Sterility Mosaic Disease-the "Green Plague" of Pigeonpea: Advances in Understanding the Etiology, Transmission and Control of a Major Virus Disease

Salient Findings on Host Range, Resistance Screening, and Molecular Studies on Sterility Mosaic Disease of Pigeonpea Induced by Pigeonpea sterility mosaic viruses (PPSMV-I and PPSMV-II)

Two distinct emaraviruses, Pigeonpea sterility mosaic virus-I (PPSMV-I) and Pigeonpea sterility mosaic virus-II (PPSMV-II) were found to be associated with sterility mosaic disease (SMD) of pigeonpea [Cajanus cajan (L.) Millsp.]. The host range of both these viruses and their vector are narrow, confined to Nicotiana benthamiana identified through mechanical transmission, and to Phaseolus vulgaris cvs. Top Crop, Kintoki, and Bountiful (F: Fabaceae) through mite transmission. A weed host Chrozophora rottleri (F: Euphorbiaceae) was also infected and tested positive for both the viruses in RT-PCR. Among the wild Cajanus species tested, Cajanus platycarpus accessions 15661, 15668, and 15671, and Cajanus scarabaeoides accessions 15683, 15686, and 15922 were infected by both the viruses and mite vector suggesting possible sources of SMD inoculum. Though accession 15666 of C. platycarpus, 15696 of C. scarabaeoides, and 15639 of Cajanus lanceolatus were infected by both the viruses, no mite infestation was observed on them. Phylogenetic analysis of nucleotide sequences of RNA-1 and RNA-2 of PPSMV-I and PPSMV-II isolates in southern India revealed significant divergence especially PPSMV-II, which is closely related to the Fig mosaic virus (FMV) than PPSMV-I. In multilocation testing of pigeonpea genotypes for their broad-based resistance to SMD for two consecutive years, genotypes ICPL-16086 and ICPL-16087 showed resistance reaction (<10% incidence) in all three locations studied. Overall, the present study gives a clear idea about the host range of PPSMV-I and PPSMV-II, their molecular relationship, and sources of resistance. This information is critical for the development of reliable diagnostic tools and improved disease management strategies.

Harnessing digital technology to improve agricultural productivity?

Can improving access to mobile extension improve agricultural productivity? Recent evidence suggests both significant and insignificant ways in which SMS-based agricultural information could affect farming outcomes. It is unclear if variations in the programs' design or the methodological challenges in evaluating the programs cause wide-ranging impacts. Extension hotline services provide rapid, unambiguous information by agricultural experts over the phone, tailored to time- and crop-specific shocks. Using methods from experimental economics, we randomly distributed the hotline number to generate exogenous variation in the access to farming information. We conducted our study among 300 farmers in the South Indian state of Karnataka. Our results show that eliminating informational inefficiencies increases farmers' average yields for a high-stakes pigeon pea crop that faced adverse aggregate shock. The impact on the yield is through the adoption of cost-effective and improved farming practices. However, we do not observe any effect on the crops that were not affected by the shock. Our findings reveal that advisory recommendations customized to time- and crop-specific shocks are associated with a greater impact on agricultural productivity.

Entry of bunyaviruses into plants and vectors

The majority of plant-infecting viruses are transmitted by arthropod vectors that deliver them directly into a living plant cell. There are diverse mechanisms of transmission ranging from direct binding to the insect stylet (non-persistent transmission) to persistent-propagative transmission in which the virus replicates in the insect vector. Despite this diversity in interactions, most arthropods that serve as efficient vectors have feeding strategies that enable them to deliver the virus into the plant cell without extensive damage to the plant and thus effectively inoculate the plant. As such, the primary virus entry mechanism for plant viruses is mediated by the biological vector. Remarkably, viruses that are transmitted in a propagative manner (bunyaviruses, rhabdoviruses, and reoviruses) have developed an ability to replicate in hosts from two kingdoms. Viruses in the order Bunyavirales are of emerging importance and with the advent of new sequencing technologies, we are getting unprecedented glimpses into the diversity of these viruses. Plant-infecting bunyaviruses are transmitted in a persistent, propagative manner must enter two unique types of host cells, plant and insect. In the insect phase of the virus life cycle, the propagative viruses likely use typical cellular entry strategies to traverse cell membranes. In this review, we highlight the transmission and entry strategies of three genera of plant-infecting bunyaviruses: orthotospoviruses, tenuiviruses, and emaraviruses.

An Intimate Relationship Between Eriophyoid Mites and Their Host Plants - A Review

Eriophyoid mites (Acari Eriophyoidea) are phytophagous arthropods forming intimate relationships with their host plants. These mites are associated with annual and perennial plants including ferns, and are highly specialized with a dominant monophagy. They can be classified in different ecological classes, i.e., vagrant, gall-making and refuge-seeking species. Many of them are major pests and some of them are vectors of plant pathogens. This paper critically reviews the knowledge on eriophyoids of agricultural importance with emphasis on sources for host plant resistance to these mites. The role of species belonging to the family Eriophyidae as vectors of plant viruses is discussed. Eriophyoid-host plant interactions, the susceptibility within selected crops and main host plant tolerance/resistance mechanisms are discussed. Fundamental concepts, subjects, and problems emerged in this review are pointed out and studies are suggested to clarify some controversial points.

Molecular characterization of emaraviruses associated with Pigeonpea sterility mosaic disease

Sterility Mosaic Disease (SMD) of pigeonpea (Cajanus cajan (L.) Millspaugh) is a complex disease due to various factors including the presence of a mixed infection. Comparison of dsRNA profile and small RNA (sRNA) deep sequencing analysis of samples from three locations revealed the presence of Pigeonpea sterility mosaic virus-I and II (PPSMV-I and II) from Chevella and only PPSMV-II from Bengaluru and Coimbatore. PPSMV-I genome consisted of four while PPSMV-II encompassed six RNAs. The two viruses have modest sequence homology between their corresponding RNA 1-4 encoding RdRp, glycoprotein precursor, nucleocapsid and movement proteins and the corresponding orthologs of other emaraviruses. However, PPSMV-II is more related to Fig mosaic virus (FMV) than to PPSMV-I. ELISA based detection methodology was standardized to identify these two viruses, uniquely. Mite inoculation of sub-isolate Chevella sometimes resulted in few- to- many pigeonpea plants containing PPSMV-I alone. The study shows that (i) the N-terminal region of RdRp (SRD-1) of both the viruses contain "cap-snatching" endonuclease domain and a 13 AA cap binding site at the C-terminal, essential for viral cap-dependent transcription similar to the members of Bunyaviridae family and (ii) P4 is the movement protein and may belong to '30 K superfamily' of MPs.

Variability of Emaravirus Species Associated with Sterility Mosaic Disease of Pigeonpea in India Provides Evidence of Segment Reassortment

Sterility mosaic disease (SMD) of pigeonpea is a serious constraint for cultivation of pigeonpea in India and other South Asian countries. SMD of pigeonpea is associated with two distinct emaraviruses, Pigeonpea sterility mosaic virus 1 (PPSMV-1) and Pigeonpea sterility mosaic virus 2 (PPSMV-2), with genomes consisting of five and six negative-sense RNA segments, respectively. The recently published genome sequences of both PPSMV-1 and PPSMV-2 are from a single location, Patancheru from the state of Telangana in India. However, here we present the first report of sequence variability among 23 isolates of PPSMV-1 and PPSMV-2, collected from ten locations representing six states of India. Both PPSMV-1 and PPSMV-2 are shown to be present across India and to exhibit considerable sequence variability. Variability of RNA3 sequences was higher than the RNA4 sequences for both PPSMV-1 and PPSMV-2. Additionally, the sixth RNA segment (RNA6), previously reported to be associated with only PPSMV-2, is also associated with isolates of PPSMV-1. Multiplex reverse transcription PCR (RT-PCR) analyses show that PPSMV-1 and PPSMV-2 frequently occur as mixed infections. Further sequence analyses indicated the presence of reassortment of RNA4 between isolates of PPSMV-1 and PPSMV-2.

Pigeonpea sterility mosaic virus: a legume-infecting Emaravirus from South Asia

Pigeonpea sterility mosaic virus (PPSMV), a species of the genus Emaravirus, is the causal agent of sterility mosaic disease (SMD) of pigeonpea [Cajanus cajan (L.) Millsp]. This disease, dubbed the 'green plague', as the infected plants remain in the vegetative state without flower production, has been reported from India and a few other South-East Asian countries. SMD is estimated to result in an annual yield loss of over US$300 million in India alone. The aetiology of SMD, which remained a mystery for over 70 years, was resolved with the discovery of PPSMV in 2000 and its complete genome sequence in 2014.

AETIOLOGY AND VIRUS TRANSMISSION

SMD is characterized by stunted and bushy plants, leaves of reduced size with chlorotic rings or mosaic symptoms, and partial or complete cessation of flower production (i.e. sterility). The causal agent of the disease is PPSMV, a virus with a segmented, negative-sense, single-stranded RNA genome, transmitted in a semi-persistent manner by an eriophyid mite Aceria cajani Channabassavanna (Acari: Arthropoda). Both the virus and vector are highly specific to pigeonpea and a few of its wild relatives, such as C. scarabaeoides and C. cajanifolius. Under experimental conditions, PPSMV was transmitted to Nicotiana benthamiana by sap inoculation using fresh extract of SMD-infected leaves (but not to pigeonpea); however, purified nucleoprotein preparations are not infectious. The virus was also transmitted to French bean (Phaseolus vulgaris L.) using viruliferous eriophyid mites. PPSMV is not seed transmitted in pigeonpea or other hosts known to be infected by this virus. On the basis of the differential host reactions in different geographical locations, the occurrence of diverse PPSMV strains was suspected.

HOST RANGE AND EPIDEMIOLOGY

PPSMV can infect several genotypes of cultivated and wild relatives of pigeonpea. Experimental hosts include N. benthamiana, N. clevelandii, P. vulgaris and Chrozophora rottleri. However, pigeonpea alone and a few wild species of Cajanus were found to support the vector A. cajani. SMD is endemic in most of the pigeonpea-growing regions of India, but the incidence varies widely between regions and years. In nature, A. cajani populations were almost exclusively observed on SMD-infected pigeonpea, but not on healthy plants, indicating a strong communalistic relationship between the virus-infected plants and the vector. The epidemiology of SMD involves the virus, mite vector, cultivar and environmental conditions. Infected perennial and volunteer plants serve as a source for both the virus and its vector mites, and play an important role in the disease cycle.

GENOME ORGANIZATION, GENE FUNCTION AND TAXONOMY

The PPSMV genome contains five segments of single-stranded RNA that are predicted to encode proteins in negative sense. The ribonucleoprotein complex is encased in quasi-spherical, membrane-bound virus particles of 100-150 nm. The largest segment, RNA-1, is 7022 nucleotides in length and codes for RNA-dependent RNA polymerase (2295 amino acids); RNA-2, with a sequence length of 2223 nucleotides, codes for glycoproteins (649 amino acids); RNA-3, with a sequence length of 1442 nucleotides, codes for nucleocapsid protein (309 amino acids); RNA-4, with a sequence length of 1563 nucleotides, codes for a putative movement protein p4 (362 amino acids); and RNA-5, with a sequence length of 1689 nucleotides, codes for p5 (474 amino acids), a protein with unknown function. PPSMV was recently classified as a species in the genus Emaravirus, a genus whose members show features resembling those of members of the genera Tospovirus (Family: Bunyaviridae) and Tenuivirus, both of which comprise single-stranded RNA viruses that encode proteins by an ambisense strategy.

SMD CONTROL

The disease is mainly controlled using SMD-resistant cultivars. However, the occurrence of distinct strains/isolates of PPSMV in different locations makes it difficult to incorporate broad-spectrum resistance. Studies on the inheritance of SMD resistance in different cultivars against different isolates of PPSMV indicate that the resistance is mostly governed by recessive genes, although there are contrasting interpretations of the data. Genetic engineering through RNA-interference (RNAi) and resistant gene-based strategies are some of the potential approaches for the transgenic control of SMD. Seed treatment or soil and foliar application of a number of organophosphorus-based insecticides or acaricides, which are recommended for the management of the vector mites, are seldom practised because of prohibitive costs and also their risks to human health and the environment.

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.

Deep sequencing of dsRNAs recovered from mosaic-diseased pigeonpea reveals the presence of a novel emaravirus: pigeonpea sterility mosaic virus 2

Deep-sequencing analysis of double-stranded RNA extracted from a mosaic-diseased pigeonpea plant (Cajanus cajan L., family Fabaceae) revealed the complete sequence of six emaravirus-like negative-sense RNA segments of 7009, 2229, 1335, 1491, 1833 and 1194 nucleotides in size. In the order from RNA1 to RNA6, these genomic RNAs contained ORFs coding for the RNA-dependent RNA polymerase (RdRp, p1 of 266 kDa), the glycoprotein precursor (GP, p2 of 74.5 kDa), the nucleocapsid (NC, p3 of 34.9 kDa), and the putative movement protein (MP, p4 of 40.7 kDa), while p5 (55 kDa) and p6 (27 kDa) had unknown functions. All RNA segments showed distant relationships to viruses of the genus Emaravirus, and in particular to pigeonpea sterility mosaic virus (PPSMV), with which they shared nucleotide sequence identity ranging from 48.5 % (RNA3) to 62.5 % (RNA1). In phylogenetic trees constructed from the sequences of the proteins encoded by RNA1, RNA2 and RNA3 (p1, p2 and p3), this new viral entity showed a consistent grouping with fig mosaic virus (FMV) and rose rosette virus (RRV), which formed a cluster of their own, clearly distinct from PPSMV-1. In experimental greenhouse trials, this novel virus was successfully transmitted to pigeonpea and French bean seedlings by the eriophyid mite Aceria cajani. Preliminary surveys conducted in the Hyderabad region (India) showed that the virus in question is widespread in pigeonpea plants affected by sterility mosaic disease (86.4 %) but is absent in symptomless plants. Based on molecular, biological and epidemiological features, this novel virus is the second emaravirus infecting pigeonpea, for which the provisional name pigeonpea sterility mosaic virus 2 (PPSMV-2) is proposed.

Inheritance of pigeonpea sterility mosaic disease resistance in pigeonpea

A comprehensive study was conducted using PPSMV resistant (BSMR 736) and susceptible (ICP 8863) genotypes to develop a segregating population and understand the inheritance of PPSMV resistance. The observed segregation was comparable to 13 (susceptible): 3 (resistant). Hence, the inheritance was controlled by two genes, SV1 and SV2, with inhibitory gene interaction.

Emaravirus: a novel genus of multipartite, negative strand RNA plant viruses

Ringspot symptoms in European mountain ash (Sorbus aucuparia L.), fig mosaic, rose rosette, raspberry leaf blotch, pigeonpea sterility mosaic (Cajanus cajan) and High Plains disease of maize and wheat were found to be associated with viruses that share several characteristics. They all have single-stranded multipartite RNA genomes of negative orientation. In some cases, double membrane-bound virus-like particles of 80 to 200 nm in diameter were found in infected tissue. Furthermore, at least five of these viruses were shown to be vectored by eriophyid mites. Sequences of European mountain ash ringspot-associated virus (EMARaV), Fig mosaic virus (FMV), rose rosette virus (RRV), raspberry leaf blotch virus (RLBV), pigeonpea sterility mosaic virus and High Plains virus strongly support their potential phylogenetic relationship. Therefore, after characterization of EMARaV, the novel genus Emaravirus was established, and FMV was the second virus species assigned to this genus. The recently sequenced RRV and RLBV are supposed to be additional members of this new group of plant RNA viruses.

Wheat curl mite resistance: interactions of mite feeding with wheat streak mosaic virus infection

The majority of plant viruses are dependent on arthropod vectors for spread between plants. Wheat streak mosaic virus (family Potyviridae, genus Tritimovirus, WSMV) is transmitted by the wheat curl mite, Aceria tosichella Keifer, and this virus and vector cause extensive yield losses in most major wheat (Triticum aestivum L.)-growing regions of the world. Many cultivars in use are susceptible to this vector-virus complex, and yield losses of 10-99% have been documented. wheat curl mite resistance genes have been identified in goat grass, Aegilops tauschii (Coss) Schmal., and transferred to hexaploid wheat, but very few varieties contain effectively wheat curl mite resistance, due to virulent wheat curl mite populations. However, wheat curl mite resistance remains an effective strategy to reduce losses due to WSMV. The goal of our project was to identify the most effective, reproducible, and rapid method for assessing wheat curl mite resistance. We also wanted to determine whether mite resistance is affected by WSMV infection, because the pathogen and pest commonly occur together. Single and group wheat curl mite infestations produced similar amounts of leaf rolling and folding on wheat curl mite-susceptible wheat varieties that were independent of initial wheat curl mite infestation. This finding will allow accurate, efficient, large-scale screening of wheat germplasm for wheat curl mite resistance by infesting plants with sections of wheat leaf tissue containing mixed stages of wheat curl mite. The wheat curl mite-resistant breeding line 'OK05312' displayed antibiosis (reduced wheat curl mite population development). The effect of WSMV infection on wheat curl mite reproduction was genotype-dependent. Mite populations increased on infected wheat curl mite- and WSMV-susceptible plants compared with uninfected plants, but WSMV infection had no significant effect on wheat curl mite populations on resistant plants. OK05312 is a strong source of wheat curl mite resistance for wheat breeding programs.

What's "cool" on eriophyoid mites?

Fundamental knowledge on the morphology, biology, ecology, and economic importance of Eriophyoidea has been exhaustively compiled by Lindquist et al. (Eriophyoid mites--their biology, natural enemies and control; Elsevier, 1996). Since that time, the number of recognized species and the economic importance of the taxon have increased substantially. The aim of this paper is to analyze and briefly review new findings from eriophyoid mites' literature after Lindquist et al. book, stressing persistent gaps and needs. Much recent attention has been given to sampling and detection, taxonomy and systematics, faunistic surveys, internal morphology, rearing techniques, biological and ecological aspects, biomolecular studies, and virus vectoring. Recommendations are made for integrating research and promoting broader dissemination of data among specialists and non-specialists.

Interaction of the mite Aceria mangiferae with Fusarium mangiferae, the causal agent of mango malformation disease

The role of the mango bud mite, Aceria mangiferae, in carrying conidia of Fusarium mangiferae, vectoring them into potential infection sites, and assisting fungal infection and dissemination was studied. Following the mite's exposure to a green fluorescent protein-marked isolate, conidia were observed clinging to the mite's body. Agar plugs bearing either bud mites or the pathogen were placed on leaves near the apical buds of potted mango plants. Conidia were found in bud bracts only when both mites and conidia were co-inoculated on the plant, demonstrating that the mite vectored the conidia into the apical bud. Potted mango plants were inoculated with conidia in the presence or absence of mites. Frequency and severity of infected buds were significantly higher in the presence of mites, revealing their significant role in the fungal infection process. Conidia and mite presence were monitored with traps in a diseased orchard over a 2-year period. No windborne bud mites bearing conidia were found; however, high numbers of windborne conidia were detected in the traps. These results suggest that A. mangiferae can carry and vector conidia between buds and assist in fungal penetration but does not play a role in the aerial dissemination of conidia between trees.