Plant epigenome alterations: an emergent player in viroid-host interactions

Virus Research 318 (2022) 198844 Available online 6 June 2022 0168-1702/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Plant epigenome alterations: an emergent player in viroid-host interactions Gustavo G´ omez a , * , Joan Marquez-Molins a , German Martinez b , Vicente Pallas c a Institute for Integrative Systems Biology (I2SysBio), Consejo Superior de Investigaciones Científicas (CSIC) - Universitat de Val` encia (UV), Parc Científic, Cat. Agustín Escardino 9, 46980 Paterna, Spain. b Department of Plant Biology, Swedish University of Agricultural Sciences and Linnean Center for Plant Biology, SE-750 07 Uppsala, Sweden c Instituto de Biología Molecular y Celular de Plantas (IBMCP), Consejo Superior de Investigaciones Científicas (CSIC) - Universitat Polit` ecnica de Val` encia, CPI 8E, Av. de los Naranjos s/n, 46022 Valencia, Spain ARTICLE INFO Key words: Viroid-host interaction DNA methylation Viroid-induced pathogenesis Epigenetic plant-response to infections RNA-dependent DNA-Methylation ABSTRACT It is well known that viroids promote significant alterations at diverse host regulatory levels. However the mechanisms by which these tiny RNAs subvert endogenous regulatory networks remain a to a large extent unsolved question. In the last years diverse studies have revealed the existence of a close interplay between viroid infection and host DNA methylation, suggesting that the modulation of the endogenous transcriptional activity by epigenetic alterations of the host genome may emerge as a novel player in plant-viroid interactions. Here, we summarize the more relevant findings related to alteration of the host DNA methylome in response to viroid infection and discus the potential strategies which may be exploited by these non-conventional pathogenic RNAs to hijack and redesign the plant epigenome. 1. ANTECEDENTS Constrained by a naked and minimal genome (250 - 400 nt), viroids have evolved into versatile molecular entities capable of subvert the host-cell machinery at diverse functional levels to guarantee their infectious cycle (G´ omez and Pall´ as, 2013; Gago-Zachert, 2016; Adkar-- Purushothama and Perreault, 2020; Navarro et al., 2021a). In certain viroid-host interactions, this crosstalk can trigger phenotypic alterations recognized as symptoms (Navarro et al., 2021a). Although several studies have demonstrated that during the infectious process, viroids promote significant alterations at diverse plant regulatory levels, the mechanisms by which these tiny RNAs, with null or residual coding activity, hijack endogenous regulatory networks remain an unsolved question (G´ omez and Pall´ as, 2013). It is well established that a massive transcriptional reprograming constitutes the common characteristic response of plants to adverse environmental conditions, including pathogen infection (Waters et al., 2017; Wang et al., 2020b). Indeed, host transcriptional reprogramming is a central part of plant defense upon pathogen recognition (Tsuda and Somssich, 2015; Pesti et al., 2019). In agreement with this association between transcriptional reprogramming and defense against pathogens, significant changes at the transcriptomic level have been characterized in diverse viroid-host pathosystems including members of both Pospiviroidae and Avsunviroidae families (Owens et al., 2012; Rizza et al., 2012; Herranz et al., 2013; Katsarou et al., 2016a; Kappagantu et al., 2017; Xia et al., 2017; Zheng et al., 2017; Thibaut and Claude, 2018; ˇ Stajner et al., 2019; Takino et al., 2019; Wang et al., 2019b; Wiesyk et al., 2019)(Rizza et al., 2012; Herranz et al., 2013; Katsarou et al., 2016b; Kappagantu et al., 2017; Xia et al., 2017; ˇ Stajner et al., 2019; Wiesyk et al., 2019). Transcriptional reprogramming is consequence of dynamic changes at a deeper level in the genome, and several regulatory layers (operating at both transcriptional and post-transcriptional levels) can modulate the complexity and/or intensity of the transcriptional response to infection (Tsuda and Somssich, 2015; Annacondia et al., 2018; Annacondia and Martinez, 2021). One of those regulatory layers is composed by epigenetic mechanisms, such as DNA methylation and histone modification, that modulate the gene accessibility to the transcriptional machinery and are a key component of the plant response to pathogens (Tsuda and Somssich, 2015; Annacondia et al., 2018; Hannan Parker et al., 2022). DNA methylation is an epigenetic mark that modulates gene and transposable element (TE) expression and, by hence, regulates diverse processes such as genome stability and gene imprinting. In plants, DNA methylation is found in the CG, CHG and CHH sequence contexts (where H is A, C or T). This phenomenon plays a prominent role in the regulation of the gene expression through Transcriptional Gene Silencing * Correspondence author. E-mail address: [email protected] (G. G´ omez). Contents lists available at ScienceDirect Virus Research journal homepage: www.elsevier.com/locate/virusres https://doi.org/10.1016/j.virusres.2022.198844 Received 12 April 2022; Received in revised form 31 May 2022; Accepted 5 June 2022 Virus Research 318 (2022) 198844 2 (TGS) (Zhang et al., 2018). Diverse studies support the existence of a close interplay between infection and host DNA methylation in plants infected by virus (Raja et al., 2008; Yang et al., 2013; Wang et al., 2019a; Corrˆ ea et al., 2020), bacteria (Pavet et al., 2006; Agorio and Vera, 2007; H. et al., 2012; Agn` es et al., 2013; Deleris et al., 2016) and viroids (Martinez et al., 2014). All these studies suggest that the modulation of the endogenous transcriptional activity by epigenetic mechanisms of the host genome may constitute an emerging (and poorly described) player in plant-pathogen interactions (Zhu et al., 2016). The first association between viroid infection and DNA methylation was reported at the end of the past century by researchers studying potato spindle tuber viroid (PSTVd) infection in tobacco plants transformed with a partial non-infectious cDNA of the PSTVd genome (Wassenegger et al., 1994). Although this analysis did not describe a direct relationship between viroid infection and epigenetic changes associated with the pathogen, it led to the discovery of the RNA-directed DNA-methylation (RdDM) phenomenon. In their analyses, Wassenegger et al (1994) showed that PSTVd transgenes became methylated at the DNA level when the transgenic plants where infected with the full viroid sequence. This was the first evidence that RdDM existed in any organism and it was crucial to the research on this topic (Wassenegger and Dalakouras, 2021). The current RdDM model propose that RNA polymerase IV (POL IV)-derived 24-nucleotide siRNAs (canonical RdDM) or, in exceptional cases, POL II-derived 21/22-nucleotide siRNAs (non-canonical RdDM), are loaded onto AGO4/6 or 9 and guide domains-rearranged methyltransferase 2 (DRM2) to methylate cognate DNA, most likely through a process wherein siRNAs directly interact with DNA or interact with the nascent transcripts produced by POL V (Erdmann and Picard, 2020; Gallego-Bartolom´ e, 2020). In the last years, several pieces of evidence point to this mechanism as a previously ignored player in viroid-host interactions. Here, we briefly summarize the more relevant findings related to the alteration of the host DNA methylome in response to viroid infection (Table 1) and discus the potential strategies which may be exploited by these nonconventional pathogenic RNAs to subvert and redesign the plant epigenome. 2. VIROID INFECTION AND HOST EPIGENETIC CHANGES As mentioned above, the initial experimental evidence supporting the link between viroid infection and DNA methylation was provided by the observation that PSTVd replication was associated to specific de novo methylation of a PSTVd-cDNA integrated as a transgene in tobacco plants (Wassenegger et al., 1994). However, it was not until the year 2014 when the direct interference of viroid infection with the methylation of host sequences was demonstrated. Employing hop stunt viroid (HSVd) as a pathogenic model (Marquez-Molins et al., 2021), it was established that viroid infection induces the hypo-methylation of the promoter region of the ribosomal genes in cucumber plants (Martinez et al., 2014). This alteration of the methylation pattern, predominantly affected cytosines in the CG context and was associated with an increased transcription and accumulation of ribosomal RNA (rRNA) precursors. Similar results were obtained analyzing transgenic N. benthamiana plants constitutively expressing dimeric transcripts of HSVd (Castellano et al., 2015). Additionally, the analysis of HSVd effects in the host male gametophyte of infected cucumber plants, showed that this viroid induced a dynamic demethylation of repetitive DNA regions (rRNA genes and TEs) in cucumber gametophytic cells (Castellano et al., 2016a). Altogether, these results support the notion that the plant epigenetic changes associated to HSVd infection constitute a regulatory phenomenon that is not restricted to a unique host-viroid interaction (cucumber or N. benthamiana) or plant tissue (vegetative or reproductive) and that those changes affect different type of repetitive sequences (rRNA and TEs). Surprisingly, aiming to elucidate the molecular basis of the alterations in the host DNA methylation associated to HSVd infection, it was demonstrated, that in infected cucumber plants, mature forms of HSVd are able to interact in vivo with Histone Deacetylase 6 (HDA6) (Castellano et al., 2016b). Furthermore, HDA6 expression was increased in HSVd-infected plants and the transient overexpression of HDA6 reverted the hypomethylation status of rDNA in infected plants. In Arabidopsis, HDA6 is recognized as a component of the RdDM pathway, involved in the maintenance and de novo DNA methylation of Transposable Elements (TEs), rRNA genes and transgenes (Aufsatz et al., 2002; Probst et al., 2004; May et al., 2005; To et al., 2011; Liu et al., 2012 and 2012b; Hristova et al., 2015) via interactions with Methyltransferase 1 (MET1). Based on these results, it was proposed that the host epigenetic alterations associated to the infection were related with the recruitment and functional inactivation of cucumber HDA6 by HSVd (Castellano et al., 2016b). Interestingly, the lack of HDA6 activity has been associated with spurious Pol II transcription of nonconventional rDNA templates (usually transcribed by RNA Pol I) (Earley et al., 2010). Similarly, in HSVd-infected plants, it was reported an increased accumulation of rRNAs precursors (pre-rRNAs) and sRNAs derived of ribosomal RNA (rb-sRNAs) indicating an unusual transcriptional environment (Martinez et al., 2014; Castellano et al., 2015; Castellano et al., 2016a; Castellano et al., 2016b). Therefore, it was proposed that the recruitment of HDA6 mediated by HSVd may promote a spurious POL II activity of rRNA repeats and in parallel favor the transcription of non-canonical templates such as viroid genomic RNA (Castellano et al., 2016b). Further studies of the relationship between epigenetic regulation and viroid infection have shown different scenarios. For example, in the potato-PSTVd pathosystem, infection with PSTVd was associated with the hypermethylation of the promoter regions of certain potato genes and their consequent transcriptional repression in N. benthamiana (Lv et al., 2016). In this work, using transgenic N. benthamiana plants overexpressing GFP transcripts (line 16c) as experimental host, it was demonstrated that PSTVd replication enhanced GFP silencing by the hypermethylation of the promoter region of this transgene (Lv et al., 2016). The authors suggested that the interference of PSTVd in the host methylation pathway might be attributable to Virp1, a bromodomain-containing viroid-binding protein (Martínez de Alba et al., 2003) required for PSTVd replication (Kalantidis et al., 2007). Similarly, in the tomato-PSTVd pathosystem, a strong activation of host DNA methylation was observed (Torchetti et al., 2016). In particular, Table 1 Detail of the the studies related to viroid-induced DNA methylation. Year Description Host Reference 1994 PSTVd replication is associated to specific de novo methylation of a PSTVd-cDNA integrated as a transgene tobacco Wassenegger et al., 1994 2014 HSVd infection induces the hypomethylation of rDNA in vegetative tissues cucumber Martinez et al., 2014 2015 Constitutive expression of HSVd transcripts is associated to host epigenetic alterations N. benthamiana Castellano et al., 2015 2016 HSVd infection induced demethylation of repetitive DNA (rDNA and TEs) in gametophytic cells cucumber Castellano et al., 2016a 2016 Host epigenetic alterations associated to infection are related with the recruitment and functional inactivation of HDA6 by HSVd mature forms cucumber Castellano et al., 2016b 2016 PSTVd infection is associated with the hypermethylation and transcriptional repression of certain host genes potato Lv et al., 2016 2016 PSTVd infection up-regulated the expression of host genes involved in the DNA methylation pathway tomato Torchetti et al., 2016 G. G´ omez et al. Virus Research 318 (2022) 198844 3 viroid replication up-regulated the expression of key genes involved in the maintenance of DNA methylation (such as Methyltransferase 1 (MET1), Chromomethylase 3 (CMT3) and Domains Rearranged Methylase 2 (DRM2) and histone methylation (like the H3K9 histone methyltransferase KRYPTONITE/SUVH4, KYP). Altogether these results support the notion that (at least in the analyzed host) PSTVd infection may be associated to a general increase of the DNA methylation levels in the host genome. However, it remains unknown if the overexpression of methylation-related genes occurs indirectly as a host response or if the viroid might interfere by interacting with any host factor to promote these alterations. Future analysis of the global DNA-methylation and histone profile landscapes and their dynamism during PSTVd, HSVd or other viroid infection will help to shed light into the overall dynamism of viroid-induced epigenetic changes and their potential connection to transcriptional changes. 3. POTENTIAL WAYS OF PROMOTING VIROID-ASSOCIATED HOST EPIGENETIC CHANGES A growing body of evidence link pathogen invasion with alteration of epigenetic modifications in the genome of animal and plant cells (G´ omez-Díaz et al., 2012; Annacondia et al., 2018; Wang et al., 2019a; Tsai and Cullen, 2020). In the case of plants, the interaction of other molecular pathogens such as viruses with the epigenetic pathways of their hosts, has provided the field with good examples of how viroids might be mediating their induced epigenetic changes. For example, plant DNA viruses of the family Geminiviridae are known to interfere with the host DNA methylation machinery (Rodríguez-Negrete et al., 2013; Yang et al., 2013) to avoid the methylation of their genome, and the consequent transcriptional repression (Raja et al., 2008). This alteration is mediated by the activity of viral proteins (termed viral RNA silencing suppressors, VSRs) that suppress the host epigenetic silencing activity against the viral genomes. As a side-effect of this avoidance of epigenetic silencing in their own viral genomes, Geminivirus-VSRs induce the transcriptional reactivation of TEs in the host genome (Rodríguez-Negrete et al., 2013; Yang et al., 2013). Similarly, RNA viruses generally code for VSRs (Csorba et al., 2015) and, some of these,for example the 2b protein of cucumber mosaic virus, have been shown to interact with siRNAs from the RdDM pathway altering the methylation levels of its target genes (Hamera et al., 2012). Nevertheless, other viruses such as pelargonium line pattern virus (PLPV), which do not have any known interaction with the RdDM pathway through its silencing suppressor, was also able to induce methylation changes in its host genome (P´ erez-Ca˜ nam´ as et al., 2020). Devoid of the ability to encode any protein homologous to VSRs, it is reasonable to assume that viroid might redesign the host epigenome during infection through alternative Figure 1. Potential mechanisms of interference with epigenetic routes in viroid infected plants. The HSVd-mediated recruitment of host-HDA6 is related to epigenetic changes observed in infected cucumber plants (Castellano, Pallas and Gomez, 2016b). The specific de novo methylation (mediated by vd-sRNAs) of PSTVd-cDNA sequences inserted in host-genome was originally described in PSTVd-infected plants (Wassenegger et al., 1994). Alternatively, has been recently proposed that “de novo” methylation step would be triggered by long dsRNAs rather than canonical siRNAs (Wassenegger and Dalakouras, 2021). The possibility that the epigenetic changes observed during viroid infection might be part of the transcriptional defense program of response to stress (Chang et al., 2020), cannot be discarded. G. G´ omez et al. Virus Research 318 (2022) 198844 4 mechanisms. According to the experimental evidence obtained from diverse viroid-host interactions, we envision two alternative mechanisms to explain the interference of these pathogenic RNAs with the host epigenetic pathways, i) Direct Interaction: through the involvement of viroid RNAs (mature forms and/or vd-sRNAs) and ii) Indirect Interaction: by means of the unknown functional regulation of certain host factors (Figure 1). 3.1. Involvement of viroid RNAs (Direct interaction) The early detection of vd-sRNAs recovered from plants infected by members of both Pospiviroidae (Itaya et al., 2001; Papaefthimiou et al., 2001) and Avsunviroidae (Martinez de Alba et al., 2002) families, supported the notion that the vd-sRNAs could guide the silencing of plant-endogenous transcripts (Papaefthimiou et al., 2001; Wang et al., 2004; G´ omez et al., 2009). Although this hypothesis was validated in diverse viroid-host interactions (Wang et al., 2011; Navarro et al., 2012; Adkar-Purushothama et al., 2015; Avina-Padilla et al., 2015; Adkar-- Purushothama et al., 2017; Adkar-Purushothama et al., 2018; Adkar-- Purushothama and Perreault, 2018; Bao et al., 2019; Navarro et al., 2021b), it cannot be excluded the possibility that DCL4/2-synthesized 21-22 nt vd-sRNAs could be incorporated into non-canonical RdDM pathways or that spurious DCL3 activity could generate 24 nt length vd-sRNAs that could be incorporated into the endogenous RdDM pathway, and that both events could guide the specific methylation of complementary regions in the host genome (Figure 1). This idea is supported by the first observation of the specific de novo methylation mediated by vd-sRNAs (generated during PSTVd replication) of full or partial length sequences of viroid cDNA inserted in the genome of tobacco plants (Wassenegger et al., 1994). Additionally, a plausible explanation for epigenetic changes taking place during viroid infection could be the colonization of the host AGO proteins, which are known to be influenced by anomalous presence of siRNAs (McCue et al., 2013; Minoia et al., 2014; Annacondia and Martinez, 2021). Hence, alteration of host RNA silencing pathways, including the RdDM pathway, might lead to altered DNA methylation levels and their associated histone marks. Alternatively, diverse observations (Dalakouras and Wassenegger, 2013) have prompted the proposal of an extended model in which the de novo methylation step would be triggered by long (>50 bp) dsRNAs rather than canonical siRNAs (Wassenegger and Dalakouras, 2021). Although the involvement of longer viroid-derived sequences, might be containing specific structural motifs (Wüsthoff and Steger, 2022), significantly reduces the probability that homologous host DNA regions can exist, this last possibility cannot be completely ruled out (Figure 1). It must be noted that both DCL4/2and DCL3-produced 21/22 and 24 nt TE-derived sRNAs are known to initiate silencing of “retrovirus-like” transcriptionally active TEs (Marí-Ord´ o˜ nez et al., 2013; Nuthikattu et al., 2013). Several studies performed in diverse viroid-hosts pathosystems have evidenced that viroid of both families are capable to subvert host factors in order to adapt the cell environment to their functional needs predominantly related to the intracellular compartmentalization (Martínez de Alba et al., 2003), replication (Mühlbach and S¨ anger, 1979; Navarro et al., 2000; Gas et al., 2007; Nohales et al., 2012a; Nohales et al., 2012b; Seo et al., 2021) and export to vascular tissue for long-distance trafficking (G´ omez and Pall´ as, 2001; Owens et al., 2001; G´ omez and Pall´ as, 2004; Morozov et al., 2014). Considering this experimental evidence, it can be envisioned a scenario in which viroids might modulate the host epigenetic landscape by interfering with the functional activity of certain host factors involved in the DNA methylation pathway (Figure 1). The mechanisms exploited by a viroid to functionally alter a host factor may be the direct physical interaction, as it was described for the interaction HSVd-HDA6 in cucumber plants (Castellano et al., 2016b), or by inducing the differential accumulation of host proteins directly (Torchetti et al., 2016) or indirectly (Lv et al., 2016) involved in the epigenetic pathways that maintain the stability of the plant genome. 3.2. Unknown regulation of host factors (Indirect interaction) Another plausible scenario would be that the epigenetic changes observed during viroid infection were just part of the transcriptional defense program orchestrated during stress (Figure 1). Defense genes such as nucleotide binding site and leucine-rich repeat domains (NBSLRR) proteins are localized in heterochromatic regions of plant genomes (Lee and Yeom, 2015) and are activated at the transcriptional level in epigenetic mutants (Stokes et al., 2002; L´ opez S´ anchez et al., 2016). Additionally, DNA methylation changes are connected to the accessibility of transcriptional regulators such as transcription factors to genomic regions (O’Malley et al., 2016), which are well-known players in the transcriptional response to stresses (Fujita et al., 2006; Chang et al., 2020). It is interesting to note that actually several genes from diverse epigenetic pathways that respond to bacterial infection contain transcription factor binding domains in their promoter regions (Agn` es et al., 2013). Future research will help to solve these unsolved questions that are basic to understand the interplay between epigenetic regulation and viroid biology. 4. REMARKS AND FUTURE PERSPECTIVES The results obtained in the last years evidence a close interaction between the pathogenesis process induced by HSVd and PSTVd (two representative members of the Pospiviroidae family) and the DNA methylation level of the host genome. However, up to date, only viroidinduced changes affecting particular plant genes have been analyzed and the global impact on DNA methylation at a genomic scale is yet unknown. Consequently, deciphering the effects exerted by these plantpathogenic RNAs on the plant epigenome (at both DNA methylation and histone modification levels) as well as identifying subverted hostendogenous mechanisms that might regulate these epigenetic changes emerge as relevant challenges for viroid research in the near future. Another aspect to be elucidated is the specificity of the viroid-host interactions that produce epigenetic alterations, or in other words, whether some DNA methylation changes might be a common phenomenon triggered by nuclear-replicating viroids, or specific to certain viroid/host combinations. For example, the current results in the field (from a very limited number of studies) indicate that while HSVd induced the demethylation of cucumber (Martinez et al., 2014; Castellano et al., 2016a) and N. benthamiana (Castellano et al., 2015) genes, an opposite situation (strong hypermethylation of the analyzed genes) was observed in PSTVd infected tomato (Torchetti et al., 2016) and potato (Lv et al., 2016) plants. However, both effects may not be necessarily contradictory, since due to the absence of information about the temporal evolution of the host-epigenome during the infection and the intrinsically different target regions analyzed, it cannot be discarded that the different methylation effects associated to these two nuclear viroids might be consequence of a specific temporal stage of a dynamic host response. Alternatively, it should be considered the possibility that this functional inconsistence may be due, as mentioned above, to the limited number of host genes analized in both pathosystems. Additionally, it would be interesting to compare the potential methylation effects between severe and mild variants of a specific viroid to try to correlate these epigenomic alterations with their corresponding pathogenic processes. It is well known that induction of the viroid infection responses occurs earlier and is stronger in plants infected with a severe variant (HSVd-g54) than in those infected with a mild variant (HSVd-h) (Xia et al., 2017). Another interesting issue to be considered is whether the infectious process triggered by viroids of the Avsunviroidae family could be associated to host epigenetic changes. Although these pathogenic RNAs replicate in the chloroplast, where no RdDM activity has been reported (Wang et al., 2020a), it is thought that they possess a nuclear processing G. G´ omez et al. Virus Research 318 (2022) 198844 5 step before being targeted to the chloroplasts (G´ omez and Pall´ as, 2012a; G´ omez and Pall´ as, 2012b; Baek et al., 2017) and Avsunviroidae infection is associated to the recovery of vd-sRNAs (Martinez de Alba et al., 2002). Finally, it is important to remark the that the first evidence linking viroid infection and epigenetic alterations in the endogenous host-DNA was only eight years ago (Martinez et al., 2014) and the road traveled so far is short. Future technical advances such as increased depth of sequencing technologies and availability of genome sequences from non-model species will help to understand these alterations to a broader extent. Consequently, much more work is needed to clarify the molecular basis and the functional impact (for both the pathogen and the plant) of the remodeling of the host epigenome in response to viroid infection. FUNDING This work was supported by the Agencia Estatal de Investigacion (AEI) (co-supported by FEDER) Grants PID2019-104126RB-I00 (GG) and PID2020-115571RB-I00 (VP). J.M.M. was recipient of a predoctoral contract ACIF-2017-114 from the Generalitat Valenciana. The funders had no role in the experiment design, data analysis, decision to publish, or preparation of the manuscript. CRediT authorship contribution statement Gustavo G´ omez: Visualization, Conceptualization, Writing – review & editing. Joan Marquez-Molins: Visualization, Conceptualization, Writing – review & editing. German Martinez: Writing – review & editing. Vicente Pallas: Writing – review & editing. Declaration of Competing Interest All the authors declare no conflict interests. REFERENCES Adkar-Purushothama, CR, Brosseau, C, Gigu` ere, T, Sano, T, Moffett, P, Perreault, J-P, 2015. Small RNA Derived from the Virulence Modulating Region of the Potato spindle tuber viroid Silences callose synthase Genes of Tomato Plants. Plant Cell 27, 2178–2194. Adkar-Purushothama, CR, Iyer, PS, Perreault, J-P, 2017. Potato spindle tuber viroid infection triggers degradation of chloride channel protein CLC-b-like and Ribosomal protein S3a-like mRNAs in tomato plants. Sci Rep 7, 8341. 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