Effects of the mutation of selected genes of cotton leaf curl Kokhran virus on infectivity, symptoms and the maintenance of cotton leaf curl Multan betasatellite

Dominance of Cotton leaf curl Multan virus-Rajasthan strain associated with third epidemic of cotton leaf curl disease in Pakistan

Cotton (Gossypium hirsutum) is an economically potent crop in many countries including Pakistan, India, and China. For the last three decades, cotton production is under the constant stress of cotton leaf curl disease (CLCuD) caused by begomoviruses/satellites complex that is transmitted through the insect pest, whitefly (Bemisia tabaci). In 2018, we identified a highly recombinant strain; Cotton leaf curl Multan virus-Rajasthan (CLCuMuV-Raj), associated with the Cotton leaf curl Multan betasatellite-Vehari (CLCuMuBVeh). This strain is dominant in cotton-growing hub areas of central Punjab, Pakistan, causing the third epidemic of CLCuD. In the present study, we have explored the CLCuD diversity from central to southern districts of Punjab (Faisalabad, Lodhran, Bahawalpur, Rahimyar Khan) and the major cotton-growing region of Sindh (Tandojam), Pakistan for 2 years (2020-2021). Interestingly, we found same virus (CLCuMuV-Raj) and associated betasatellite (CLCuMuBVeh) strain that was previously reported with the third epidemic in the central Punjab region. Furthermore, we found minor mutations in two genes of CLCuMuV-Raj C4 and C1 in 2020 and 2021 respectively as compared to its isolates in 2018, which exhibited virus evolution. Surprisingly, we did not find these mutations in CLCuMuV-Raj isolates identified from Sindh province. The findings of the current study represent the stability of CLCuMuV-Raj and its spread toward the Sindh province where previously Cotton leaf curl Kokhran virus (CLCuKoV) and Cotton leaf curl Shahdadpur virus (CLCuShV) have been reported. The findings of the current study demand future research on CLCuD complex to explore the possible reasons for prevalence in the field and how the virus-host-vector compatible interaction can be broken to develop resistant cultivars.

A comprehensive review on Gossypium hirsutum resistance against cotton leaf curl virus

Cotton (Gossypium hirsutum L.) is a significant fiber crop. Being a major contributor to the textile industry requires continuous care and attention. Cotton is subjected to various biotic and abiotic constraints. Among these, biotic factors including cotton leaf curl virus (CLCuV) are dominant. CLCuV is a notorious disease of cotton and is acquired, carried, and transmitted by the whitefly (Bemisia tabaci). A cotton plant affected with CLCuV may show a wide range of symptoms such as yellowing of leaves, thickening of veins, upward or downward curling, formation of enations, and stunted growth. Though there are many efforts to protect the crop from CLCuV, long-term results are not yet obtained as CLCuV strains are capable of mutating and overcoming plant resistance. However, systemic-induced resistance using a gene-based approach remained effective until new virulent strains of CLCuV (like Cotton Leaf Curl Burewala Virus and others) came into existence. Disease control by biological means and the development of CLCuV-resistant cotton varieties are in progress. In this review, we first discussed in detail the evolution of cotton and CLCuV strains, the transmission mechanism of CLCuV, the genetic architecture of CLCuV vectors, and the use of pathogen and nonpathogen-based approaches to control CLCuD. Next, we delineate the uses of cutting-edge technologies like genome editing (with a special focus on CRISPR-Cas), next-generation technologies, and their application in cotton genomics and speed breeding to develop CLCuD resistant cotton germplasm in a short time. Finally, we delve into the current obstacles related to cotton genome editing and explore forthcoming pathways for enhancing precision in genome editing through the utilization of advanced genome editing technologies. These endeavors aim to enhance cotton's resilience against CLCuD.

Cotton leaf curl Multan betasatellite impaired ToLCNDV ability to maintain cotton leaf curl Multan alphasatellite

Abstract Alphasatellites (family Alphasatellitidae) are circular, single-stranded (ss) DNA molecules of ~1350 nucleotide in size that have been characterized in both the Old and New Worlds. Alphasatellites have inherent ability to self-replicate, which is accomplished by a single protein, replication-associated protein (Rep). Although the precise function of alphasatellite is yet unknown, and these consider dispensable for infectivity, however, their Rep protein functions as a suppressor of host defence. While alphasatellites are most frequently associated with begomoviruses, particularly with monopartite than bipartite begomoviruses, they have recently been found associated with mastreviruses. The in planta maintenance of alphasatellites by helper geminivirus is still an enigma, with no available study on the topic. This study aimed to investigate whether a widely distributed bipartite begomovirus, tomato leaf curl New Delhi virus (ToLCNDV), can maintain cotton leaf curl Multan alphasatellite (CLCuMuA) in the presence or absence of cotton leaf curl Multan betasatellite (CLCuMuB). The findings of this study demonstrated that ToLCNDV or its DNA A could maintain CLCuMuA in Nicotiana benthamiana plants. However, the presence of CLCuMuB interferes with the maintenance of CLCuMuA, and mutations in the CP of ToLCNDV further reduces it. Our study highlighted that the maintenance of alphasatellites is impaired in the presence of a betasatellite by ToLCNDV. Further investigation is needed to unravel all the interactions between a helper virus and an alphasatellites.

Transreplication Preference of the Tomato Leaf Curl Joydebpur Virus for a Noncognate Betasatellite through Iteron Resemblance on Nicotiana bethamiana

Pepper plants (Capsicum annuum) with severe leaf curl symptoms were collected in 2013 from Bangalore, Karnataka, India. The detection results showed a co-infection between the tomato leaf curl Joydebpur virus (ToLCJoV) and tomato leaf curl Bangladesh betasatellite (ToLCBDB) through the sequencing analysis of PCR amplicons. To pinpoint the molecular mechanism of this uncommon combination, infectious clones of ToLCJoV and two different betasatellites-ToLCBDB and tomato leaf curl Joydebpur betasatellite (ToLCJoB)-were constructed and tested for their infectivity in Nicotiana benthamiana. Together, we conducted various combined agroinoculation studies to compare the interaction of ToLCJoV with non-cognate and cognate betasatellites. The natural non-cognate interaction between ToLCJoV and ToLCBDB showed severe symptoms compared to the mild symptoms of a cognate combination (ToLCJoV × ToLCJoB) in infected plants. A sequence comparison among betasatellites and their helper virus wasperformed and the iteron resemblances in ToLCBDB as well as ToLCJoB clones were processed. Mutant betasatellites that comprised iteron modifications revealed that changes in iteron sequences could disturb the transreplication process between betasatellites and their helper virus. Our study might provide an important consideration for determining the efficiency of transreplication activity between betasatellites and their helper virus.

qPCR Assay as a Tool for Examining Cotton Resistance to the Virus Complex Causing CLCuD: Yield Loss Inversely Correlates with Betasatellite, Not Virus, DNA Titer

Cotton leaf curl disease (CLCuD) is a significant constraint to the economies of Pakistan and India. The disease is caused by different begomoviruses (genus Begomovirus, family Geminiviridae) in association with a disease-specific betasatellite. However, another satellite-like molecule, alphasatellite, is occasionally found associated with this disease complex. A quantitative real-time PCR assay for the virus/satellite components causing CLCuD was used to investigate the performance of selected cotton varieties in the 2014-2015 National Coordinated Varietal Trials (NCVT) in Pakistan. The DNA levels of virus and satellites in cotton plants were determined for five cotton varieties across three geographic locations and compared with seed cotton yield (SCY) as a measure of the plant performance. The highest virus titer was detected in B-10 (0.972 ng·µg^-1) from Vehari and the lowest in B-3 (0.006 ng·µg^-1) from Faisalabad. Likewise, the highest alphasatellite titer was found in B-1 (0.055 ng·µg^-1) from Vehari and the lowest in B-1 and B-2 (0.001 ng·µg^-1) from Faisalabad. The highest betasatellite titer was found in B-23 (1.156 ng·µg^-1) from Faisalabad and the lowest in B-12 (0.072 ng·µg^-1) from Multan. Virus/satellite DNA levels, symptoms, and SCY were found to be highly variable between the varieties and between the locations. Nevertheless, statistical analysis of the results suggested that betasatellite DNA levels, rather than virus or alphasatellite DNA levels, were the important variable in plant performance, having an inverse relationship with SCY (-0.447). This quantitative assay will be useful in breeding programs for development of virus resistant plants and varietal trials, such as the NCVT, to select suitable varieties of cotton with mild (preferably no) symptoms and low (preferably no) virus/satellite. At present, no such molecular techniques are used in resistance breeding programs or varietal trials in Pakistan.

An Insight into Emerging Begomoviruses and their Satellite Complex causing Papaya Leaf Curl Disease

Papaya leaf curl disease (PaLCD) was primarily detected in India and causes major economic damage to agriculture crops grown globally, seriously threatening food security. Begomoviruses are communicated by the vector Bemisia tabaci, and their transmission efficiency and persistence in the vector are the highest, exhibiting the widest host range due to adaptation and evolution. Symptoms induced during PaLCD include leaf curl, leaf yellowing, interveinal chlorosis, and reduced fruit quality and yield. Consequently, plants have evolved several multi-layered defense mechanisms to resist Begomovirus infection and distribution. Subsequently, Begomovirus genomes organise circular ssDNA of size ~2.5-2.7 kb of overlapping viral transcripts and carry six-seven ORFs encoding multifunctional proteins, which are precisely evolved by the viruses to maintain the genome-constraint and develop complex but integrated interactions with a variety of host components to expand and facilitate successful infection cycles, i.e., suppression of host defense strategies. Geographical distribution is continuing to increase due to the advent and evolution of new Begomoviruses, and sweep to new regions is a future scenario. This review summarizes the current information on the biological functions of papaya-infecting Begomoviruses and their encoded proteins in transmission through vectors and modulating host-mediated responses, which may improve our understanding of how to challenge these significant plant viruses by revealing new information on the development of antiviral approaches against Begomoviruses associated with PaLCD.

Natural occurrence of mesta yellow vein mosaic virus and DNA-satellites in ornamental sunflower (Helianthus spp.) in Pakistan

Weeds and ornamental plants serve as a reservoir for geminiviruses and contribute to their dissemination, genome recombination and/or satellite capture. Ornamental sunflower (Helianthus spp.) plants exhibiting mild leaf curl symptoms were subjected to begomovirus and DNA-satellites isolation. The full-length genome of the isolated begomovirus clone (Od1-A) showed 96.8% nucleotide (nt) sequence identity with mesta yellow vein mosaic virus (MeYVMV; accession no. FR772081) whereas, alphasatellite (Od1-a) and betasatellite (Od1-b) clones showed their highest nt sequence identities at 97.4% and 98.2% with ageratum enation alphasatellite (AEA; accession no. FR772085) and papaya leaf curl betasatellite (PaLCuB; accession. no. LN878112), respectively. The evolutionary relationships, average evolutionary divergence and the recombination events were also inferred. The MeYVMV exhibited 9.5% average evolutionary divergence and its CP and Rep had 9.3% and 12.2%, concomitantly; the alphasatellite and the betasatellite had 8.3% and 5.2%, respectively. The nt substitution rates (site^-1 year⁻¹) were found to be 6.983 × 10^-04 and 5.702 × 10^-05 in the CP and Rep of MeYVMV, respectively. The dN/dS ratio and the Tajima D value of MeYVMV CP demonstrated its possible role in host switching. The absolute quantification of the begomovirus demonstrated that mild symptoms might have a correlation with low virus titer. This is the first identification of MeYVMV and associated DNA-satellites from ornamental sunflower in Pakistan. The role of sequence divergence, recombination and importance of MeYVMV along with DNA-satellites in extending its host range is discussed.

Small but mighty: Functional landscape of the versatile geminivirus-encoded C4 protein

The fast-paced evolution of viruses enables them to quickly adapt to the organisms they infect by constantly exploring the potential functional landscape of the proteins encoded in their genomes. Geminiviruses, DNA viruses infecting plants and causing devastating crop diseases worldwide, produce a limited number of multifunctional proteins that mediate the manipulation of the cellular environment to the virus’ advantage. Among the proteins produced by the members of this family, C4, the smallest one described to date, is emerging as a powerful viral effector with unexpected versatility. C4 is the only geminiviral protein consistently subjected to positive selection and displays a number of dynamic subcellular localizations, interacting partners, and functions, which can vary between viral species. In this review, we aim to summarize our current knowledge on this remarkable viral protein, encompassing the different aspects of its multilayered diversity, and discuss what it can teach us about geminivirus evolution, invasion requirements, and virulence strategies.

Engineering tolerance to CLCuD in transgenic Gossypium hirsutum cv. HS6 expressing Cotton leaf curl Multan virus-C4 intron hairpin

Cotton leaf curl disease (CLCuD), caused by begomoviruses in combination with betasatellite molecule, has adversely affected cotton industry of Indian subcontinent. To devise a CLCuD-control strategy, RNAi-mediated approach was followed in this study. Gossypium hirsutum cv. HS6 plants were transformed with intron-hairpin RNAi (ihpRNAi-C4) construct carrying silencing suppressor C4 gene of Cotton leaf curl Multan virus (CLCuMuV). Efficacy of the construct in imparting CLCuD resistance was evaluated in transgenic (T0, T1) cotton lines. Accumulation of CLCuMuV/betasatellite and attenuation of CLCuD symptoms in the transgenic lines were monitored at different times interval after virus inoculation. Northern hybridization revealed the expression of C4-gene derived siRNA. Expression of the ihpRNAi transcript was recorded higher in transgenic lines expressing siRNA which supposedly targeted the C4 gene. A significant delay in detection of virus as well as betasatellite was observed in the transgenic lines. At 30 days post inoculation (dpi), none of the lines tested positive. At 45 dpi, however, it could be detected in few lines having much lower titre as compared to non-transformed control plants. Notably, till 60 dpi, no significant progression of the virus/betasatellite DNA was observed and the plants did not exhibit any characteristic CLCuD symptoms. A tolerance phenomenon leading to escape of CLCuD symptoms in the transformed cotton was described.

Construction of Infectious Clones of Begomoviruses: Strategies, Techniques and Applications

Simple Summary

Begomovirus has a wide host range and threatens a significant amount of economic damage to many important crops such as tomatoes, beans, cassava, squash and cotton. There are many efforts directed at controlling this disease including the use of insecticides to control the insect vector as well as screening the resistant varieties. The use of synthetic virus or infectious clones approaches has allowed plant virologists to characterize and exploit the genome virus at the molecular and biological levels. By exploiting the DNA of the virus using the infectious clones strategy, the viral genome can be manipulated at specific regions to study functional genes for host–virus interactions. Thus, this review will provide an overview of the strategy to construct infectious clones of Begomovirus. The significance of established infectious clones in Begomovirus study will also be discussed.

Abstract

Begomovirus has become a potential threat to the agriculture sector. It causes significant losses to several economically important crops. Given this considerable loss, the development of tools to study viral genomes and function is needed. Infectious clones approaches and applications have allowed the direct exploitation of virus genomes. Infectious clones of DNA viruses are the critical instrument for functional characterization of the notable and newly discovered virus. Understanding of structure and composition of viruses has contributed to the evolution of molecular plant pathology. Therefore, this review provides extensive guidelines on the strategy to construct infectious clones of Begomovirus. Also, this technique’s impacts and benefits in controlling and understanding the Begomovirus infection will be discussed.

Exogenous application of bio-stimulant and growth retardant improved the productivity of cotton cultivars under different planting arrangement

Optimum planting arrangement is an important attribute for efficient utilization of available resources and to obtain high yield of cotton. Application of plant growth promoter and retardant on cotton in improved planting density are the innovative techniques in the establishment of more productive cotton crop. Therefore, we planned a field study to assess the role of bio-stimulant and growth retardant in the resource utilization efficiency of cotton cultivars planted under variable row spacing at Agronomic Research Area Bahauddin Zakariya University and Usmania Agricultural Farm Shujabad during Kharif 2012. Experimental treatments consisted of cotton genotypes viz. CIM-573 and CIM-598, cultivated under conventional (75 cm), medium (50 cm) and ultra-narrow row spacing (25 cm) with foliar spray of bio-stimulant (moringa leaf extract) and growth retardant (mepiquate chloride), either sole or in combination, keeping distilled water as a control. Exogenously applied MLE alone and MLE + MC significantly enhanced the number of squares, flowers and green bolls per plant leading to higher cotton seed and lint yield of CIM 598 cultivar cultivated under conventional row spacing. While application of MC alone and MLE + MC produced maximum micronaire value, fiber strength and fiber uniformity ratio of CIM 573 cultivar cultivated under conventional row spacing. The results suggested that application of MLE is a possible approach to enhance the cotton productivity and the use of MC to enhance the fiber quality attributes under conventional row spacing.

Characterization of Pathogenicity-Associated V2 Protein of Tobacco Curly Shoot Virus

V2 proteins encoded by some whitefly-transmitted geminiviruses were reported to be functionally important proteins. However, the functions of the V2 protein of tobacco curly shoot virus (TbCSV), a monopartite begomovirus that causes leaf curl disease on tomato and tobacco in China, remains to be characterized. In our report, an Agrobacterium infiltration-mediated transient expression assay indicated that TbCSV V2 can suppress local and systemic RNA silencing and the deletion analyses demonstrated that the amino acid region 1-92 of V2, including the five predicted α-helices, are required for local RNA silencing suppression. Site-directed substitutions showed that the conserved basic and ring-structured amino acids in TbCSV V2 are critical for its suppressor activity. Potato virus X-mediated heteroexpression of TbCSV V2 in Nicotiana benthamiana induced hypersensitive response-like (HR-like) cell death and systemic necrosis in a manner independent of V2's suppressor activity. Furthermore, TbCSV infectious clone mutant with untranslated V2 protein (TbCSV∆V2) could not induce visual symptoms, and coinfection with betasatellite (TbCSB) could obviously elevate the viral accumulation and symptom development. Interestingly, symptom recovery occurred at 15 days postinoculation (dpi) and onward in TbCSV∆V2/TbCSB-inoculated plants. The presented work contributes to understanding the RNA silencing suppression activity of TbCSV V2 and extends our knowledge of the multifunctional role of begomovirus-encoded V2 proteins during viral infections.

Frequent occurrence of Mungbean yellow mosaic India virus in tomato leaf curl disease affected tomato in Oman

Next generation sequencing (NGS) of DNAs amplified by rolling circle amplification from 6 tomato (Solanum lycopersicum) plants with leaf curl symptoms identified a number of monopartite begomoviruses, including Tomato yellow leaf curl virus (TYLCV), and a betasatellite (Tomato leaf curl betasatellite [ToLCB]). Both TYLCV and ToLCB have previously been identified infecting tomato in Oman. Surprisingly the NGS results also suggested the presence of the bipartite, legume-adapted begomovirus Mungbean yellow mosaic Indian virus (MYMIV). The presence of MYMIV was confirmed by cloning and Sanger sequencing from four of the six plants. A wider analysis by PCR showed MYMIV infection of tomato in Oman to be widespread. Inoculation of plants with full-length clones showed the host range of MYMIV not to extend to Nicotiana benthamiana or tomato. Inoculation to N. benthamiana showed TYLCV to be capable of maintaining MYMIV in both the presence and absence of the betasatellite. In tomato MYMIV was only maintained by TYLCV in the presence of the betasatellite and then only at low titre and efficiency. This is the first identification of TYLCV with ToLCB and the legume adapted bipartite begomovirus MYMIV co-infecting tomato. This finding has far reaching implications. TYLCV has spread around the World from its origins in the Mediterranean/Middle East, in some instances, in live tomato planting material. The results here may suggest that begomoviruses which do not commonly infect tomato, such as MYMIV, could be spread as a passenger of TYLCV in tomato.

The Malvastrum Yellow Vein Virus C4 Protein Promotes Disease Symptom Development and Enhances Virus Accumulation in Plants

The begomovirus C4 protein is required for disease symptom development during virus infection in host plants. It can reprogram the cell cycle process for more efficient virus accumulation. In this study, we showed that the Malvastrum yellow vein virus (MaYVV) C4 protein could cause leaf up-ward curling and flower malformation, and increase virus accumulation in plants using PVX-based transient expression technology. We also demonstrated that, in the presence of its cognate betasatellite DNA (MaYVB), a mutant MaYVV, defective in producing the C4 protein (MaYVVΔC4), caused and alleviated infection in Nicotiana benthamiana. Transgenic plants expressing the MaYVV C4 protein showed upward leaf curling and uneven leaf lamina growth. Microscopic analysis showed that the epidermal cells of the C4 transgenic leaves were much smaller than those in the wild type (WT) leaves, and the mesophyll cells size and arrangement of transgenic plants was significantly altered. Inoculation of C4 transgenic plants with MaYVV or MaYVVΔC4 alone or associated with MaYVB showed that the transgenic C4 protein could promote viral and betasatellite accumulation and rescue the accumulation defect of MaYVVΔC4. Other transient expression assays also confirmed that the MaYVV C4 protein could suppress silencing of a GFP gene. In summary, our results indicate that the MaYVV C4 protein is a determinant of disease symptom and viral DNA accumulation. This protein can also function as a suppressor of RNA silencing and alter cell division and expansion.

Maintenance of Cotton Leaf Curl Multan Betasatellite by Tomato Leaf Curl New Delhi Virus-Analysis by Mutation

Viruses of the genus Begomovirus (family Geminiviridae) are economically important phytopathogens that are transmitted plant-to-plant by the whitefly Bemisia tabaci. Most Old World (OW) begomoviruses are monopartite and many of these interact with symptoms and host range determining betasatellites. Tomato leaf curl New Delhi virus (ToLCNDV) is one of only a few OW begomoviruses with a bipartite genome (components known as DNA A and DNA B). Four genes [AV2, coat protein (CP), transcriptional-activator protein (TrAP), and AC4] of ToLCNDV were mutated and the effects of the mutations on infectivity, symptoms and the ability to maintain Cotton leaf curl Multan betasatellite (CLCuMuB) were investigated. Infectivity and virus/betasatellite DNA titer were assessed by Southern blot hybridization, PCR, and quantitative PCR. The results showed TrAP of ToLCNDV to be essential for maintenance of CLCuMuB and AV2 to be important only in the presence of the DNA B. AC4 was found to be important for the maintenance of CLCuMuB in the presence of, but indispensable in the absence of, the DNA B. Rather than being required for maintenance, the CP was shown to possibly interfere with maintenance of the betasatellite. The findings show that the interaction between a bipartite begomovirus and a betasatellite is more complex than just trans-replication. Clearly, multiple levels of interactions are present and such associations can cause additional significant losses to crops although the interaction may not be stable.

V2 from a curtovirus is a suppressor of post-transcriptional gene silencing

The suppression of gene silencing is a key mechanism for the success of viral infection in plants. DNA viruses from the Geminiviridae family encode several proteins that suppress transcriptional and post-transcriptional gene silencing (TGS/PTGS). In Begomovirus, the most abundant genus of this family, three out of six genome-encoded proteins, namely C2, C4 and V2, have been shown to suppress PTGS, with V2 being the strongest PTGS suppressor in transient assays. Beet curly top virus (BCTV), the model species for the Curtovirus genus, is able to infect the widest range of plants among geminiviruses. In this genus, only one protein, C2/L2, has been described as inhibiting PTGS. We show here that, despite the lack of sequence homology with its begomoviral counterpart, BCTV V2 acts as a potent PTGS suppressor, possibly by impairing the RDR6 (RNA-dependent RNA polymerase 6)/suppressor of gene silencing 3 (SGS3) pathway.

Further changes in the cotton leaf curl disease complex: an indication of things to come?

Cotton leaf curl disease (CLCuD) has been a problem for cotton production in Pakistan and India since the early 1990s. The disease is caused by begomoviruses associated with a specific satellite, the cotton leaf curl Multan betasatellite (CLCuMB). In 2001, resistance introduced into cotton was broken by a recombinant begomovirus, Cotton leaf curl Kokhran virus strain Burewala (CLCuKoV-Bur). Unusually, in resistant cotton, this virus lacked an intact transcriptional activator protein (TrAP) gene, with the capacity to encode only 35 of the usual ~134 amino acids. Recently, isolates of CLCuKoV-Bur with a longer, but still truncated, TrAP gene have been identified in cotton breeding lines lacking the earlier resistance. This suggests that more pathogenic viruses with a full TrAP could return to cotton if the earlier resistance is not maintained in ongoing breeding efforts to produce CLCuD-resistant cotton varieties. This conclusion is supported by recent studies showing the reappearance of pre-resistance-breaking begomoviruses, with full-length TrAP genes, in cotton.

Genetics and Genomics of Cotton Leaf Curl Disease, Its Viral Causal Agents and Whitefly Vector: A Way Forward to Sustain Cotton Fiber Security

Cotton leaf curl disease (CLCuD) after its first epidemic in 1912 in Nigeria, has spread to different cotton growing countries including United States, Pakistan, India, and China. The disease is of viral origin-transmitted by the whitefly Bemisia tabaci, which is difficult to control because of the prevalence of multiple virulent viral strains or related species. The problem is further complicated as the CLCuD causing virus complex has a higher recombination rate. The availability of alternate host crops like tomato, okra, etc., and practicing mixed type farming system have further exaggerated the situation by adding synergy to the evolution of new viral strains and vectors. Efforts to control this disease using host plant resistance remained successful using two gene based-resistance that was broken by the evolution of new resistance breaking strain called Burewala virus. Development of transgenic cotton using both pathogen and non-pathogenic derived approaches are in progress. In future, screening for new forms of host resistance, use of DNA markers for the rapid incorporation of resistance into adapted cultivars overlaid with transgenics and using genome editing by CRISPR/Cas system would be instrumental in adding multiple layers of defense to control the disease-thus cotton fiber production will be sustained.

G5, a Phage Single-Stranded DNA-Binding Protein, Fused with a Nuclear Localization Signal, Attenuates Symptoms and Reduces Begomovirus-Betasatellite Accumulation in Transgenic Plants

Cotton leaf curl disease is caused by several monopartite begomoviruses and is the major threat to cotton production in the Indian subcontinent. The disease has been shown to be associated with four distinct species, including Cotton leaf curl Kokhran virus (CLCuKoV), and a specific betasatellite-Cotton leaf curl Multan betasatellite (CLCuMuB). Transgenic Nicotiana benthamiana plants were produced which constitutively express the Escherichia coli phage M13 encoded, sequence nonspecific single-stranded (ss) DNA-binding protein, G5 alone and fused with the maize opaque-2 nuclear localization signal (NLS), to evaluate resistance against CLCuKoV-CLCuMuB. Transgenic plants expressing only G5 performed poorly exhibiting symptoms of infection and high virus DNA levels upon inoculation with CLCuKoV and CLCuKoV with CLCuMuB. In contrast, plants transformed with G5 fused to the NLS developed mild symptoms and showed a reduction in virus and betasatellite DNA levels in comparison to nontransformed plants. The results show that G5 may be useful in developing broad-spectrum resistance against ssDNA viruses.

Translation, modification and cellular distribution of two AC4 variants of African cassava mosaic virus in yeast and their pathogenic potential in plants

Plant infecting geminiviruses encode a small (A)C4 protein within the open reading frame of the replication-initiator protein. In African cassava mosaic virus, two in-frame start codons may be used for the translation of a longer and a shorter AC4 variant. Both were fused to green fluorescent protein or glutathione-S-transferase genes and expressed in fission yeast. The longer variant accumulated in discrete spots in the cytoplasm, whereas the shorter variant localized to the plasma membrane. A similar expression pattern was found in plants. A myristoylation motif may promote a targeting of the shorter variant to the plasma membrane. Mass spectrometry analysis of the yeast-expressed shorter variant detected the corresponding myristoylation. The biological relevance of the second start codon was confirmed using mutated infectious clones. Whereas mutating the first start codon had no effect on the infectivity in Nicotiana benthamiana plants, the second start codon proved to be essential.

Infectivity, effects on helper viruses and whitefly transmission of the deltasatellites associated with sweepoviruses (genus Begomovirus, family Geminiviridae)

Begomoviruses (family Geminiviridae) are whitefly-transmitted viruses with single-stranded DNA genomes that are frequently associated with DNA satellites. These satellites include non-coding satellites, for which the name deltasatellites has been proposed. Although the first deltasatellite was identified in the late 1990s, little is known about the effects they have on infections of their helper begomoviruses. Recently a group of deltasatellites were identified associated with sweepoviruses, a group of phylogenetically distinct begomoviruses that infect plants of the family Convolvulaceae including sweet potato. In this work, the deltasatellites associated with sweepoviruses are shown to be transreplicated and maintained in plants by the virus with which they were identified, sweet potato leaf curl virus (SPLCV). These deltasatellites were shown generally to reduce symptom severity of the virus infection by reducing virus DNA levels. Additionally they were shown to be maintained in plants, and reduce the symptoms induced by two Old World monopartite begomoviruses, tomato yellow leaf curl virus and tomato yellow leaf curl Sardinia virus. Finally one of the satellites was shown to be transmitted plant-to-plant in the presence of SPLCV by the whitefly vector of the virus, Bemisia tabaci, being the first time a deltasatellite has been shown to be insect transmitted.

Identification of amino acid residues of the coat protein of Sri Lankan cassava mosaic virus affecting symptom production and viral titer in Nicotiana benthamiana

Sri Lankan cassava mosaic virus (SLCMV) is bipartite begomovirus infecting cassava in India and Sri Lanka. Interestingly, the DNA-A component of the SLCMV alone is able to infect Nicotiana benthamiana causing symptoms of upward leaf rolling and stunting. One of the differences between monopartite and bipartite begomoviruses is the requirement of Coat Protein (CP) for infectivity; CP being essential for the former, but dispensable in the latter. This investigation was aimed to determine the importance of CP in the infectivity of the bipartite SLCMV, behaving as a monopartite virus in N. benthamiana. We tested CP-null mutants, single amino acid replacement mutants and double, triple and quadruple combinations of the above in SLCMV DNA-A, for infectivity, symptom development and viral DNA accumulation in N. benthamiana. While CP-null mutants were non-infectious, a majority of the single amino acid replacement mutants and their combinations retained infectivity, some with attenuated symptoms and reduced viral titers. Some of the combined mutations restored the attenuated symptoms to wild type levels. Some of the mutations were predicted to cause changes in the secondary structure of the CP, which roughly correlated with the attenuation of symptoms and the reduction in viral titers.

The 35-amino acid C2 protein of Cotton leaf curl Kokhran virus, Burewala, implicated in resistance breaking in cotton, retains some activities of the full-length protein

With one exception, all the begomoviruses characterized so far encode an ~134-amino acid (aa) (A)C2 protein. The exception is the "Burewala" strain of Cotton leaf curl Kokhran virus (CLCuKoV-Bu), associated with resistance breaking in cotton across Pakistan and northwestern India, that encodes a truncated 35-aa C2. The C2 protein encoded by begomoviruses performs multiple functions including suppression of post-transcriptional gene silencing (PTGS), modulating microRNA (miRNA) expression and may be a pathogenicity determinant. The study described here was designed to investigate whether the CLCuKoV-Bu 35-aa C2 retains the activities of the full-length C2 protein. The results showed the 35-aa C2 of CLCuKoV-Bu acts as a pathogenicity determinant, suppresses PTGS and upregulates miRNA expression when expressed from a Potato virus X vector in Nicotiana benthamiana. The symptoms induced by expression of full-length C2 were more severe than those induced by the 35-aa C2. The accumulation of most developmental miRNAs decreases with the full-length C2 protein and increases with the 35-aa peptide of CLCuKoV-Bu. The study also revealed that 35-aa peptide of CLCuKoV-Bu maintains suppressor of silencing activity at a level equal to that of full-length C2. The significance of the results with respect to virus fitness and resistance breaking is discussed.

CRISPR/Cas9: A Tool to Circumscribe Cotton Leaf Curl Disease

The begomoviruses (family Geminiviridae) associated with cotton leaf curl disease (CLCuD) pose a major threat to cotton productivity in South-East Asia including Pakistan and India. These viruses have single-stranded, circular DNA genome, of ∼2800 nt in size, encapsidated in twinned icosa-hedera, transmitted by ubiquitous whitefly and are associated with satellite molecules referred to as alpha- and betasatellite. To circumvent the proliferation of these viruses numerous techniques, ranging from conventional breeding to molecular approaches have been applied. Such devised strategies worked perfectly well for a short time period and then viruses relapse due to various reasons including multiple infections, where related viruses synergistically interact with each other, virus proliferation and evolution. Another shortcoming is, until now, that all molecular biology approaches are devised to control only helper begomoviruses but not to control associated satellites. Despite the fact that satellites could add various functions to helper begomoviruses, they remain ignored. Such conditions necessitate a very comprehensive technique that can offer best controlling strategy not only against helper begomoviruses but also their associated DNA-satellites. In the current scenario clustered regulatory interspaced short palindromic repeats (CRISPR)/CRISPR associated nuclease 9 (Cas9) has proved to be versatile technique that has very recently been deployed successfully to control different geminiviruses. The CRISPR/Cas9 system has been proved to be a comprehensive technique to control different geminiviruses, however, like previously used techniques, only a single virus is targeted and hitherto it has not been deployed to control begomovirus complexes associated with DNA-satellites. Here in this article, we proposed an inimitable, unique, and broad spectrum controlling method based on multiplexed CRISPR/Cas9 system where a cassette of sgRNA is designed to target not only the whole CLCuD-associated begomovirus complex but also the associated satellite molecules.

Diversity in Betasatellites Associated with Cotton Leaf Curl Disease During Source-To-Sink Movement Through a Resistant Host

Cotton leaf curl is devastating disease of cotton characterized by leaf curling, vein darkening and enations. The disease symptoms are induced by DNA satellite known as Cotton leaf curl Multan betasatellite (CLCuMuB), dominant betasatellite in cotton but another betasatellite known as Chili leaf curl betasatellite (ChLCB) is also found associated with the disease. Grafting experiment was performed to determine if host plant resistance is determinant of dominant population of betasatellite in cotton (several distinct strains of CLCuMuB are associated with the disease). Infected scion of Gossypium hirsutum collected from field (the source) was grafted on G. arboreum, a diploid cotton species, resistant to the disease. A healthy scion of G. hirsutum (sink) was grafted at the top of G. arboreum to determine the movement of virus/betasatellite to upper susceptible scion of G. hirsutum. Symptoms of disease appeared in the upper scion and presence of virus/betasatellite in the upper scion was confirmed via molecular techniques, showing that virus/betasatellite was able to move to upper scion through resistant G. arboreum. However, no symptoms appeared on G. arboreum. Betasatelites were cloned and sequenced from lower scion, upper scion and G. arboreum which show that the lower scion contained both CLCuMuB and ChLCB, however only ChLCB was found in G. arboreum. The upper scion contained CLCuMuB with a deletion of 78 nucleotides (nt) in the non-coding region between A-rich sequence and βC1 gene and insertion of 27 nt in the middle of βC1 ORF. This study may help in investigating molecular basis of resistance in G. arboreum.

Genetic variability of Cotton leaf curl betasatellite in Northern India

Cotton is an important crop and its production is affected by various disease pathogens. Monopartite begomovirus associated betasatellites cause Cotton leaf curl disease (CLCuD) in Northern India. In order to access the occurrence and genetic variability of Cotton leaf curl betasatellites, an extensive field survey was conducted in states of Rajasthan, Punjab and Haryana. We selected the betasatellite sequence for analysis as they are reported as important for disease severity and sequence variability. Based on the field observations, the disease incidence ranged from 30% to 80% during the survey. Full genome and DNA β were amplified from various samples while no amplicon was obtained in some samples. The nucleotide sequence homology ranged from 90.0% to 98.7% with Cotton leaf curl virus (CLCuV), 55.2-55.5% with Bhendi yellow vein mosaic virus, 55.8% with Okra leaf curl virus and 51.70% with Tomato leaf curl virus isolates. The lowest similarity (47.8%) was found in CLCuV-Sudan isolate. Phylogenetic analysis showed that analyzed isolates formed a close cluster with various CLCuV isolates reported earlier. The analysis results show sequence variation in Cotton leaf curl betasatellite which could be the result of recombination. The results obtained by genome amplification and sequence variability indicate that some new variants are circulating and causing leaf curl disease in Rajasthan, Punjab and Haryana.

Regional changes in the sequence of cotton leaf curl multan betasatellite

Cotton leaf curl disease (CLCuD) in Pakistan and northwestern India is caused by monopartite begomoviruses in association with an essential, disease-specific satellite, Cotton leaf curl Multan betasatellite (CLCuMB). Following a recent upsurge in CLCuD problems in Sindh province (southern Pakistan), sequences of clones of CLCuMB were obtained from Sindh and Punjab province (central Pakistan), where CLCuD has been a problem since the mid-1980s. The sequences were compared to all sequences of CLCuMB available in the databases. Analysis of the sequences shows extensive sequence variation in CLCuMB, most likely resulting from recombination. The range of sequence variants differ between Sindh, the Punjab and northwestern India. The possible significance of the findings with respect to movement of the CLCuD between the three regions is discussed. Additionally, the lack of sequence variation within the only coding sequence of CLCuMB suggests that the betasatellite is not involved in resistance breaking which became a problem after 2001 in the Punjab and subsequently also in northwestern India.

Effects of genetic changes to the begomovirus/betasatellite complex causing cotton leaf curl disease in South Asia post-resistance breaking

Cotton leaf curl disease (CLCuD) has been a problem for cotton production across Pakistan and north-eastern India since the early 1990s. The appearance of the disease has been attributed to the introduction, and near monoculture of highly susceptible cotton varieties. During the intervening period the genetic make-up of the virus(es) causing the disease has changed dramatically. The most prominent of these changes has been in response to the introduction of CLCuD-resistant cotton varieties in the late 1990s, which provided a brief respite from the losses due to the disease. During the 1990s the disease was shown to be caused by multiple begomoviruses and a single, disease-specific betasatellite. Post-resistance breaking the complex encompassed only a single begomovirus, Cotton leaf curl Burewala virus (CLCuBuV), and a recombinant version of the betasatellite. Surprisingly CLCuBuV lacks an intact transcriptional-activator protein (TrAP) gene. The TrAP gene is found in all begomoviruses and encodes a product of ∼134 amino acids that is important in virus-host interactions; being a suppressor of post-transcriptional gene silencing (host defence) and a transcription factor that modulates host gene expression, including microRNA genes. Recent studies have highlighted the differences between CLCuBuV and the earlier viruses that are part of on-going efforts to define the molecular basis for resistance breaking in cotton.

A recombinant begomovirus resulting from exchange of the C4 gene

A begomovirus isolated from Malvastrum coromandelianum and tomato originating from Yunnan province (China) was shown to be representative of a new begomovirus species, for which the name tomato leaf curl Yunnan virus (TLCYnV) is proposed. TLCYnV has high levels of sequence identity to tomato yellow leaf curl China virus (TYLCCNV) across the whole genome, except for sequences encompassing the C4 gene. Agrobacterium-mediated inoculation showed TLCYnV to be highly infectious to a range of plant species but poorly infectious to M. coromandelianum. In contrast to TYLCCNV, TLCYnV was shown to infect tomato in the absence of a betasatellite. In field-collected samples, TLCYnV was identified most frequently in tomato in which it was not associated with a betasatellite. Transgenic expression in Nicotiana benthamiana showed that the C4 protein of TYLCCNV did not induce developmental abnormalities, whereas the C4 of TLCYnV induced severe developmental abnormalities, reminiscent of virus symptoms. The genome of TLCYnV was shown to be significantly less methylated in plants than that of TYLCCNV and the C4 protein of TLCYnV was shown to suppress post-transcriptional gene silencing and transcriptional gene silencing more effectively than the C4 of TYLCCNV. The results indicate that TLCYnV evolved from TYLCCNV by recombination, acquiring a more virulent C4, allowing it to dispense with the requirement for a betasatellite. The implications of these findings in relation to the evolution of monopartite begomoviruses are discussed.

Artificial microRNA-mediated resistance against the monopartite begomovirus Cotton leaf curl Burewala virus

BACKGROUND

Cotton leaf curl disease, caused by single-stranded DNA viruses of the genus Begomovirus (family Geminiviridae), is a major constraint to cotton cultivation across Pakistan and north-western India. At this time only cotton varieties with moderate tolerance are available to counter the disease. microRNAs (miRNAs) are a class of endogenous small RNA molecules that play an important role in plant development, signal transduction, and response to biotic and a biotic stress. Studies have shown that miRNAs can be engineered to alter their target specificity. Such artificial miRNAs (amiRNAs) have been shown to provide resistance against plant-infecting viruses.

RESULTS

Two amiRNA constructs, based on the sequence of cotton miRNA169a, were produced containing 21 nt of the V2 gene sequence of Cotton leaf curl Burewala virus (CLCuBuV) and transformed into Nicotiana benthamiana. The first amiRNA construct (P1C) maintained the miR169a sequence with the exception of the replaced 21 nt whereas in the second (P1D) the sequence of the miRNA169a backbone was altered to restore some of the hydrogen bonding of the mature miRNA duplex. P1C transgenic plants showed good resistance when challenge with CLCuBV; plants being asymptomatic with low viral DNA levels. The resistance to heterologous viruses was lower and correlated with the numbers of sequence mismatches between the amiRNA and the V2 gene sequence. P1D plants showed overall poorer resistance to challenge with all viruses tested.

CONCLUSIONS

The results show that the amiRNA approach can deliver efficient resistance in plants against a monopartite begomoviruses and that this has the potential to be broad-spectrum, providing protection from a number of viruses. Additionally the findings indicate that the levels of resistance depend upon the levels of complementarity between the amiRNA and the target sequence and the sequence of the miRNA backbone, consistent with earlier studies.