been estimated to be USD133.4 billion over the period from 1996 to 2013, and have been divided roughly 50% each to farmers in developed and developing countries. The environmental benefits include contributing to the practice of minimal till agriculture and a decrease in the use of pesticides. But what are the downsides of this technology? In this review I look at some of the problems related to weeds becoming resistant to glyphosate (the main ingredient that is used on herbicide tolerant crops), how these can be overcome and whether glyphosate can cause cancer. I also discuss the problem of insects becoming resistant to the toxins that are used in insect resistant crops and how these are being addressed. I look at what scientists around the world are saying on this topic and then consider GM crops that are in the pipeline of benefit to developing countries and whether any of these are likely to be commercialised in the foreseeable future.

Utilization of engineered resistance to viruses in crops of the developing world, with emphasis on sub-Saharan Africa

Viral diseases in crop plants constitute a major obstacle to food security in the developing world. Subsistence crops, including cassava, sweetpotato, potato, banana, papaya, common bean, rice and maize are often infected with RNA and/or DNA viruses that cannot be controlled with pesticides. Hence, healthy planting materials and virus-resistant cultivars are essential for high yields of good quality. However, resistance genes are not available for all viral diseases of crop plants. Therefore, virus resistance engineered in plants using modern biotechnology methods is an important addition to the crop production toolbox.

Updates in inducible transgene expression using viral vectors: from transient to stable expression

The prospect of economically producing useful biologics in plants has greatly increased with the advent of viral vectors. The ability of viral vectors to amplify transgene expression has seen them develop into robust transient platforms for the high-level, rapid production of recombinant proteins. To adapt these systems to stably transformed plants, new ways of deconstructing the virus machinery and linking its expression and replication to chemically controlled promoters have been developed. The more advanced of these stable, inducible hyper-expression vectors provide both activated and amplified heterologous transgene expression. Such systems could be deployed in broad acre crops and provide a pathway to fully exploit the advantages of plants as a platform for the manufacture of a wide spectrum of products.