The expanding field of plant virus ecology: Historical foundations, knowledge gaps, and research directions

Indirect Effects of One Plant Pathogen on the Transmission of a Second Pathogen and the Behavior of its Potato Psyllid Vector

Plant pathogens can influence the behavior and performance of insect herbivores. Studies of these associations typically focus on tripartite interactions between a plant host, a plant pathogen, and its insect vector. An unrelated herbivore or pathogen might influence such interactions. This study used a model system consisting of Tobacco mosaic virus (TMV), the psyllid Bactericera cockerelli Sulc, and tomatoes to investigate multipartite interactions among a pathogen, a nonvector, and a plant host, and determine whether shifts in host physiology were behind potential interactions. Additionally, the ability of TMV to affect the success of another pathogen, 'Candidatus Liberibacter solanacearum,' which is transmitted by the psyllid, was studied. In choice trials, psyllids preferred nearly fourfold noninfected plants to TMV-infected plants. No-choice bioassays demonstrated that there was no difference in psyllid development between TMV-infected and control plants; oviposition was twice as high on control plants. Following inoculation by psyllids, 'Candidatus Liberibacter solanacearum' titers were lower in TMV-infected plants than control plants. TMV-infected plants had lower levels of amino acids and sugars but little differences in phenolics and terpenoids, relative to control plants. Possibly, these changes in sugars are associated with a reduction in psyllid attractiveness in TMV-infected tomatoes resulting in decreased infection of 'Candidatus Liberibacter solanacearum.'

Infection of host plants by Cucumber mosaic virus increases the susceptibility of Myzus persicae aphids to the parasitoid Aphidius colemani

Plant viruses can profoundly alter the phenotypes of their host plants, with potentially far-reaching implications for ecology. Yet few studies have explored the indirect, host-mediated, effects of plant viruses on non-vector insects. We examined how infection of Cucurbita pepo plants by Cucumber mosaic virus (CMV) impacted the susceptibility of aphids (Myzus persicae) to attack by the parasitoid wasp Aphidius colemani. In semi-natural foraging assays, we observed higher rates of aphid parasitism on infected plants compared to healthy plants. Subsequent experiments revealed that this difference is not explained by different attack rates on plants differing in infection status, but rather by the fact that parasitoid larvae successfully complete their development more often when aphid hosts feed on infected plants. This suggests that the reduced nutritional quality of infected plants as host for aphids--documented in previous studies--compromises their ability to mount effective defenses against parasitism. Furthermore, our current findings indicate that the aphid diet during parasitoid development (rather than prior to wasp oviposition) is a key factor influencing resistance. These findings complement our previous work showing that CMV-induced changes in host plant chemistry alter patterns of aphid recruitment and dispersal in ways conducive to virus transmission.

Plants, viruses and the environment: Ecology and mutualism

Since the discovery of Tobacco mosaic virus nearly 120 years ago, most studies on viruses have focused on their roles as pathogens. Virus ecology takes a different look at viruses, from the standpoint of how they affect their hosts׳ interactions with the environment. Using the framework of symbiotic relationships helps put the true nature of viruses into perspective. Plants clearly have a long history of relationships with viruses that have shaped their evolution. In wild plants viruses are common but usually asymptomatic. In experimental studies plant viruses are sometimes mutualists rather than pathogens. Virus ecology is closely tied to the ecology of their vectors, and the behavior of insects, critical for transmission of many plant viruses, is impacted by virus-plant interactions. Virulence is probable not beneficial for most host-virus interactions, hence commensal and mutualistic relationships are almost certainly common, in spite of the paucity of literature on beneficial viruses.

Detection of plant viruses in natural environments by using RNA-Seq

Sequencing of RNA by next generation sequencers, RNA-Seq, is revolutionizing virus detection. In addition to the unbiased detection of various viruses from wild plants in natural environments, RNA-Seq also allows for the parallel collection of host plant transcriptome data. Host transcriptome data are highly valuable for studying the responses of hosts to viral infections, as well as viral host manipulation. When detecting viruses using RNA-Seq, it is critical to choose appropriate methods for the removal of rRNA from total RNA. Although viruses with polyadenylated genomes can be detected by RNA-Seq following mRNA purification using oligo-dT beads, viruses with non-polyadenylated genomes are not effectively detected. However, such viruses can be detected by RNA-Seq using the rRNA selective depression method. The high-throughput and cost-effective method of RNA-Seq library preparation which is described here allows us to detect a broad range of viruses in wild plants.

Current impact and future directions of high throughput sequencing in plant virus diagnostics

The ability to provide a fast, inexpensive and reliable diagnostic for any given viral infection is a key parameter in efforts to fight and control these ubiquitous pathogens. The recent developments of high-throughput sequencing (also called Next Generation Sequencing - NGS) technologies and bioinformatics have drastically changed the research on viral pathogens. It is now raising a growing interest for virus diagnostics. This review provides a snapshot vision on the current use and impact of high throughput sequencing approaches in plant virus characterization. More specifically, this review highlights the potential of these new technologies and their interplay with current protocols in the future of molecular diagnostic of plant viruses. The current limitations that will need to be addressed for a wider adoption of high-throughput sequencing in plant virus diagnostics are thoroughly discussed.

Effects of introduced and indigenous viruses on native plants: exploring their disease causing potential at the agro-ecological interface

The ever increasing movement of viruses around the world poses a major threat to plants growing in cultivated and natural ecosystems. Both generalist and specialist viruses move via trade in plants and plant products. Their potential to damage cultivated plants is well understood, but little attention has been given to the threat such viruses pose to plant biodiversity. To address this, we studied their impact, and that of indigenous viruses, on native plants from a global biodiversity hot spot in an isolated region where agriculture is very recent (<185 years), making it possible to distinguish between introduced and indigenous viruses readily. To establish their potential to cause severe or mild systemic symptoms in different native plant species, we used introduced generalist and specialist viruses, and indigenous viruses, to inoculate plants of 15 native species belonging to eight families. We also measured resulting losses in biomass and reproductive ability for some host-virus combinations. In addition, we sampled native plants growing over a wide area to increase knowledge of natural infection with introduced viruses. The results suggest that generalist introduced viruses and indigenous viruses from other hosts pose a greater potential threat than introduced specialist viruses to populations of native plants encountered for the first time. Some introduced generalist viruses infected plants in more families than others and so pose a greater potential threat to biodiversity. The indigenous viruses tested were often surprisingly virulent when they infected native plant species they were not adapted to. These results are relevant to managing virus disease in new encounter scenarios at the agro-ecological interface between managed and natural vegetation, and within other disturbed natural vegetation situations. They are also relevant for establishing conservation policies for endangered plant species and avoiding spread of damaging viruses to undisturbed natural vegetation beyond the agro-ecological interface.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

Donkey orchid symptomless virus: a viral 'platypus' from Australian terrestrial orchids

Complete and partial genome sequences of two isolates of an unusual new plant virus, designated Donkey orchid symptomless virus (DOSV) were identified using a high-throughput sequencing approach. The virus was identified from asymptomatic plants of Australian terrestrial orchid Diuris longifolia (Common donkey orchid) growing in a remnant forest patch near Perth, western Australia. DOSV was identified from two D. longifolia plants of 264 tested, and from at least one plant of 129 Caladenia latifolia (pink fairy orchid) plants tested. Phylogenetic analysis of the genome revealed open reading frames (ORF) encoding seven putative proteins of apparently disparate origins. A 69-kDa protein (ORF1) that overlapped the replicase shared low identity with MPs of plant tymoviruses (Tymoviridae). A 157-kDa replicase (ORF2) and 22-kDa coat protein (ORF4) shared 32% and 40% amino acid identity, respectively, with homologous proteins encoded by members of the plant virus family Alphaflexiviridae. A 44-kDa protein (ORF3) shared low identity with myosin and an autophagy protein from Squirrelpox virus. A 27-kDa protein (ORF5) shared no identity with described proteins. A 14-kDa protein (ORF6) shared limited sequence identity (26%) over a limited region of the envelope glycoprotein precursor of mammal-infecting Crimea-Congo hemorrhagic fever virus (Bunyaviridae). The putative 25-kDa movement protein (MP) (ORF7) shared limited (27%) identity with 3A-like MPs of members of the plant-infecting Tombusviridae and Virgaviridae. Transmissibility was shown when DOSV systemically infected Nicotiana benthamiana plants. Structure and organization of the domains within the putative replicase of DOSV suggests a common evolutionary origin with ‘potexvirus-like’ replicases of viruses within the Alphaflexiviridae and Tymoviridae, and the CP appears to be ancestral to CPs of allexiviruses (Alphaflexiviridae). The MP shares an evolutionary history with MPs of dianthoviruses, but the other putative proteins are distant from plant viruses. DOSV is not readily classified in current lower order virus taxa.

Biosecurity implications of new technology and discovery in plant virus research

Human activity is causing new encounters between viruses and plants. Anthropogenic interventions include changing land use, decreasing biodiversity, trade, the introduction of new plant and vector species to native landscapes, and changing atmospheric and climatic conditions. The discovery of thousands of new viruses, especially those associated with healthy-appearing native plants, is shifting the paradigm for their role within the ecosystem from foe to friend. The cost of new plant virus incursions can be high and result in the loss of trade and/or production for short or extended periods. We present and justify three recommendations for plant biosecurity to improve communication about plant viruses, assist with the identification of viruses and their impacts, and protect the high economic, social, environmental, and cultural value of our respective nations' unique flora: 1) As part of the burden of proof, countries and jurisdictions should identify what pests already exist in, and which pests pose a risk to, their native flora; 2) Plant virus sequences not associated with a recognized virus infection are designated as "uncultured virus" and tentatively named using the host plant species of greatest known prevalence, the word "virus," a general location identifier, and a serial number; and 3) Invest in basic research to determine the ecology of known and new viruses with existing and potential new plant hosts and vectors and develop host-virus pathogenicity prediction tools. These recommendations have implications for researchers, risk analysts, biosecurity authorities, and policy makers at both a national and an international level.

Plant viruses alter insect behavior to enhance their spread

Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. In plant systems, such effects are largely restricted to vectors, because they are mobile and may exhibit preferences dependent upon plant host infection status. Here we report the first evidence that acquisition of a plant virus directly alters host selection behavior by its insect vector. We show that the aphid Rhopalosiphum padi, after acquiring Barley yellow dwarf virus (BYDV) during in vitro feeding, prefers noninfected wheat plants, while noninfective aphids also fed in vitro prefer BYDV-infected plants. This behavioral change should promote pathogen spread since noninfective vector preference for infected plants will promote acquisition, while infective vector preference for noninfected hosts will promote transmission. We propose the “Vector Manipulation Hypothesis” to explain the evolution of strategies in plant pathogens to enhance their spread to new hosts. Our findings have implications for disease and vector management.