Basavaprabhu L. Patil

Light intensity and temperature affect systemic spread of silencing signal in transient agroinfiltration studies

RNA silencing is a sequence-specific post-transcriptional gene inactivation mechanism that operates in diverse organisms and that can extend beyond its site of initiation, owing to the movement of the silencing signal, called non-autonomous gene silencing. Previous studies have shown that several factors manifest the movement of the silencing signal, such as the size (21 or 24 nucleotides) of the secondary small interfering RNA (siRNA) produced, the steady-state concentration of siRNAs and their cognate messenger RNA (mRNA) or a change in the sink-source status of plant parts affecting phloem translocation. Our study shows that both light intensity and temperature have a significant impact on the systemic movement of the silencing signal in transient agroinfiltration studies in Nicotiana benthamiana. At higher light intensities (≥ 450 μE/m(2)/s) and higher temperatures (≥ 30 °C), gene silencing was localized to leaf tissue that was infiltrated, without any systemic spread. Interestingly, in these light and temperature conditions (≥ 450 μE/m(2) /s and ≥ 30 °C), the N. benthamiana plants showed recovery from the viral symptoms. However, the reduced systemic silencing and reduced viral symptom severity at higher light intensities were caused by a change in the sink-source status of the plant, ultimately affecting the phloem translocation of small RNAs or the viral genome. In contrast, at lower light intensities (<300 μE/m(2)/s) with a constant temperature of 25 °C, there was strong systemic movement of the silencing signal in the N. benthamiana plants and reduced recovery from virus infections. The accumulation of gene-specific siRNAs was reduced at higher temperature as a result of a reduction in the accumulation of transcript on transient agroinfiltration of RNA interference (RNAi) constructs, mostly because of poor T-DNA transfer activity of Agrobacterium, possibly also accompanied by reduced phloem translocation.

Artificial microRNA-derived resistance to Cassava brown streak disease

Highlights • amiRNAs were produced for imparting resistance to Cassava brown streak disease.• transgenic plants were produced expressing amiRNAs against CBSV and UCBSV.• amiRNAs targeting conserved sequences of P1, NIb and CP genes were efficacious.• levels of resistance to CBSD correlated with accumulation of detectable miRNA.

Tropical food legumes: virus diseases of economic importance and their control.

Diverse array of food legume crops (Fabaceae: Papilionoideae) have been adopted worldwide for their protein-rich seed. Choice of legumes and their importance vary in different parts of the world. The economically important legumes are severely affected by a range of virus diseases causing significant economic losses due to reduction in grain production, poor quality seed, and costs incurred in phytosanitation and disease control. The majority of the viruses infecting legumes are vectored by insects, and several of them are also seed transmitted, thus assuming importance in the quarantine and in the epidemiology. This review is focused on the economically important viruses of soybean, groundnut, common bean, cowpea, pigeonpea, mungbean, urdbean, chickpea, pea, faba bean, and lentil and begomovirus diseases of three minor tropical food legumes (hyacinth bean, horse gram, and lima bean). Aspects included are geographic distribution, impact on crop growth and yields, virus characteristics, diagnosis of causal viruses, disease epidemiology, and options for control. Effectiveness of selection and planting with virus-free seed, phytosanitation, manipulation of crop cultural and agronomic practices, control of virus vectors and host plant resistance, and potential of transgenic resistance for legume virus disease control are discussed.

Pigeonpea Sterility Mosaic Emaraviruses: A Journey from a Mysterious Virus to Classic Emaraviruses

Sterility mosaic disease (SMD) of pigeonpea (Cajanus cajan (L.) Millsp) first reported in 1931, is an economically most important viral disease, which is endemic to India, Nepal, Bangladesh and Myanmar. SMD was long suspected to be a viral disease, however its causal agent, pigeonpea sterility mosaic virus (PPSMV) was discovered only in 2000. In 2013, the full genome of PPSMV was sequenced and based on the genome organization the virus was assigned to the genus Emaravirus. In 2015, association of another distinct emaravirus in SMD affected pigeonpea was discovered and it was named as pigeonpea sterility mosaic virus 2. As per the latest ICTV classification, these two pigeonpea infecting emaraviruses are renamed and recognised as two different virus species, Pigeonpea sterility mosaic emaravirus 1 and Pigeonpea sterility mosaic emaravirus 2 under the family Fimoviridae of the order Bunyavirales. These two emaraviruses involved in SMD are transmitted in a semi-persistent manner by an eriophyid mite, Aceria cajani Channabassavanna (Acari: Arthropoda). These viruses and its eriophyid vector are highly specific to pigeonpea and its wild relatives. This chapter presents the review of the studies conducted on SMD of pigeonpea and PPSMVs in India.

Current Knowledge of Viruses Infecting Papaya and Their Transgenic Management

Papaya (Carica papaya L.), native to the South American continent, is an important horticultural crop cultivated across the tropical and subtropical regions of the globe. Papaya is rich source of vitamin-C and globally it is ranked fourth in total fruit production, next only to bananas, oranges and mangoes. India is the leading producer of papaya and both India and Brazil put together account for more than 50 % of global papaya production. Multiple pests and pathogens are known to inflict damage to papaya, of which viral diseases are the most damaging ones. Of all the viral diseases, papaya ring spot virus (PRSV) belonging to the Potyviridae family is most important one, followed by the viruses belonging to the Geminiviridae family causing leaf curl disease in papaya. Other viral diseases of papaya are Papaya meleira virus (PMeV), Papaya mosaic virus (PapMV), Papaya lethal yellowing virus (PLYV) and several other viruses are known to infect papaya, but may not be of economical significance. Management of viral diseases in papaya is very crucial to accomplish a good harvest, and of all the management practices, genetic engineering papaya for virus resistance is most promising and successful. The PRSV resistant transgenic papaya varieties “SunUp” and “Rainbow” developed by the University of Hawaii and extensively cultivated in the Hawaii islands of United States is the most successful field application of transgenic technology. Since there is significant sequence variation in the PRSV strains from different parts of the world and many more diverse range of viruses are known to infect papaya, there is an urgent need to develop region specific virus resistant papaya.

Novel Strategies for Engineering Resistance to Plant Viral Diseases

Abstract Viruses are the most significant class of plant pathogens leading to huge economic losses causing significant loss in yield and the quality of almost all the cultivated crop plants. Several cultural practices have been adopted to check crop losses due to the viral diseases. Since it is not possible to control plant viral diseases through application of chemicals, genetically resistant plant varieties are being used as one of the most viable option to control the plant viral diseases. In the recent past, RNA silencing has expanded the horizon of defensive mechanism against exogenous nucleic acids, including viruses. In addition, modifications in the plant DNA at targeted loci in the genome, that is, genome editing, have also become a prominent tool to confer resistance against plant viruses.

Plant Viruses: Evolution and Management

Viruses are very small pathogenic particles made up of nucleoprotein (nucleic acid and protein). The study of plant viruses is so important because they cause diseases to the economically important crops. They cause a great loss to the quality and quantity of the crops. Plant viruses show various types of symptoms such as colour breaking, chlorosis, mottling, vein clearing, vein bending, leaf curl, decrease in size, distorted growth, etc. The plant viruses are very simple and are very host specifi c.

Viral Diseases of Banana and Their Management

Banana (Musa spp.) is one of the most important staple food crops which provide nutrition and income for the millions of farmers worldwide, especially in tropical regions. Asia being the major continent for banana production contributes more than half of the world banana production (101.9 million tonnes). Banana is vegetatively propagated using suckers or through tissue culture plants which grow, mature and fruit without seasonality throughout the year. Viral diseases are considered a major concern for banana production because of their effects on yield and quality as well as limitations to germplasm multiplication and the international germplasm exchange. There are many (about 20) different viruses reported to infect banana worldwide. However, the economically most important viruses are: Banana bunchy top virus (BBTV), Banana streak viruses (BSV), Banana bract mosaic virus (BBrMV) and Cucumber mosaic virus (CMV). Among these, BBTV and BSV are major threats for banana production. Of the two, BSV exist as episomal and endogenous forms and more widely spread worldwide than BBTV, though later is so far most economically damaging virus contributing to a yield reduction of up to 100 %. Due to lack of durable virus resistance in the Musa spp., measures such as phytosanitation, use of virus free planting material, strict regulation on movement of infected planting materials are effective means to control viral diseases in banana. Studies of several decades on the biology, epidemiology, survival, spread, sequence integration into the host genome of banana viruses and their integrated management strategies are summarized in this review.