Difference of physiological characters in dark green islands and yellow leaf tissue of Cucumber mosaic virus (CMV)-infected Nicotiana tabacum leaves

Catalases mediate tobacco resistance to virus infection through crosstalk between salicylic acid and auxin signaling pathways

Catalases (CATs) play important roles in plant growth, development and defense responses. Previous studies have shown that CATs exhibit different or even opposite effects on plant immunity in different plant-pathogen interactions, but little is known about the mechanisms. In this study, Nicotiana tabacum plants with overexpression or knockout of CAT genes, tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV) were employed to investigate the role of CAT in compatible plant-virus interactions. The results showed that there were dynamic changes in the effect of CAT on N. tabacum defense responses. Overexpression of catalase 1 (CAT1) and catalase 3 (CAT3) improved N. tabacum resistance in the early stage of virus infection but depressed it during the late stages of pathogenesis, especially in CAT3 overexpressing plants. The lower level of electrolyte leakage, lower contents of malonaldehyde and hydrogen peroxide (H2 O2 ), higher activities of antioxidant enzymes and improved functions of photosystem II corresponded to the milder symptoms and higher resistance of infected tobacco plants. In addition, the infection of TMV and CMV resulted in expression changes of CATs in tobacco plants, and pretreatment with H2 O2 facilitated TMV and CMV infection. Further experiments showed that the content of salicylic acid (SA) and the expression of genes related to SA signaling pathway were positively correlated with plant resistance, whereas auxin and its related signaling pathway were related to the viral susceptibility of plants. Taken together, our results demonstrated that CAT1 and CAT3 mediated tobacco resistance to virus infection through crosstalk between SA and auxin signaling pathways.

Tobamovirus infection aggravates gray mold disease caused by Botrytis cinerea by manipulating the salicylic acid pathway in tomato

Botrytis cinerea is the causative agent of gray mold disease, and infects more than 1400 plant species, including important crop plants. In tomato, B. cinerea causes severe damage in greenhouses and post-harvest storage and transport. Plant viruses of the Tobamovirus genus cause significant damage to various crop species. In recent years, the tobamovirus tomato brown rugose fruit virus (ToBRFV) has significantly affected the global tomato industry. Most studies of plant-microbe interactions focus on the interaction between the plant host and a single pathogen, however, in agricultural or natural environments, plants are routinely exposed to multiple pathogens. Here, we examined how preceding tobamovirus infection affects the response of tomato to subsequent infection by B. cinerea. We found that infection with the tobamoviruses tomato mosaic virus (ToMV) or ToBRFV resulted in increased susceptibility to B. cinerea. Analysis of the immune response of tobamovirus-infected plants revealed hyper-accumulation of endogenous salicylic acid (SA), upregulation of SA-responsive transcripts, and activation of SA-mediated immunity. Deficiency in SA biosynthesis decreased tobamovirus-mediated susceptibility to B. cinerea, while exogenous application of SA enhanced B. cinerea symptoms. These results suggest that tobamovirus-mediated accumulation of SA increases the plants' susceptibility to B. cinerea, and provide evidence for a new risk caused by tobamovirus infection in agriculture.

Soybean balanced the growth and defense in response to SMV infection under different light intensities

Light is essential for the growth and defense of soybean. It is not clear how soybeans adjust their defenses to different light environments with different cropping patterns. The mechanism of soybean response to Soybean mosaic virus (SMV) infection under different light intensities was analyzed by RNA-seq sequencing method. Enrichment analysis illustrated that most defense-related genes were down-regulated in the dark and the shade, and up-regulated under hard light and normal light. Soybean can resist SMV infection mainly by activating salicylic acid signaling pathway. Light is essential for activating salicylic acid defense signaling pathways. With the increase of light intensity, the oxidative damage of soybean leaves was aggravated, which promoted the infection of virus. When light was insufficient, the growth of soybean was weak, and the plant-pathogen interaction pathway, MAPK pathway and hormone defense pathway in infected soybean was inhibited. Under hard light, some defense genes in infected soybean were down-regulated to reduce the degree of oxidative damage. The expression of differentially expressed genes was verified by real-time fluorescence quantitative RT-PCR. In order to adapt to the change of light intensity, soybean balanced allocation of resources between growth and defense through a series regulation of gene expression. The results of this study will provide a theoretical basis for the research of SMV resistance in intercropping soybean.

Elimination of BBTV via a systemic in vitro electrotherapy approach

Banana bunchy top virus (BBTV) is the most destructive etiological agent limiting banana cultivation areas globally. This study attempted BBTV elimination by traditional shoot-tip culture (control) and alternative shoot-tip + electrotherapy (treated) techniques. Shoot-tip culture from Musa acuminata cv. 'Grand Naine' infected sources were exposed to 100 mA electric current for different time intervals (20-60 min). Virus indexing (via PCR) and genetic fidelity (by ISSR assay) from the cultures were tested, alongside the physio-biochemical parameters. Exposure of electric current for less than 50 min was ineffective for BBTV elimination. Still, a rise in the duration from 50 min or more led to eradicating the virus from some explants. Elimination of BBTV was complete from 100 % of explants exposed to 100 mA for 60 min, as confirmed by lack of BBTV detection even at six months after acclimatization. In the control treatment, the maximum efficiency of BBTV elimination was 28 % after eight subcultures. On the other hand, improved survival % was observed in the treated culture. Moreover, homogenous ISSR patterns were there between the treated and the mother plant and similar physio-biochemical activities were seen in electro-exposed cultures and healthy ones. Thus, the study reports complete BBTV-elimination from banana with international compliances, for the first time, via electrotherapy while maintaining genomic template and biochemical stability.

Reactive Oxygen Species Contribute to Symptomless, Extreme Resistance to Potato virus X in Tobacco

Here we show that in tobacco (Nicotiana tabacum cultivar Samsun NN Rx1) the development of Rx1 gene-mediated, symptomless, extreme resistance to Potato virus X (PVX) is preceded by an early, intensive accumulation of the reactive oxygen species (ROS) superoxide (O2·-), evident between 1 and 6 h after inoculation and associated with increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activities. This suggests a direct contribution of this ROS to virus restriction during symptomless, extreme resistance. Superoxide inhibition in PVX-inoculated leaves by infiltration of antioxidants (superoxide dismutase [SOD] and catalase [CAT]) partially suppresses extreme resistance in parallel with the appearance of localized leaf necrosis resembling a hypersensitive resistance (HR) response. F1 progeny from crosses of Rx1 and ferritin overproducer (deficient in production of the ROS OH·) tobaccos also display a suppressed extreme resistance to PVX, because significantly increased virus levels are coupled to HR, suggesting a role of the hydroxyl radical (OH·) in this symptomless antiviral defense. In addition, treatment of PVX-susceptible tobacco with a superoxide-generating agent (riboflavin/methionine) results in HR-like symptoms and reduced PVX titers. Finally, by comparing defense responses during PVX-elicited symptomless, extreme resistance and HR-type resistance elicited by Tobacco mosaic virus, we conclude that defense reactions typical of an HR (e.g., induction of cell death/ROS-regulator genes and antioxidants) are early and transient in the course of extreme resistance. Our results demonstrate the contribution of early accumulation of ROS (superoxide, OH·) in limiting PVX replication during symptomless extreme resistance and support earlier findings that virus-elicited HR represents a delayed, slower resistance response than symptomless, extreme resistance.

Superoxide (O2.-) accumulation contributes to symptomless (type I) nonhost resistance of plants to biotrophic pathogens

Nonhost resistance is the most common form of disease resistance exhibited by plants against most pathogenic microorganisms. Type I nonhost resistance is symptomless (i.e. no macroscopically visible cell/tissue death), implying an early halt of pathogen growth. The timing/speed of defences is much more rapid during type I nonhost resistance than during type II nonhost and host ("gene-for-gene") resistance associated with a hypersensitive response (localized necrosis, HR). However, the mechanism(s) underlying symptomless (type I) nonhost resistance is not entirely understood. Here we assessed accumulation dynamics of the reactive oxygen species superoxide (O₂^.-) during interactions of plants with a range of biotrophic and hemibiotrophic pathogens resulting in susceptibility, symptomless nonhost resistance or host resistance with HR. Our results show that the timing of macroscopically detectable superoxide accumulation (1-4 days after inoculation, DAI) is always associated with the speed of the defense response (symptomless nonhost resistance vs. host resistance with HR) in inoculated leaves. The relatively early (1 DAI) superoxide accumulation during symptomless nonhost resistance of barley to wheat powdery mildew (Blumeria graminis f. sp. tritici) is localized to mesophyll chloroplasts of inoculated leaves and coupled to enhanced NADPH oxidase (EC 1.6.3.1) activity and transient increases in expression of genes regulating superoxide levels and cell death (superoxide dismutase, HvSOD1 and BAX inhibitor-1, HvBI-1). Importantly, the partial suppression of symptomless nonhost resistance of barley to wheat powdery mildew by heat shock (49 °C, 45 s) and antioxidant (SOD and catalase) treatments points to a functional role of superoxide in symptomless (type I) nonhost resistance.

RNA-seq approach to analysis of gene expression profiles in dark green islands and light green tissues of Cucumber mosaic virus-infected Nicotiana tabacum

Dark green islands (DGIs) surrounded by light green tissues (LGTs) are common leaf symptoms of plants that are systemically infected by various viruses that induce leaf mosaic in infected plants. The inoculation of Cucumber mosaic virus (CMV) in Nicotiana tabacum produced a commonly occurring sequence of classic patterns of DGIs and LGTs. Previous studies confirmed that there are significant differences between DGIs and LGTs in terms of physiology, biochemistry and molecular biology, but the mechanisms by which DGIs form remain unclear. To investigate the global gene expression changes that occur in these special tissues, individual differential gene expression tag libraries were constructed from three total RNA samples isolated from DGIs, LGTs and control plants (CK) and were sequenced using an Illumina HiSeqTM 2000. An analysis of differentially expressed genes (DEGs) and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed. These analyses revealed the differences between DGIs, LGTs and CK. GO enrichment and KEGG pathway analyses suggested that several pathways related to photosynthesis and chlorophyll metabolism were enriched in DGIs compared to LGTs and CK. Several pathways related to apoptosis were significantly up-regulated in LGTs compared to DGIs. Additionally, we identified sets of DEGs that may be related to the formation or development of DGIs and LGTs. Our systematic analyses provide comprehensive transcriptomic information regarding DGIs and LGTs in CMV-infected N. tabacum. These data will help characterize the detailed mechanisms of DGI and LGT formation.

Characterisation of the dark green islands of cucumber mosaic virus infected Nicotiana tabacum

KEY MESSAGE

There are significant differences between the DGIs and LGTs. Additionally, most of the characteristics indicate that the DGIs are more similar to recovered tissue and can resist viral attacks. Dark green islands (DGIs) surrounded by light green tissues (LGTs) are common leaf symptoms of plants that are systemically infected by various mosaic viruses. We performed cytological, physiological and molecular biological analyses of the DGIs and LGTs in cucumber mosaic virus-infected Nicotiana tabacum leaves. Our results indicated that the DGIs contained less virus than did the LGTs. Compared to the LGTs, the DGIs contained higher levels of the metabolites involved in plant defence. The contents of reduced glutathione and ascorbic acid were increased in the DGIs to reach levels that were even higher than those of control plants. Moreover, hormone measurements and quantitative real-time PCR analysis revealed that the endogenous salicylic acid, ethylene and defence genes mediated these elevations by playing positive roles in the regulation of the DGIs responses to viral infection. The accumulation of cytokinin was also much greater in the DGIs than in the LGTs. Finally, northern blotting analysis indicated that the accumulation of viral small interfering RNAs was decreased in the DGIs compared to the LGTs. Taken together, these results suggest that DGIs might represent leaf areas that have recovered from viral infection due to locally enhanced defence responses.

A broad-spectrum, efficient and nontransgenic approach to control plant viruses by application of salicylic acid and jasmonic acid

Plant viruses cause many diseases that lead to significant economic losses. However, most of the approaches to control plant viruses, including transgenic processes or drugs are plant-species-limited or virus-species-limited, and not very effective. We introduce an application of jasmonic acid (JA) and salicylic acid (SA), a broad-spectrum, efficient and nontransgenic method, to improve plant resistance to RNA viruses. Applying 0.06 mM JA and then 0.1 mM SA 24 h later, enhanced resistance to Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV) and Turnip crinkle virus (TCV) in Arabidopsis, tobacco, tomato and hot pepper. The inhibition efficiency to virus replication usually achieved up to 80-90%. The putative molecular mechanism was investigated. Some possible factors affecting the synergism of JA and SA have been defined, including WRKY53, WRKY70, PDF1.2, MPK4, MPK2, MPK3, MPK5, MPK12, MPK14, MKK1, MKK2, and MKK6. All genes involving in the synergism of JA and SA were investigated. This approach is safe to human beings and environmentally friendly and shows potential as a strong tool for crop protection against plant viruses.