Co-infection of Sweet Orange with Severe and Mild Strains of Citrus tristeza virus Is Overwhelmingly Dominated by the Severe Strain on Both the Transcriptional and Biological Levels

Citrus tristeza is one of the most destructive citrus diseases and is caused by the phloem-restricted Closterovirus, Citrus tristeza virus. Mild strain CTV-B2 does not cause obvious symptoms on indicators whereas severe strain CTV-B6 causes symptoms, including stem pitting, cupping, yellowing, and stiffening of leaves, and vein corking. Our laboratory has previously characterized changes in transcription in sweet orange separately infected with CTV-B2 and CTV-B6. In the present study, transcriptome analysis of Citrus sinensis in response to double infection by CTV-B2 and CTV-B6 was carried out. Four hundred and eleven transcripts were up-regulated and 356 transcripts were down-regulated prior to the onset of symptoms. Repressed genes were overwhelmingly associated with photosynthesis, and carbon and nucleic acid metabolism. Expression of genes related to the glycolytic, oxidative pentose phosphate (OPP), tricarboxylic acid cycle (TCA) pathways, tetrapyrrole synthesis, redox homeostasis, nucleotide metabolism, protein synthesis and post translational protein modification and folding, and cell organization were all reduced. Ribosomal composition was also greatly altered in response to infection by CTV-B2/CTV-B6. Genes that were induced were related to cell wall structure, secondary and hormone metabolism, responses to biotic stress, regulation of transcription, signaling, and secondary metabolism. Transport systems dedicated to metal ions were especially disturbed and ZIPs (Zinc Transporter Precursors) showed different expression patterns in response to co-infection by CTV-B2/CTV-B6 and single infection by CTV-B2. Host plants experienced root decline that may have contributed to Zn, Fe, and other nutrient deficiencies. Though defense responses, such as, strengthening of the cell wall, alteration of hormone metabolism, secondary metabolites, and signaling pathways, were activated, these defense responses did not suppress the spread of the pathogens and the development of symptoms. The mild strain CTV-B2 did not provide a useful level of cross-protection to citrus against the severe strain CTV-B6.


INTRODUCTION
Citrus tristeza has historically been one of the most destructive and globally distributed citrus diseases and is still responsible for tremendous economic losses to the citrus industries worldwide (Bennett and Costa, 1949;Moreno et al., 2008). The disease is caused by Citrus tristeza virus (CTV), a member of the Closteroviridae, with single-stranded and positive-sense genomic RNA (gRNA) of about ∼19 kb in size (Karasev et al., 1995;Moreno et al., 2008). CTV is phloem-restricted and long-distance spread of the virus is via movement of infected plants or propagation of infected buds. It is also transmitted locally by several aphid species in a semi-persistent mode, most notably by Toxoptera citricida (Kirkaldy). Strains of CTV can be classified as mild to severe based on the type and intensity of symptoms caused in different citrus hosts. Mild CTV strains do not cause symptoms on most indicators and usually do not bring about economic loss, while severe CTV strains induce distinct disease syndromes, including quick decline and death of sweet orange on sour orange rootstocks, or stem pitting (SP) of sweet orange and grapefruit scions when grown on other rootstocks, seedling yellows (SY) in sour orange, and vein clearing in "Mexican" lime seedlings (Dawson et al., 2013). Based on phylogenetic analysis of nucleotide sequences of the most variable genomic region, CTV has been divided into seven genotypes: T3, T30, T36, VT, B165/T68, RB, and HA16-5 (Dawson et al., 2015). It is interesting that complete symptoms of Tristeza disease were observed in transgenic Mexican lime plants that expressed the product of CTV ORF 11, an RNA binding protein designated p23 (Ghorbel et al., 2001).
Physiological and cellular changes caused by virus infection are accompanied by differential gene expression in plants.
Understanding host responses to infection by pathogens facilitates the understanding of plant-pathogen interactions and also provides insights for new control strategies. Changes in the transcriptomes of susceptible Mexican lime (Gandía et al., 2007;Liu et al., 2012;Yang et al., 2013) and sweet orange (Cheng et al., 2016) and resistant trifoliate orange (Cristofani-Yaly et al., 2007) in response to the infection by mild or severe CTV strains have been well documented. Changes in gene expression in resistant, tolerant, and susceptible hosts in response to CTV were also compared to huanglongbing (HLB; Bowman and Albrecht, 2015). Citrus hosts are commonly infected with more than one CTV strain, sometimes as many as seven strains in a single tree (Roy et al., 2010). Some mild CTV strains have played an important role in protecting susceptible commercial varieties in citrus industries worldwide by cross protection (Grant and Costa, 1951), but failure is common (Roistacher and Dodds, 1993) because the presumptive protecting strain must be selected to match the genotype of the strains of CTV in the local environment (Folimonova and Achor, 2010).
Sweet orange (Citrus sinensis) is susceptible to CTV and accounts for ∼60% of citrus production (Moreno et al., 2008;Xu et al., 2013). In a previous study, transcriptional changes in sweet orange separately infected with CTV-B2 and CTV-B6 revealed a disturbance of circadian rhythm and ionic homeostasis, as well as activation of defense responses including modification of cell walls. The regulation of transcription, hormone, and secondary metabolism, are also affected by infection by CTV, but with differing patterns depending on the strain of CTV (Fu et al., 2016). In this study, transcriptome data were collected from sweet orange co-infected with both mild strain CTV-B2 and severe strain CTV-B6. This provides new insight into the host response to simultaneous infection with mild and severe strains of CTV and how these changes are correlated with the host response to infection by single strains of CTV.

Plant Materials and Inoculation with CTV
CTV mild strain B2 (T30 genotype, Florida) and severe strain B6 (mixed genotype, SY568, California; Vives et al., 2005;Ruiz-Ruiz et al., 2006) are maintained in planta as part of the Exotic Pathogens of Citrus Collection (EPCC) at the USDA-ARS Beltsville Agricultural Research Center (BARC) in Beltsville, MD. Both CTV-B2 and CTV-B6 were graft-inoculated into ten "Valencia" sweet orange seedlings simultaneously. Ten trees were mock-inoculated with their own buds as the healthy control.

Extraction of RNA and Detection of CTV
Inoculated plants were tested for the presence of CTV specific amplicons by RT-PCR as described (Roy et al., 2010). RNA extracts were prepared for RNA-Seq immediately after the trees became RT-PCR positive and before any symptoms developed from three young, soft, not fully expanded leaves of uniform size. Leaves were frozen in liquid nitrogen and kept at −80 • C until the extraction of RNA and assessment for quantity and quality with Qubit (Thermo-Fisher, Pittsburgh, PA) and Bioanalyzer (Agilent, Santa Clara, CA) instruments. Three qualified RNA replicates, each of three leaves from different plants, for CTV-B2/CTV-B6 and healthy citrus were sent to Otogenetics (Norcross, GA, USA) for paired-end sequencing with the Illumina HiSeq 2500 platform.

Statistical Analyses and RT-qPCR Verification
Raw reads obtained by Illumina HiSeq 2500 were filtered to exclude low complexity reads. Clean reads from the nine libraries were aligned to the reference genome (Xu et al., 2013) with Bowtie (Langmead, 2010) and fold changes (log 2 FC) for each transcript were evaluated by DEseq2 (Love et al., 2014). Differentially expressed transcripts (DETs) were filtered with cutoff values, Padj ≤ 0.1, |log 2 FC| ≥ 1 and e-value ≤ e −5 . DETs involved in pathways were enriched with Panther (Mi et al., 2009) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) data bases. A summary of gene expression patterns was visualized with MapMan software (Thimm et al., 2004). Twenty genes were selected and transcripts were amplified by RT-qPCR to validate the RNASeq analysis as described (Fu et al., 2016). Primers used for RT-qPCR were designed by Real-time PCR online tool (Integrated DNA Technology, IDT, Table S1).
In order to estimate the relative concentration of CTV-B2 and CTV-B6 genomes in co-infected plants, RNA-Seq libraries from trees co-inoculated with CTV-B2 and CTV-B6 and mock inoculated trees were searched for sequences homologous to the p23 protein of reference CTV T30 genome (AF260651) and the reference CTV T318A genome (DQ151548). The 100 bp paired end reads were mapped to the reference genomes under stringent conditions, allowing a maximum of 1 mismatch per read duo to the high homology between the p23 protein of CTV-B2 and CTV-B6.

Infection and Symptoms
Ten doubly-inoculated trees became positive for both mild CTV-B2 and severe CTV-B6 based on RT-PCR after 2-4 months ( Figure 1). Symptoms developed 5-6 months after inoculation. Trees infected with CTV-B2/CTV-B6 were much smaller and stunted than the healthy controls, with leaf chlorosis, vein corking, and vein curling (Figure 2) increasing as time passed. Symptoms of the co-inoculated plants 12 months after coinoculation were indistinguishable from those produced by plants infected with CTV-B6 alone. In the plants co-inoculated with CTV-B2 and CTV-B6, the p23 transcript of CTV-B6 was found 300 times more frequently than the CTV-B2 homolog. Sequences homologous to p23 were not found in the mock-inoculated controls, nor when other libraries that contained only CTV-B2 or CTV-B6 were searched for the heterologous p23 sequence.  Overview of RNA-Seq 38 to 47 million raw reads were obtained from each inoculated tree with approximate 61% (average of three replicates) of these reads successfully aligned to the C. sinensis reference genome (Xu et al., 2013; Figure S1). Compared with mock-inoculated healthy trees, a total of 767 transcripts were differentially expressed: 411 were induced and 356 were repressed (Padj ≤ 0.1; Figure S2). DETs were found to be enriched for different FIGURE 4 | Metabolic overview of young asymptomatic sweet orange leaves in response to co-infection by Citrus tristeza virus strains CTV-B2 and CTV-B6. Green and red squares are shaded to represent the degree of up-and down-regulation of transcripts in response to CTV-B2 and CTV-B6 co-infection, respectively. Gray circles represent transcripts without differential expression between infected and healthy plants.
Frontiers in Plant Science | www.frontiersin.org biological processes (level 2) with Panther based on orthologs of Arabidopsis thaliana. The distribution of functional categories for up-and down-regulated genes was similar: metabolic processes, cellular processes, localization, biological regulation, and response to stimulus (Figure 3). The set of down-regulated genes had relatively more genes related to cellular component FIGURE 5 | Overview of biotic stress pathways in young asymptomatic sweet orange leaves in response to co-infection by Citrus tristeza virus strains CTV-B2 and CTV-B6. Green and red squares are shaded to represent the degree of up-and down-regulation of transcripts in response to CTV-B2 and CTV-B6 co-infection, respectively. Gray circles represent transcripts in pathways without change between infected and healthy plants. organization/biogenesis but less for localization (Figure 3). In an overview of metabolic and biotic stress pathways, genes related to light reactions, oxidative pentose phosphate (OPP), tricarboxylic acid cycle (TCA), tetrapyrrole synthesis, nucleotide metabolism, carbohydrate, and lipid metabolism were mostly repressed (Figure 4), whereas the genes associated with cell wall synthesis, secondary and hormone metabolism, pathogenesis-related (PR) proteins, and signaling were mostly induced ( Figure 5). A full list of differentially expressed transcripts is shown in more detail (Table S2). The expression levels of selected DETs ranged from −2 to 8 (log 2 FC). Similar expression patterns were obtained for all evaluated transcripts by both RNA-seq and RT-qPCR techniques. The dissociation pattern and single peak obtained after RT-qPCR verified the specificity of RT-qPCR primers ( Figure S3). A high Spearman's rho value (0.82) indicated good correlation of gene expression between the RNA-seq and RT-qPCR results and confirmed the reliability and accuracy of the RNA-seq data in our study ( Figure 6).

Alteration of Ribosomal Composition
A large group of genes encoding ribosomal proteins were repressed, including components of the 30S, 40S, 50S, and 60S subunits, indicating an extensive reprogramming of ribosomal synthesis ( Table 2). Ribosomal proteins (RPs) play roles in metabolism, growth and cell division, but also function in developmental processes as regulatory components (Byrne, 2009).

Cell Wall Barriers Were Enhanced
The expression of a majority of genes for cell wall metabolism was up-regulated: Genes encoding CESA9 (cellulose synthase A9) and PRP4 (cell wall proline-rich protein 4), as well as XTH28 (xyloglucan endotransglycosylase) and XTH30 were all up-regulated and the expression level of EXLB3 (expansin-like B3 precursor) was up-regulated ∼7 fold (Table 3).

Infection and Symptoms
Both strains of CTV were able to establish infections in the inoculated sweet orange plants, indicating that simultaneous inoculation of CTV-B2 and CTV-B6 did not prevent infection and establishment of either strain in sweet orange. We isolated RNA from the inoculated plants after infection was established but before symptoms were expressed. The assessment of gene expression prior to the onset of visible symptoms allowed identification of changes in gene expression that led to the symptoms that were subsequently observed. Symptoms of infection by strain CTV-B2 are very mild and difficult to observe. However, in our doubly inoculated plants the symptoms were extensive and severe. CTV-B6 has a complex genome based on components of several genotypes (Ruiz-Ruiz et al., 2006), and the nucleotide sequence of the p23 gene is 98.5% identical to the reference strains T318A. In our co-inoculated plant, the p23 transcript of CTV-B6 was found much more frequently than the CTV-B2 homolog, consistent with the symptoms induced in the plants (Ghorbel et al., 2001). In cases where strains are simultaneously present in sweet orange, the genotype of the severe component often becomes dominant (Sambade et al., 2007). This is evidence that CTV-B6 is the dominant strain when trees are co-infected with mild CTV-B2 and the severe, virulent CTV-B6, and that CTV-B2 is not able to protect sweet orange from CTV-B6.

Photosynthesis, Nucleic Acid, and Iron Metabolism
Genes that encode proteins that are components of both the light harvesting complex and that contribute to the assembly of tetrapyrroles and chlorophyll were down regulated. In addition to photosynthesis, tetrapyrroles play crucial roles in a broad range of biological processes including respiration and assimilation of nitrogen/sulfur in higher plants (Tanaka et al., 2011). In our study, genes associated with photosynthesis and tetrapyrrole metabolism were mostly down-regulated in response to CTV-B2/CTV-B6. HEMC, HEME2, and HEMF1 are enzyme intermediates in tetrapyrrole synthesis and their downregulation indicates a disturbance of tetrapyrrole synthesis, as well as chlorophyll synthesis in host plants, because chlorophyll biosynthesis has been suggested to depend upon a balance between the methyl erythritol phosphate and tetrapyrrole pathways (Kim et al., 2013). The PsbP protein is indispensible for the regulation/stabilization (Ifuku et al., 2005) and photo autotrophy (Yi et al., 2007) of the PSII complex, as well as normal thylakoid membrane architecture (Yi et al., 2009) in higher plants. PsbP-like proteins (PPLs) are PsbP homologs and PPL1 is required for efficient repair of photo damaged PSII in higher plants (Ishihara et al., 2007). These alterations in gene expression patterns are consistent with altered thylakoid membrane structure and function in response to CTV infection. The decrease of several photosynthetic proteins (i.e., PSAD, PsbP, and LHCA) would also cause defects in photosynthetic performance (Romani et al., 2012). Symptoms such as, yellowing and chlorosis of leaves caused by CTV-B6 or CTV-B2/CTV-B6 may be due in part to the decrease of the PsbP andPPLs, as observed in experiments with PsbP-deficient tobacco (Nicotiana tabacum) which had pale-green-colored leaves (Yi et al., 2009). The decrease in the levels of these proteins limits plant growth as there is less carbohydrate synthesized as a result of the repressed photosynthesis.
Iron metabolism was also dramatically altered in response to infection by CTV-B2 and CTV-B6, which likely contributed to the alterations photosynthetic processes observed. As we observed with infection of citrus with CTV-B2/CTV-B6, genes related to photosynthesis and tetrapyrrole metabolism were also dramatically repressed in Arabidopsis leaves in response to iron (Fe) deficiency (Rodríguez-Celma et al., 2013). Iron has crucial effects on respiration and tetrapyrrole metabolism and its deficiency compromises chlorophyll synthesis, causing chlorosis in developing leaves and decreased photosynthetic activity. The iron deficiency may be exacerbated by the decline of feeder roots induced by CTV-B6, which limit the uptake of iron from the rhizosphere.
The overwhelming down-regulation of genes related to DNA synthesis is also in concert with iron deficiency as iron is required in many enzymatic reactions in the DNA replication pathway. Hexameric mini chromosome maintenance proteins (MCMs) have important roles in DNA replication in plants (Ni et al., (Ni et al., 2009). The down regulation of MCM2 is consistent with the decline in feeder roots seen as the CTV syndrome develops. MCM2 was also down-regulated in rice with limited nutrients (Cho et al., 2008). In A. thaliana, MCM3 is present in various oligomeric forms including as a homohexamer and possesses 3 ′ to 5 ′ helicase and ATPase activities in vitro (Rizvi et al., 2015). MCM7 protein is localized in the nucleus and is required for DNA replication and cytokinesis at an early stage of Arabidopsis development (Springer et al., 2000;Holding and Springer, 2002). The repression of MCM2, MCM3, MCM5, and MCM7 was associated with viral infection previously (Choi et al., 2015), but detailed functional studies have not been carried out. The down-regulation of photosynthetic capability, tetrapyrrole and lipid metabolism and DNA replication are all associated with iron deficiency. Similar symptoms of leaf yellowing following disturbances in heavy metal metabolism are found in citrus suffering from huanglongbing (HLB). In the case of HLB, foliar sprays of nutritional supplements alleviate symptoms (Spann and Schumann, 2009;Stansly et al., 2013), and may be worthwhile in the case of severe CTV as well.

Alteration of Ribosomal Composition
Reduction of shoot growth and cell proliferation were found in ribosomal protein mutants (Byrne, 2009;Horiguchi et al., 2011Horiguchi et al., , 2012. Many chloroplast ribosomal protein genes were strongly down-regulated in a related study in response to co-infection with CTV-B6 and "Candidatus Liberibacter asiaticus, " causal agent of citrus huanglongbing (Bové, 2006;submitted), and were also greatly repressed in response to CTV-B2/CTV-B6, including RPS17, RPL9, RPL17, and RPL18 ( Table 2). Gene expression in chloroplasts is primarily controlled through the regulation of translation, mediated by the ribosome. Great alterations of ribosomal composition were observed in leaves and roots of Arabidopsis in response to iron and phosphate deficiency (Rodríguez-Celma et al., 2013;Wang et al., 2013). Gene expression in chloroplasts was limited and further affected the photosynthesis capacity.

Cell Wall Barriers Were Enhanced
Cellulose is the most abundant and strongest component of cell walls and it is targeted for degradation by pathogens to facilitate penetration of and movement between plant cells. Genes associated with cellulose synthase were induced by double infection with CTV-B2/CTV-B6 and by single infection with CTV-B2 but not by CTV-B6 alone (Fu et al., 2016). XTH gene expression and enzyme activity was positively associated with cell wall elongation (Shin et al., 2006) and strengthening (Antosiewicz et al., 1997). Expansin activity is most often in association with cell-wall loosening in growing cells (Lee et al., 2001), and expansin proteins are distributed throughout cell walls (Zhang and Hasenstein, 2000;Cosgrove et al., 2002). Both XTH and EXLB are also correlated with cell wall modification, which is an important aspect of plant responses to biotic and abiotic stresses, and is likely to be the relevant context in this study. The expression of EXLB3 was also up-regulated by single infection with CTV-B2 (Table 4), but not as strongly as in response to double infection with CTV-B2/CTV-B6. The stronger upregulation of EXLB3 may be stimulated by the presence of virulent CTV-B6. The up-regulation of XTH and EXLB gene expression when under virus attack can modulate cell wall growth and equip the plant with a stronger physical barrier to deter feeding by the vector aphids. The remodeling and thickening of cell walls is also correlated with the symptoms of stiff and thickened leaves and vein corking (Figure 2).

Transportation System
Induction of transporters was also observed in a previous study (Fu et al., 2016). The up-regulation of sugar, amino acid, sulfate, phosphate, and metal and oligopeptide transporters denoted a shortage of these substances and a disturbance of the transportation system in young leaves prior to the development of symptoms. SUC2 (sucrose-proton symporter 2), encoding a sucrose/proton symporter was up-regulated and is necessary for phloem loading and is also fundamental for phloem transport and plant productivity (Srivastava et al., 2009). Ammonium is the preferred form of nitrogen for uptake by plants (Gazzarrini et al., 1999) as it requires less energy for assimilation into amino acids (Bloom et al., 1992). The assimilation of ammonium from the soil solution is regulated by ammonium transporters (AMT) and AtAMT1;1 is expressed in roots and leaves (Engineer and Kranz, 2007). The differential down-regulation level of phloem proteins in response to co-infection with CTV-B2/CTV-B6 and single infection with CTV-B2 are consistent with the restructuring of phloem, as well as with cell-to-cell and long distance movement of viruses (Imlau et al., 1999;Gómez and Pallás, 2004).
The up-regulation of ZIP11 was also observed in response to single infections with CTV-B2 or CTV-B6 (Table 4; Fu et al., 2016). However, the expression of ZIP5, as well as ZIP1, ZIP4, and was down-regulated by CTV-B2 in a single infection ( Table 4). Zinc is a key element possessing both structural and functional roles in plants and about 4% of all predicted proteins in Arabidopsis contain one or more zinc fingers (Kawagashira et al., 2001). A large number of enzymes contain zinc binding sites, such as, copper-zinc superoxide dismutase (Hacisalihoglu et al., 2003) and zinc metallo proteases (Ståhl et al., 2002). The differential expression patterns of ZIP1, ZIP4, and ZIP5 in response to co-infection by CTV-B2/CTV-B6 and infection by CTV-B2 alone is probably due to severe, virulent CTV-B6, which affects the plant profoundly, inducing chlorosis of leaves, collapse of phloem and root decline.
Iron is also an essential metal nutrient required for a number of critical cellular functions such as, the synthesis of heme and heme-dependent oxygen transport, iron-dependent enzymatic reactions, photosynthesis and ribonucleotide synthesis (Straus, 1994;Kobayashi and Nishizawa, 2012). FRO6 (ferric reduction oxidase 6) was up-regulated in response to CTV-B2/CTV-B6 infection. FRO6 is involved in metal binding/acquisition and is responsible for regulating the reduction of iron from ferric to ferrous at the plasma membrane of leaf cells in response to iron deficiency (Maynes, 2013). Compared to roots of ironsufficient plants, very high concentrations of manganese, zinc and copper accumulated in roots of iron-deficient Arabidopsis plants (Korshunova et al., 1999;Connolly et al., 2002;Vert et al., 2002). Hence, the high up-regulation of zinc transporters may be a secondary effect of iron deficiency.

Up-Regulation of Hormone and Secondary Metabolism and Signaling Associated Defense Responses
Auxin regulates gene expression and facilitates elongation of shoot cells by activating the auxin response factor (ARF) family of transcription factors. Auxin is perceived by TIR1/AFB (transport inhibitor response1/auxin signaling F-BOX) family of F-box proteins (Salehin et al., 2015). In the presence of auxin, members of TIR1/AFB family of proteins regulate the polyubiquitylation and proteasomal degradation of Aux/IAA transcriptional repressors (Dharmasiri et al., 2005;Greenham et al., 2011). ARFs are repressed by the Aux/IAA proteins. Some of the most common symptoms produced by plant viruses are leaf curling, chlorosis, and stunting, are related to disruptions of plant hormone production, accumulation, and sensing (Jameson and Clarke, 2002;Padmanabhan et al., 2005). The auxin system has been directly disrupted by viral components in other systems. Auxin signaling was disrupted by the Tobacco mosaic virus (TMV) 126 KDa replication protein via an interaction with select Aux/IAA family members (Padmanabhan et al., 2005(Padmanabhan et al., , 2006, which are removed by auxin-induced degradation. The up regulation of AFB5 and down regulation of ARF18 and ARF19 are signs of alterations in auxin metabolism, which may be associated with disease symptoms such as, stunting and leaf curling. Ethylene is a vital growth regulator and is also important for biotic and abiotic stress responses (Schaller, 2012). Ethylene is perceived by a family of five membrane-associated receptors (ETR1, ETR2, ERS1, ERS2, and EIN4). The ethylene receptors are involved in transcriptional as well as post-transcriptional regulatory mechanisms (O'Malley et al., 2005;Chen et al., 2007;Kevany et al., 2007). The ethylene receptors function as negative regulators of ethylene responses and their function is inactivated through ethylene binding (Bleecker and Kende, 2000;Chang and Stadler, 2001). The up regulation of ERS1 and EIN4 imply enhanced ethylene production, which is an early and active response of plants to virus attack in association with the induction of defense responses (Boller, 1991).
Gibberellins are master regulators for plant growth in response to both abiotic (temperature, salt, and light) and biotic stress (Claeys et al., 2014;Colebrook et al., 2014). A giberellinresponsive pathway regulates photosynthesis (Xie et al., 2016). The up-regulation of AFB5, auxin-responsive protein, ERS1 and EIN4, and gibberellin-responsive protein are changes of hormone signaling pathways in plants under pathogen stress and lead to the induction of the following defense actions.
Terpenoids are a structurally diverse group of plant secondary metabolites and are involved in both direct and indirect plant defenses. Terpene synthases (TPSs) play an important role in the synthesis of volatile terpenes. The expression of TPS21 can be induced by gibberellin and jasmonate and the induction of TPS21 increases the emission of sesquiterpenes. The plant cuticle plays a vital role in the interactions between the plant and environment stresses, including water stress. Alkanes are prominent components of cuticular wax and the alkane-forming pathway is controlled by the co-expression of CER1/CER3 (eceriferum; Bernard et al., 2012). The increased expression of CER1/CER3 increases the amount of wax on the surface of the leaves. It is probable that increasing expression of these genes in association with hormone and signaling pathways mediated by secondary metabolites enhanced the tolerance of the plants to water stress and may deter feeding by the aphid vector of CTV.
When mild CTV-B2 and severe CTV-B6 were inoculated simultaneously, both mild and severe CTV strains successfully established in citrus plants. The sweet orange plants co-infected with both CTV strains showed symptoms typical of CTV-B6, indicating virulent CTV-B6 is the dominant strain and that neither virus affected the other's establishment in sweet orange. The failure of CTV-B2 to cross protect is likely because the two virus strains are from different genotypes Folimonova, 2013). The photosynthetic and carbon metabolism capacities of host plants were greatly reduced, and the composition of the ribosome was greatly altered in response to co-infection. These responses may be attributed to deficiency of iron, and secondarily to zinc deficiency. Host plant metal ion transport systems were generally disturbed and ZIPs showed very different expression patterns in response to co-infection by CTV-B2/CTV-B6 and to single strain infection by CTV-B2, which was due to the presence of virulent CTV-B6.
The outcome of a plant-virus interaction from antagonism to mutualism is determined by the environment, host genotype, and pathogens. Virulence is the negative impact of parasite infection on host fitness (Read, 1994;Little et al., 2010) and is determined by both the host and pathogen. Host defenses may decrease virulence through either resistance or tolerance. The former limits the multiplication of the parasite and the latter decreases damage to the host in spite of the amount of multiplication by the pathogen. C. sinensis is generally susceptible to CTV. Though defense responses, such as, strengthening of cell walls, altered hormone metabolism, increased production of secondary metabolites and signaling proteins were induced, these did not suppress the spread of the pathogens and the development of symptoms caused by CTV-B2/CTV-B6, or strain CTV-B6 alone, although they were able to induce tolerance to strain CTV-B2 (Fu et al., 2016).

AUTHOR CONTRIBUTIONS
SF: performed the laboratory experiments and drafted the manuscript; SF and JS: collected data and carried out all data analyses; JH: designed the study, oversaw the research and cowrote the manuscript; CZ: revised the manuscript.