Transgenic expression of coat protein gene of Rice tungro bacilliform virus in rice reduces the accumulation of viral DNA in inoculated plants

Control of Plant Viral Diseases by CRISPR/Cas9: Resistance Mechanisms, Strategies and Challenges in Food Crops

Protecting food crops from viral pathogens is a significant challenge for agriculture. An integral approach to genome-editing, known as CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats and CRISPR associated protein 9), is used to produce virus-resistant cultivars. The CRISPR/Cas9 tool is an essential part of modern plant breeding due to its attractive features. Advances in plant breeding programs due to the incorporation of Cas9 have enabled the development of cultivars with heritable resistance to plant viruses. The resistance to viral DNA and RNA is generally provided using the Cas9 endonuclease and sgRNAs (single-guide RNAs) complex, targeting particular virus and host plant genomes by interrupting the viral cleavage or altering the plant host genome, thus reducing the replication ability of the virus. In this review, the CRISPR/Cas9 system and its application to staple food crops resistance against several destructive plant viruses are briefly described. We outline the key findings of recent Cas9 applications, including enhanced virus resistance, genetic mechanisms, research strategies, and challenges in economically important and globally cultivated food crop species. The research outcome of this emerging molecular technology can extend the development of agriculture and food security. We also describe the information gaps and address the unanswered concerns relating to plant viral resistance mediated by CRISPR/Cas9.

Evaluation of virus resistance and agronomic performance of rice cultivar ASD 16 after transfer of transgene against Rice tungro bacilliform virus by backcross breeding

Severe losses of rice yield in south and southeast Asia are caused by Rice tungro disease (RTD) induced by mixed infection of Rice tungro bacilliform virus (RTBV) and Rice tungro spherical virus (RTSV). In order to develop transgene-based resistance against RTBV, one of its genes, ORF IV, was used to generate transgenic resistance based on RNA-interference in the easily transformed rice variety Pusa Basmati-1, and the transgene was subsequently introgressed to rice variety ASD 16, a variety popular in southern India, using transgene marker-assisted selection. Here, we report the evaluation of BC3F4 and BC3F5 generation rice plants for resistance to RTBV as well as for agronomic traits under glasshouse conditions. The BC3F4 and BC3F5 generation rice plants tested showed variable levels of resistance, which was manifested by an average of twofold amelioration in height reduction, 1.5-fold decrease in the reduction in chlorophyll content, and 100- to 10,000-fold reduction in the titers of RTBV, but no reduction of RTSV titers, in three backcrossed lines when compared with the ASD 16 parent. Agronomic traits of some of the backcrossed lines recorded substantial improvements when compared with the ASD 16 parental line after inoculation by RTBV and RTSV. This work represents an important step in transferring RTD resistance to a susceptible popular rice variety, hence enhancing its yield in areas threatened by the disease.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Bacilliform DNA-containing plant viruses in the tropics: commonalities within a genetically diverse group

Plant viruses, possessing a bacilliform shape and containing double-stranded DNA, are emerging as important pathogens in a number of agricultural and horticultural crops in the tropics. They have been reported from a large number of countries in African and Asian continents, as well as from islands from the Pacific region. The viruses, belonging to two genera, Badnavirus and Tungrovirus, within the family Caulimoviridae, have genomes displaying a common plan, yet are highly variable, sometimes even between isolates of the same virus. In this article, we summarize the current knowledge with a view to revealing the common features embedded within the genetic diversity of this group of viruses.

TAXONOMY

Virus; order Unassigned; family Caulimoviridae; genera Badnavirus and Tungrovirus; species Banana streak viruses, Bougainvillea spectabilis chlorotic vein banding virus, Cacao swollen shoot virus, Citrus yellow mosaic badnavirus, Dioscorea bacilliform viruses, Rice tungro bacilliform virus, Sugarcane bacilliform viruses and Taro bacilliform virus.

MICROBIOLOGICAL PROPERTIES

Bacilliform in shape; length, 60-900 nm; width, 35-50 nm; circular double-stranded DNA of approximately 7.5 kbp with one or more single-stranded discontinuities.

HOST RANGE

Each virus generally limited to its own host, including banana, bougainvillea, black pepper, cacao, citrus species, Dioscorea alata, rice, sugarcane and taro.

DISEASE SYMPTOMS

Foliar streaking in banana and sugarcane, swelling of shoots in cacao, yellow mosaic in leaves and stems in citrus, brown spot in the tubers in yam and yellow-orange discoloration and stunting in rice.

USEFUL WEBSITES

http://www.dpvweb.net.

Delay in virus accumulation and low virus transmission from transgenic rice plants expressing Rice tungro spherical virus RNA

Rice tungro, a devastating viral disease of rice in South and Southeast Asia, is caused by the joint infection of a DNA virus, Rice tungro bacilliform virus (RTBV) and an RNA virus Rice tungro spherical virus (RTSV). RTBV and RTSV are transmitted exclusively by the insect vector Green leafhopper (GLH). RTSV is necessary for the transmission of RTBV. To obtain transgenic resistance against RTSV, indica rice plants were transformed using DNA constructs designed to express an untranslatable sense or anti-sense RTSV RNA. Progeny of primary transformants showing low copies of the integrated transgenes and accumulating the corresponding transcripts at low levels were challenged with viruliferous GLH. Three out of four transgenic plant lines expressing untranslatable RTSV RNA in the sense orientation and two out of the four lines expressing an RTSV gene in the anti-sense orientation showed delayed buildup of RTSV RNA over time. Transmission of RTBV from the above lines was reduced significantly.

Engineering cotton (Gossypium hirsutum L.) for resistance to cotton leaf curl disease using viral truncated AC1 DNA sequences

Several important biological processes are performed by distinct functional domains found on replication-associated protein (Rep) encoded by AC1 of geminiviruses. Two truncated forms of replicase (tAC1) gene, capable of expressing only the N-terminal 669 bp (5'AC1) and C-terminal 783 bp (3'AC1) nucleotides cloned under transcriptional control of the CaMV35S were introduced into cotton (Gossypium hirsutum L.) using LBA4404 strain of Agrobacterium tumefaciens to make use of an interference strategy for impairing cotton leaf curl virus (CLCuV) infection in transgenic cotton. Compared with nontransformed control, we observed that transgenic cotton plants overexpressing either N-terminal (5'AC1) or C-terminal (3'AC1) sequences confer resistance to CLCuV by inhibiting replication of viral genomic and β satellite DNA components. Molecular analysis by Northern blot hybridization revealed high transgene expression in early and late growth stages associated with inhibition of CLCuV replication. Of the eight T(1) transgenic lines tested, six had delayed and minor symptoms as compared to nontransformed control lines which developed disease symptoms after 2-3 weeks of whitefly-mediated viral delivery. Virus biological assay and growth of T(2) plants proved that transgenic cotton plants overexpressing 5'- and 3'AC1 displayed high resistance level up to 72, 81%, respectively, as compared to non-transformed control plants following inoculation with viruliferous whiteflies giving significantly high cotton seed yield. Progeny analysis of these plants by polymerase chain reaction (PCR), Southern blotting and virus biological assay showed stable transgene, integration, inheritance and cotton leaf curl disease (CLCuD) resistance in two of the eight transgenic lines having single or two transgene insertions. Transgenic cotton expressing partial AC1 gene of CLCuV can be used as virus resistance source in cotton breeding programs aiming to improve virus resistance in cotton crop.

Engineering pathogen resistance in crop plants: current trends and future prospects

Transgenic crops are now grown commercially in 25 countries worldwide. Although pathogens represent major constraints for the growth of many crops, only a tiny proportion of these transgenic crops carry disease resistance traits. Nevertheless, transgenic disease-resistant plants represent approximately 10% of the total number of approved field trials in North America, a proportion that has remained constant for 15 years. In this review, we explore the socioeconomic and biological reasons for the paradox that although technically useful solutions now exist for providing transgenic disease resistance, very few new crops have been introduced to the global market. For bacteria and fungi, the majority of transgenic crops in trials express antimicrobial proteins. For viruses, three-quarters of the transgenics express coat protein (CP) genes. There is a notable trend toward more biologically sophisticated solutions involving components of signal transduction pathways regulating plant defenses. For viruses, RNA interference is increasingly being used.