Inducible transgenic tobacco system to study the mechanisms underlying chlorosis mediated by the silencing of chloroplast heat shock protein 90

Insights Into the Effect of Rice Stripe Virus P2 on Rice Defense by Comparative Proteomic Analysis

Rice stripe virus (RSV) has a serious effect on rice production. Our previous research had shown that RSV P2 plays important roles in RSV infection, so in order to further understand the effect of P2 on rice, we used Tandem Mass Tag (TMT) quantitative proteomics experimental system to analyze the changes of protein in transgenic rice expressing P2 for the first time. The results of proteomics showed that a total of 4,767 proteins were identified, including 198 up-regulated proteins and 120 down-regulated proteins. Functional classification results showed that differentially expressed proteins (DEPs) were mainly localized in chloroplasts and mainly involved in the metabolic pathways. Functional enrichment results showed that DEPs are mainly involved in RNA processing and splicing. We also verified the expression of several DEPs at the mRNA level and the interaction of a transcription factor (B7EPB8) with RSV P2. This research is the first time to use proteomics technology to explore the mechanism of RSV infection in rice with the RSV P2 as breakthrough point. Our findings provide valuable information for the study of RSV P2 and RSV infection mechanism.

Global Screening and Functional Identification of Major HSPs Involved in PVY Infection in Potato

HSP40 (also known as DnaJ), HSP70, and HSP90 are major heat shock protein (HSP) families that play critical roles in plant growth and development and stress adaption. Recently, several members of the three HSP families were reported to be widely involved in the plant host-virus interactions. However, their global expression profiles and core members recruited by viruses are largely unknown. In this study, a total of 89 StDnaJs were identified from a genome-wide survey, and their classification, phylogenetic relationships, chromosomal locations, and gene duplication events were further analyzed. Together with 20 StHSP70s and 7 StHSP90s previously identified in the potato genome, the global expression patterns of the members in 3 HSP families were investigated in 2 potato cultivars during Potato virus Y (PVY) infection using RNA-seq data. Of them, 16 genes (including 8 StDnaJs, 6 StHSP70s, and 2 StHSP90s) were significantly up- or downregulated. Further analysis using qRT-PCR demonstrated that 7 of the 16 genes (StDnaJ06, StDnaJ17, StDnaJ21, StDnaJ63, StHSP70-6, StHSP70-19, and StHSP90.5) were remarkably upregulated in the potato cultivar 'Eshu 3' after PVY infection, implying their potential roles in the potato-PVY compatible interaction. Subsequent virus-induced gene silencing (VIGS) assays showed that silencing of the homologous genes of StDnaJ17, StDnaJ21, StHSP70-6, and StHSP90.5 in Nicotiana. benthamiana plants dramatically reduced the accumulation of PVY, which indicated the four genes may function as susceptibility factors in PVY infection. This study provides candidate genes for exploring the mechanism of potato-PVY compatible interaction and benefits breeding work aiming to produce new cultivars with the ability to grow healthily under PVY infection.

Viroid pathogenesis: a critical appraisal of the role of RNA silencing in triggering the initial molecular lesion

The initial molecular lesions through which viroids, satellite RNAs and viruses trigger signal cascades resulting in plant diseases are hotly debated. Since viroids are circular non-protein-coding RNAs of ∼250-430 nucleotides, they appear very convenient to address this issue. Viroids are targeted by their host RNA silencing defense, generating viroid-derived small RNAs (vd-sRNAs) that are presumed to direct Argonaute (AGO) proteins to inactivate messenger RNAs, thus initiating disease. Here, we review the existing evidence. Viroid-induced symptoms reveal a distinction. Those attributed to vd-sRNAs from potato spindle tuber viroid and members of the family Pospiviroidae (replicating in the nucleus) are late, non-specific and systemic. In contrast, those attributed to vd-sRNAs from peach latent mosaic viroid (PLMVd) and other members of the family Avsunviroidae (replicating in plastids) are early, specific and local. Remarkably, leaf sectors expressing different PLMVd-induced chloroses accumulate viroid variants with specific pathogenic determinants. Some vd-sRNAs containing such determinant guide AGO1-mediated cleavage of mRNAs that code for proteins regulating chloroplast biogenesis/development. Therefore, the initial lesions and the expected phenotypes are connected by short signal cascades, hence supporting a cause-effect relationship. Intriguingly, one virus satellite RNA initiates disease through a similar mechanism, whereas in the Pospiviroidae and in plant viruses the situation remains uncertain.

Comparative proteomic analysis of Nicotiana benthamiana plants under Chinese wheat mosaic virus infection

BACKGROUND

Chinese wheat mosaic virus (CWMV) is a severe threat to winter wheat and is transmitted by Polymyxa graminis. The mechanisms of interactions between CWMV and plants are poorly understood. In this study, a comparative proteomics analysis based on nanoliquid chromatography mass spectrometry (MS)/MS was conducted to characterize proteomic changes in plants responding to CWMV infection.

RESULTS

In total, 2751 host proteins were identified, 1496 of which were quantified and 146 up-regulated and 244 down-regulated proteins were identified as differentially expressed proteins (DEPs). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis showed that DEPs were most strongly associated with photosynthesis antenna proteins, MAPK signaling plant and glyoxylate and dicarboxylate metabolism pathways. Subcellular localization analysis predicted that more than half of the DEPs were localized in the chloroplast, an organelle indispensable for abscisic acid (ABA) synthesis. Our results suggest that CWMV infection interrupts normal chloroplast functions and decreases ABA concentrations in Nicotiana benthamiana. Further analysis showed that the ABA pathway was suppressed during CWMV infection and that ABA treatment induced plant hosts defenses against CWMV.

CONCLUSIONS

We identified several candidate proteins expressed during CWMV infection, and the ABA pathway was strongly associated with responses to CWMV infection in N. benthamiana.

Transcriptome Analysis Shows Activation of Stress and Defense Responses by Silencing of Chlorophyll Biosynthetic Enzyme CHLI in Transgenic Tobacco

In the present study, we have shown the transcriptional changes in a chlorosis model transgenic tobacco plant, i-amiCHLI, in which an artificial micro RNA is expressed in a chemically inducible manner to silence the expression of CHLI genes encoding a subunit of a chlorophyll biosynthetic enzyme. Comparison to the inducer-treated and untreated control non-transformants and untreated i-amiCHLI revealed that 3568 and 3582 genes were up- and down-regulated, respectively, in the inducer-treated i-amiCHLI plants. Gene Ontology enrichment analysis of these differentially expressed genes indicated the upregulation of the genes related to innate immune responses, and cell death pathways, and the downregulation of genes for photosynthesis, plastid organization, and primary and secondary metabolic pathways in the inducer-treated i-amiCHLI plants. The cell death in the chlorotic tissues with a preceding H2O2 production was observed in the inducer-treated i-amiCHLI plants, confirming the activation of the immune response. The involvement of activated innate immune response in the chlorosis development was supported by the comparative expression analysis between the two transgenic chlorosis model systems, i-amiCHLI and i-hpHSP90C, in which nuclear genes encoding different chloroplast proteins were similarly silenced.

Multifaceted roles of HEAT SHOCK PROTEIN 90 molecular chaperones in plant development

HEAT SHOCK PROTEINS 90 (HSP90s) are molecular chaperones that mediate correct folding and stability of many client proteins. These chaperones act as master molecular hubs involved in multiple aspects of cellular and developmental signalling in diverse organisms. Moreover, environmental and genetic perturbations affect both HSP90s and their clients, leading to alterations of molecular networks determining respectively plant phenotypes and genotypes and contributing to a broad phenotypic plasticity. Although HSP90 interaction networks affecting the genetic basis of phenotypic variation and diversity have been thoroughly studied in animals, such studies are just starting to emerge in plants. Here, we summarize current knowledge and discuss HSP90 network functions in plant development and cellular homeostasis.

Impaired Expression of Chloroplast HSP90C Chaperone Activates Plant Defense Responses with a Possible Link to a Disease-Symptom-Like Phenotype

RNA-seq analysis of a transgenic tobacco plant, i-hpHSP90C, in which chloroplast HSP90C genes can be silenced in an artificially inducible manner resulting in the development of chlorosis, revealed the up- and downregulation of 2746 and 3490 genes, respectively. Gene ontology analysis of these differentially expressed genes indicated the upregulation of ROS-responsive genes; the activation of the innate immunity and cell death pathways; and the downregulation of genes involved in photosynthesis, plastid organization, and cell cycle. Cell death was confirmed by trypan blue staining and electrolyte leakage assay, and the H2O2 production was confirmed by diaminobenzidine staining. The results collectively suggest that the reduced levels of HSP90C chaperone lead the plant to develop chlorosis primarily through the global downregulation of chloroplast- and photosynthesis-related genes and additionally through the light-dependent production of ROS, followed by the activation of immune responses, including cell death.

Heat treatment alleviates the growth and photosynthetic impairment of transplastomic plants expressing Leishmania infantum Hsp83-Toxoplasma gondii SAG1 fusion protein

Previously, we showed that transplastomic tobacco plants expressing the LiHsp83-SAG1 fusion protein displayed a chlorotic phenotype and growth retardation, while plants expressing the SAG1 and GRA4 antigens alone did not. We conducted a comprehensive examination of the metabolic and photosynthetic parameters that could be affecting the normal growth of LiHsp83-SAG1 plants in order to understand the origin of these pleiotropic effects. These plants presented all photosynthetic pigments and parameters related to PSII efficiency significantly diminished. However, the expression of CHLI, RSSU and LHCa/b genes did not show significant differences between LiHsp83-SAG1 and control plants. Total protein, starch, and soluble sugar contents were also greatly reduced in LiHsp83-SAG1 plants. Since Hsp90 s are constitutively expressed at much higher concentrations at high temperatures, we tested if the fitness of LiHsp83-SAG1 over-expressing LiHsp83 would improve after heat treatment. LiHsp83-SAG1 plants showed an important alleviation of their phenotype and an evident recovery of the PSII function. As far as we know, this is the first report where it is demonstrated that a transplastomic line performs much better at higher temperatures. Finally, we detected that LiHsp83-SAG1 protein could be binding to key photosynthesis-related proteins at 37 °C. Our results suggest that the excess of this molecular chaperone could benefit the plant in a possible heat shock and prevent the expected denaturation of proteins. However, the LiHsp83-SAG1 protein content was weakly decreased in heat-treated plants. Therefore, we cannot rule out that the alleviation observed at 37 °C may be partially due to a reduction of the levels of the recombinant protein.

Characterization of Proteins Involved in Chloroplast Targeting Disturbed by Rice Stripe Virus by Novel Protoplast⁻Chloroplast Proteomics

Rice stripe virus (RSV) is one of the most devastating viral pathogens in rice and can also cause the general chlorosis symptom in Nicotiana benthamiana plants. The chloroplast changes associated with chlorosis symptom suggest that RSV interrupts normal chloroplast functions. Although the change of proteins of the whole cell or inside the chloroplast in response to RSV infection have been revealed by proteomics, the mechanisms resulted in chloroplast-related symptoms and the crucial factors remain to be elucidated. RSV infection caused the malformation of chloroplast structure and a global reduction of chloroplast membrane protein complexes in N. benthamiana plants. Here, both the protoplast proteome and the chloroplast proteome were acquired simultaneously upon RSV infection, and the proteins in each fraction were analyzed. In the protoplasts, 1128 proteins were identified, among which 494 proteins presented significant changes during RSV; meanwhile, 659 proteins were identified from the chloroplasts, and 279 of these chloroplast proteins presented significant change. According to the label-free LC⁻MS/MS data, 66 nucleus-encoded chloroplast-related proteins (ChRPs), which only reduced in chloroplast but not in the whole protoplast, were identified, indicating that these nuclear-encoded ChRPswere not transported to chloroplasts during RSV infection. Gene ontology (GO) enrichment analysis confirmed that RSV infection changed the biological process of protein targeting to chloroplast, where 3 crucial ChRPs (K4CSN4, K4CR23, and K4BXN9) were involved in the regulation of protein targeting into chloroplast. In addition to these 3 proteins, 41 among the 63 candidate proteins were characterized to have chloroplast transit peptides. These results indicated that RSV infection changed the biological process of protein targeting into chloroplast and the location of ChRPs through crucial protein factors, which illuminated a new layer of RSV⁻host interaction that might contribute to the symptom development.

Inducible expression of magnesium protoporphyrin chelatase subunit I (CHLI)-amiRNA provides insights into cucumber mosaic virus Y satellite RNA-induced chlorosis symptoms

Recent studies with Y satellite RNA (Y-sat) of cucumber mosaic virus have demonstrated that Y-sat modifies the disease symptoms in specific host plants through the silencing of the magnesium protoporphyrin chelatase I subunit (CHLI), which is directed by the Y-sat derived siRNA. Along with the development of peculiar yellow phenotypes, a drastic decrease in CHLI-transcripts and a higher accumulation of Y-sat derived siRNA were observed. To investigate the molecular mechanisms underlying the Y-sat-induced chlorosis, especially whether or not the reduced expression of CHLI causes the chlorosis simply through the reduced production of chlorophyll or it triggers some other mechanisms leading to the chlorosis, we have established a new experimental system with an inducible silencing mechanism. This system involves the expression of artificial microRNAs targeting of Nicotiana tabacum CHLI gene under the control of chemically inducible promoter. The CHLI mRNA levels and total chlorophyll content decreased significantly in 2 days, enabling us to analyze early events in induced chlorosis and temporary changes therein. This study revealed that the silencing of CHLI did not only result in the decreased chlorophyll content but also lead to the downregulation of chloroplast and photosynthesis-related genes expression and the upregulation of defense-related genes. Based on these results, we propose that the reduced expression of CHLI could activate unidentified signaling pathways that lead plants to chlorosis.