Effects of virus infection and growth irradiance on fitness components and photosynthetic properties of Eupatorium makinoi (Compositae)

Papaya Leaf Curl Virus (PaLCuV) Infection on Papaya (Carica papaya L.) Plants Alters Anatomical and Physiological Properties and Reduces Bioactive Components

Papaya leaves are used frequently for curing scores of ailments. The medicinal properties of papaya leaves are due to presence of certain bioactive/pharmacological compounds. However, the papaya leaf curl virus (PaLCuV), a geminivirus, is a major threat to papaya cultivation globally. During the present investigation, we observed that PaLCuV infection significantly altered the anatomy, physiology, and bioactive properties of papaya leaves. As compared to healthy leaves, the PaLCuV-infected leaves were found to have reduced stomatal density (76.83%), stomatal conductance (78.34%), photosynthesis rate (74.87%), water use efficiency (82.51%), chlorophyll (72.88%), carotenoid (46.63%), osmolality (48.55%), and soluble sugars (70.37%). We also found lower enzymatic activity (superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT)-56.88%, 85.27%, and 74.49%, respectively). It was found that the size of guard cells (50%), transpiration rate (45.05%), intercellular CO₂ concentration (47.81%), anthocyanin (27.47%), proline content (74.17%), malondialdehyde (MDA) (106.65%), and electrolyte leakage (75.38%) was elevated in PaLCuV-infected leaves. The chlorophyll fluorescence analysis showed that the infected plant leaves had a significantly lower value of maximal quantum yield of photosystem II (PSII (Fv/Fm), photochemical quantum yield of photosystem I (PSI (Y(I)), and effective quantum yield of PSII (Y(II)). However, in non-photochemical quenching mechanisms, the proportion of energy dissipated in heat form (Y(NPQ)) was found to be significantly higher. We also tested the bioactivity of infected and healthy papaya leaf extracts on a Caenorhabditis elegans (C. elegans) model system. It was found that the crude extract of papaya leaves significantly enhanced the life span of C. elegans (29.7%) in comparison to virus-infected leaves (18.4%) on application of 100 µg/mL dose of the crude extract. Our research indicates that the PaLCuV-infected leaves not only had anatomical and physiological losses, but that pharmacological potential was also significantly decreased.

Suppression of nbe-miR166h-p5 attenuates leaf yellowing symptoms of potato virus X on Nicotiana benthamiana and reduces virus accumulation

MicroRNAs (miRNAs) play essential roles in plant development. There is increasing evidence that changed expression of miRNAs in virus-infected plants contributes to the development of viral symptoms. Here, we analysed the altered expression of miRNAs of Nicotiana benthamiana in response to Potato virus X (PVX) by Illumina Solexa sequencing. One of the 21 miRNAs significantly affected, nbe-miR166h-p5, was closely associated with viral symptoms. Using the Tobacco rattle virus-based miRNA suppression (VbMS) system, we found that the suppression of nbe-miR166h-p5 in plants caused leaves to turn dark green with increased chlorophyll. When PVX was inoculated on nbe-miR166h-p5-suppressed plants, the leaf yellowing symptom of PVX was largely attenuated with less reduction in chlorophyll content, and the accumulation of PVX was decreased. nbe-miR166h-p5 was also up-regulated in plants infected by Turnip mosaic virus (TuMV), and its suppression attenuated the leaf yellowing symptom of TuMV and decreased viral accumulation. Three potential targets of nbe-miR166h-p5 were identified. The results indicate the association of nbe-miR166h-p5 with symptoms of PVX and also with those of TuMV, providing useful information on the relationship between miRNA and viral infection.

Blue Native/SDS-PAGE and iTRAQ-Based Chloroplasts Proteomics Analysis of Nicotiana tabacum Leaves Infected with M Strain of Cucumber Mosaic Virus Reveals Several Proteins Involved in Chlorosis Symptoms

Virus infection in plants involves necrosis, chlorosis, and mosaic. The M strain of cucumber mosaic virus (M-CMV) has six distinct symptoms: vein clearing, mosaic, chlorosis, partial green recovery, complete green recovery, and secondary mosaic. Chlorosis indicates the loss of chlorophyll which is highly abundant in plant leaves and plays essential roles in photosynthesis. Blue native/SDS-PAGE combined with mass spectrum was performed to detect the location of virus, and proteomic analysis of chloroplast isolated from virus-infected plants was performed to quantify the changes of individual proteins in order to gain a global view of the total chloroplast protein dynamics during the virus infection. Among the 438 proteins quantified, 33 showed a more than twofold change in abundance, of which 22 are involved in the light-dependent reactions and five in the Calvin cycle. The dynamic change of these proteins indicates that light-dependent reactions are down-accumulated, and the Calvin cycle was up-accumulated during virus infection. In addition to the proteins involved in photosynthesis, tubulin was up-accumulated in virus-infected plant, which might contribute to the autophagic process during plant infection. In conclusion, this extensive proteomic investigation on intact chloroplasts of virus-infected tobacco leaves provided some important novel information on chlorosis mechanisms induced by virus infection.

Chlorophyll fluorescence lifetime imaging provides new insight into the chlorosis induced by plant virus infection

KEY MESSAGE

Leaf chlorosis induced by plant virus infection has a short fluorescence lifetime, which reflects damaged photosynthetic complexes and degraded chloroplasts. Plant viruses often induce chlorosis and necrosis, which are intimately related to photosynthetic functions. Chlorophyll fluorescence lifetime measurement is a valuable noninvasive tool for analyzing photosynthetic processes and is a sensitive indicator of the environment surrounding the fluorescent molecules. In this study, our central goal was to explore the effect of viral infection on photosynthesis by employing chlorophyll fluorescence lifetime imaging (FLIM), steady-state fluorescence, non-photochemical quenching (NPQ), transmission electron microscopy (TEM), and pigment analysis. The data indicated that the chlorophyll fluorescence lifetime of chlorotic leaves was significantly shorter than that of healthy control leaves, and the fitted short lifetime component of chlorophyll fluorescence of chlorotic leaves was dominant. This dominant short lifetime component may result from damage to the structure of thylakoid, which was confirmed by TEM. The NPQ value of chlorotic leaves was slightly higher than that of healthy green leaves, which can be explained by increased neoxanthin, lutein and violaxanthin content relative to chlorophyll a. The difference in NPQ is slight, but FLIM can provide simple and direct characterization of PSII structure and photosynthetic function. Therefore, this technique shows great potential as a simple and rapid method for studying mechanisms of plant virus infection.

Identification and regulation of host genes related to Rice stripe virus symptom production

Viral infections cause plant chlorosis, stunting, necrosis or other symptoms. The down-regulation of chloroplast-related genes (ChRGs) is assumed to be responsible for chlorosis. We identified the differentially expressed genes (DEGs) in Rice stripe virus (RSV)-infected Nicotiana benthamiana, and examined the contribution of 75 down-regulated DEGs to RSV symptoms by silencing them one by one using Tobacco rattle virus (TRV)-induced gene silencing. Silencing of 11 of the 75 down-regulated DEGs caused plant chlorosis, and nine of the 11 were ChRGs. Silencing of a down-regulated DEG encoding the eukaryotic translation initiation factor 4A (eIF4A) caused leaf-twisting and stunting that were visible on RSV-infected N. benthamiana. A region of RSV RNA4 was complementary to part of eIF4A mRNA and virus-derived small interfering (vsiRNAs) from that region were present in infected N. benthamiana. When expressed as artificial microRNAs, those vsiRNAs could target NbeIF4A mRNA for regulation. We provide experimental evidence supporting the association of ChRGs with chlorosis and show that eIF4A is involved in RSV symptom development. This is also the first report demonstrating that siRNA derived directly from a plant virus can target a host gene for regulation.

Why mosaic? Gene expression profiling of African cassava mosaic virus-infected cassava reveals the effect of chlorophyll degradation on symptom development

Cassava mosaic disease, caused by cassava begomoviruses, is the most serious disease for cassava in Africa. However, the pathogenesis of this disease is poorly understood. We employed high throughput digital gene expression profiling based on the Illumina Solexa sequencing technology to investigate the global transcriptional response of cassava to African cassava mosaic virus infection. We found that 3,210 genes were differentially expressed in virus-infected cassava leaves. Gene ontology term and Kyoto Encyclopedia of Genes and Genomes pathway analysis indicated that genes implicated in photosynthesis were most affected, consistent with the chlorotic symptoms observed in infected leaves. The upregulation of chlorophyll degradation genes, including the genes encoding chlorophyllase, pheophytinase, and pheophorbide a oxygenase, and downregulation of genes encoding the major apoproteins in light-harvesting complex II were confirmed by qRT-PCR. These findings, together with the reduction of chlorophyll b content and fewer grana stacks in the infected leaf cells, reveal that the degradation of chlorophyll plays an important role in African cassava mosaic virus symptom development. This study will provide a road map for future investigations into viral pathogenesis.

Reduction of leaf area and symptom severity as proxies of disease-induced plant mortality: the example of the Cauliflower mosaic virus infecting two Brassicaceae hosts

Disease induced effects on host survival are important to understand the evolution of parasitic virulence and host resistance/tolerance. Unfortunately, experiments evaluating such effects are in most cases logistically demanding justifying the measurement of survival proxies. For plant hosts commonly used proxies are leaf area and the nature and severity of visual qualitative disease symptoms. In this study we tested whether these traits are indeed correlated to the host mortality rate induced by viral infection. We infected Brassica rapa and Arabidopsis thaliana plants with different natural isolates of Cauliflower mosaic virus (CaMV) and estimated over time the development of symptoms and the relative reduction of leaf area compared to healthy plants and followed plant mortality. We observed that the mortality of infected plants was correlated with the relative reduction of leaf area of both B. rapa and A. thaliana. Measures of mortality were also correlated with the severity of visual qualitative symptoms but the magnitude of the correlations and the time frame at which they were significant depended on the host plant: stronger and earlier correlations were observed on A. thaliana.

Photosynthetic alterations of pea leaves infected systemically by pea enation mosaic virus: A coordinated decrease in efficiencies of CO(2) assimilation and photosystem II photochemistry

We have investigated photosynthetic changes of fully expanded pea leaves infected systemically by pea enation mosaic virus (PEMV) that often attacks legumes particularly in northern temperate regions. A typical compatible virus-host interaction was monitored during 40 post-inoculation days (dpi). An initial PEMV-induced decrease in photosynthetic CO(2) assimilation was detected at 15 dpi, when the virus appeared in the measured leaves. This decrease was not induced by stomata closure and corresponded with a decrease in the efficiency of photosystem II photochemistry (Φ(PSII)). Despite of a slight impairment of oxygen evolution at this stage, PSII function was not primarily responsible for the decrease in Φ(PSII). Chlorophyll fluorescence imaging revealed that Φ(PSII) started to decrease from the leaf tip to the base. More pronounced symptoms of PEMV disease appeared at later stages, when a typical mosaic and enations appeared in the infected leaves and oxidative damage of cell membranes was detected. From 30 dpi, a degradation of photosynthetic pigments accelerated, stomata were closing and corresponding pronounced decline in CO(2) assimilation was observed. A concomitant photoprotective responses, i.e. an increase in non-photochemical quenching and accumulation of de-epoxidized xanthophylls, were also detected. Interestingly, alternative electron sinks in chloroplasts were not stimulated by PEMV infection, which is in contradiction to earlier reports dealing with virus-induced plant stresses. The presented results show that the PEMV-induced alterations in mature pea leaves accelerated leaf senescence during which a decrease in Φ(PSII) took place in coordinated manner with an inhibition of CO(2) assimilation.

The potential of plant viruses to promote genotypic diversity via genotype x environment interactions

BACKGROUND AND AIMS

Genotype by environment (G × E) interactions are important for the long-term persistence of plant species in heterogeneous environments. It has often been suggested that disease is a key factor for the maintenance of genotypic diversity in plant populations. However, empirical evidence for this contention is scarce. Here virus infection is proposed as a possible candidate for maintaining genotypic diversity in their host plants.

METHODS

The effects of White clover mosaic virus (WClMV) on the performance and development of different Trifolium repens genotypes were analysed and the G × E interactions were examined with respect to genotype-specific plant responses to WClMV infection. Thus, the environment is defined as the presence or absence of the virus.

KEY RESULTS

WClMV had a negative effect on plant performance as shown by a decrease in biomass and number of ramets. These effects of virus infection differ greatly among host genotypes, representing a strong G × E interaction. Moreover, the relative fitness and associated ranking of genotypes changed significantly between control and virus treatments. This shift in relative fitness among genotypes suggests the potential for WClMV to provoke differential selection on T. repens genotypes, which may lead to negative frequency-dependent selection in host populations.

CONCLUSIONS

The apparent G × E interaction and evident repercussions for relative fitness reported in this study stress the importance of viruses for ecological and evolutionary processes and suggest an important role for viruses in shaping population dynamics and micro-evolutionary processes.

Arabidopsis thaliana as a model for the study of plant-virus co-evolution

Understanding plant-virus coevolution requires wild systems in which there is no human manipulation of either host or virus. To develop such a system, we analysed virus infection in six wild populations of Arabidopsis thaliana in Central Spain. The incidence of five virus species with different life-styles was monitored during four years, and this was analysed in relation to the demography of the host populations. Total virus incidence reached 70 per cent, which suggests a role of virus infection in the population structure and dynamics of the host, under the assumption of a host fitness cost caused by the infection. Maximum incidence occurred at early growth stages, and co-infection with different viruses was frequent, two factors often resulting in increased virulence. Experimental infections under controlled conditions with two isolates of the most prevalent viruses, cauliflower mosaic virus and cucumber mosaic virus, showed that there is genetic variation for virus accumulation, although this depended on the interaction between host and virus genotypes. Comparison of Q(ST)-based genetic differentiations between both host populations with F(ST) genetic differentiation based on putatively neutral markers suggests different selection dynamics for resistance against different virus species or genotypes. Together, these results are compatible with a hypothesis of plant-virus coevolution.

Risk assessment of genetically engineered crops: fitness effects of virus-resistance transgenes in wild Cucurbita pepo

The development of crops genetically engineered for pathogen resistance has raised concerns that crop-to-wild gene flow could release wild or weedy relatives from regulation by the pathogens targeted by the transgenes that confer resistance. Investigation of these risks has also raised questions about the impact of gene flow from conventional crops into wild plant populations. Viruses in natural plant populations can play important roles in plant fecundity and competitive interactions. Here, we show that virus-resistance transgenes and conventional crop genes can increase fecundity of wild plants under virus pressure. We asked how gene flow from a cultivated squash (Cucurbita pepo) engineered for virus resistance would affect the fecundity of wild squash (C. pepo) in the presence and absence of virus pressure. A transgenic squash cultivar was crossed and backcrossed with wild C. pepo from Arkansas. Wild C. pepo, transgenic backcross plants, and non-transgenic backcross plants were compared in field plots in Ithaca, New York, USA. The second and third generations of backcrosses (BC2 and BC3) were used in 2002 and 2003, respectively. One-half of the plants were inoculated with zucchini yellow mosaic virus (ZYMV), and one-half of the plants were maintained as healthy controls. Virus pressure dramatically decreased the fecundity of wild C. pepo plants and non-transgenic backcross plants relative to transgenic backcross plants, which showed continued functioning of the virus-resistance transgene. In 2002, non-transgenic backcross fecundity was slightly higher than wild C. pepo fecundity under virus pressure, indicating a possible benefit of conventional crop alleles, but they did not differ in 2003 when fecundity was lower in both groups. We detected no fitness costs of the transgene in the absence of the virus. If viruses play a role in the population dynamics of wild C. pepo, we predict that gene flow from transgenic, virus-resistant squash and, to a much lesser extent, conventionally bred squash would increase C. pepo fecundity. Studies such as this one, in combination with documentation of the probability of crop-to-wild gene flow and surveys of virus incidence in wild populations, can provide a solid basis for environmental risk assessments of crops genetically engineered for virus resistance.

Natural wild hosts of sweet potato feathery mottle virus show spatial differences in virus incidence and virus-like diseases in Uganda

Sweet potato feathery mottle virus (SPFMV, genus Potyvirus) is globally the most common pathogen of sweetpotato. An East African strain of SPFMV incites the severe 'sweetpotato virus disease' in plants co-infected with Sweet potato chlorotic stunt virus and threatens subsistence sweetpotato production in East Africa; however, little is known about its natural hosts and ecology. In all, 2,864 wild plants growing in sweetpotato fields or in their close proximity in Uganda were observed for virus-like symptoms and tested for SPFMV in two surveys (2004 and 2007). SPFMV was detected at different incidence in 22 Ipomoea spp., Hewittia sublobata, and Lepistemon owariensis, of which 19 species are new hosts for SPFMV. Among the SPFMV-positive plants, approximately 60% displayed virus-like symptoms. Although SPFMV incidence was similar in annual and perennial species, virus-like diseases were more common in annuals than perennials. Virus-like diseases and SPFMV were more common in the eastern agroecological zone than the western, central, and northern zones, which contrasted with known incidence of SPFMV in sweetpotato crops. The data on a large number of new natural hosts of SPFMV detected in this study provide novel insights into the ecology of SPFMV in East Africa.

Effects of Eupatorium yellow vein virus infection on photosynthetic rate, chlorophyll content and chloroplast structure in leaves of Eupatorium makinoi during leaf development

Infection of Eupatorium yellow vein geminivirus (EpYVV, formerly called tobacco leaf curl virus, TLCV) causes variegation in Eupatorium makinoi Kawahara et Yahara leaves. We examined changes in photosynthesis during leaf development to clarify what is the primary event when photosynthesis is suppressed in virus-infected E. makinoi leaves. The gas-exchange rate, leaf absorptance, chlorophyll (Chl) and nitrogen contents, leaf anatomy and chloroplast ultrastructure were compared between virus-infected and uninfected E. makinoi leaves at various developmental stages. These photosynthetic properties did not differ between infected and uninfected leaves when they were young. However, when expanded, infected leaves showed lower maximum quantum yield of photosynthetic CO₂ uptake in the incident photosynthetically active photon fluence rate (PPFR), which was attributed to their lower Chl contents. The Chla / b ratio was higher and the grana had fewer thylakoids in the infected leaves, which are features common to Chl b-deficient mutants that have defects in Chl synthesis. Our results suggested that, in E. makinoi leaves, EpYVV infection primarily impairs Chl biosynthesis. Possible mechanisms of the suppression of photosynthesis in E. makinoi leaves by virus infection are discussed.

Variability and genetic structure of plant virus populations

Populations of plant viruses, like all other living beings, are genetically heterogeneous, a property long recognized in plant virology. Only recently have the processes resulting in genetic variation and diversity in virus populations and genetic structure been analyzed quantitatively. The subject of this review is the analysis of genetic variation, its quantification in plant virus populations, and what factors and processes determine the genetic structure of these populations and its temporal change. The high potential for genetic variation in plant viruses, through either mutation or genetic exchange by recombination or reassortment of genomic segments, need not necessarily result in high diversity of virus populations. Selection by factors such as the interaction of the virus with host plants and vectors and random genetic drift may in fact reduce genetic diversity in populations. There is evidence that negative selection results in virus-encoded proteins being not more variable than those of their hosts and vectors. Evidence suggests that small population diversity, and genetic stability, is the rule. Populations of plant viruses often consist of a few genetic variants and many infrequent variants. Their distribution may provide evidence of a population that is undifferentiated, differentiated by factors such as location, host plant, or time, or that fluctuates randomly in composition, depending on the virus.

Molecular evolution of a host-range gene in geminiviruses infecting asexual populations of Eupatorium makinoi

Asexual plants of Eupatorium makinoi is frequently infected with tobacco leaf curl geminivirus (TLCV). The host range of TLCV is narrow, and ORF C4 is considered to function as a host range determinant. Using this TLCV-Eupatorium system, we tested the expectation that the rate of amino acid replacements will be accelerated in ORF C4 if resistant genes of the host plants drive molecular evolution in ORF C4. ORF C4 is entirely contained within a longer ORF C1 encoding a replication protein. We analyzed 21 sequences containing ORF C4 and a part of ORF C1. While per-site number of synonymous substitutions exceeded that of replacements in ORF C1, per-site number of replacements exceeded that of synonymous substitutions in ORF C4. However, this excess of per-site replacement in ORF C4 was mostly explained by the overlap gene nature, because most synonymous substitutions in ORF C1 change amino acid of ORF C4. In conclusion, not positive but negative selection is a predominant mode characterizing molecular evolution of ORF C4.