A cDNA from tobacco codes for an inhibitor of virus replication (IVR)-like protein

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

The Virus-Induced Transcription Factor SHE1 Interacts with and Regulates Expression of the Inhibitor of Virus Replication (IVR) in N Gene Tobacco

The transcription factor SHE1 was induced by tobacco mosaic virus (TMV) infection in tobacco cv. Samsun NN (SNN) and SHE1 inhibited TMV accumulation when expressed constitutively. To better understand the role of SHE1 in virus infection, transgenic SNN tobacco plants generated to over-express SHE1 (OEx-SHE1) or silence expression of SHE1 (si-SHE1) were infected with TMV. OEx-SHE1 affected the local lesion resistance response to TMV, whereas si-SHE1 did not. However, si-SHE1 allowed a slow systemic infection to occur in SNN tobacco. An inhibitor of virus replication (IVR) was known to reduce the accumulation of TMV in SNN tobacco. Analysis of SHE1 and IVR mRNA levels in OEx-SHE1 plants showed constitutive expression of both mRNAs, whereas both mRNAs were less expressed in si-SHE1 plants, even after TMV infection, indicating that SHE1 and IVR were associated with a common signaling pathway. SHE1 and IVR interacted with each other in four different assay systems. The yeast two-hybrid assay also delimited sequences required for the interaction of these two proteins to the SHE1 central 58-79% region and the IVR C-terminal 50% of the protein sequences. This suggests that SHE is a transcription factor involved in the induction of IVR and that IVR binds to SHE1 to regulate its own synthesis.

The Secret Life of the Inhibitor of Virus Replication

The inhibitor of virus replication (IVR) is an inducible protein that is not virus-target-specific and can be induced by several viruses. The GenBank was interrogated for sequences closely related to the tobacco IVR. Various RNA fragments from tobacco, tomato, and potato and their genomic DNA contained IVR-like sequences. However, IVRs were part of larger proteins encoded by these genomic DNA sequences, which were identified in Arabidopsis as being related to the cyclosome protein designated anaphase-promoting complex 7 (APC7). Sequence analysis of the putative APC7s of nine plant species showed proteins of 558-561 amino acids highly conserved in sequence containing at least six protein-binding elements of 34 amino acids called tetratricopeptide repeats (TPRs), which form helix-turn-helix structures. The structures of Arabidopsis APC7 and the tobacco IVR proteins were modeled using the AlphaFold program and superimposed, showing that IVR had the same structure as the C-terminal 34% of APC7, indicating that IVR was a product of the APC7 gene. Based on the presence of various transcription factor binding sites in the APC7 sequences upstream of the IVR coding sequences, we propose that IVR could be expressed by these APC7 gene sequences involving the transcription factor SHE1.

A wheat aminocyclopropane-1-carboxylate oxidase gene, TaACO1, negatively regulates salinity stress in Arabidopsis thaliana

TaACO1 could catalyze ACC into ethylene in vitro. Constitutive expression of TaACO1 in Arabidopsis conferred salt sensitivity, and TaACO1 regulates salt stress mainly via the DREB1/CBF signal transduction pathway. Ethylene signaling plays essential roles in mediating plant responses to biotic and abiotic stresses, besides regulating plant growth and development. The roles of ethylene biosynthesis in abiotic stress, however, remain elusive. In this study, an aminocyclopropane-1-carboxylate oxidase gene, TaACO1, affecting the terminal step in ethylene biosynthesis, was isolated from a salt-tolerant bread wheat introgression line Shanrong No. 3 (SR3) and its effect on salt-stress response was examined. Purified recombinant protein of TaACO1 heterogenously expressed in Escherchia coli could catalyze ACC into ethylene in vitro. TaACO1 transcripts were down-regulated by salt, drought, oxidative stress and ABA. TaACO1-transgenic plants conferred salt sensitivity as judged from the seed germination, cotyledon greening and the relative root growth under salt stress. Constitutive expression of TaACO1 in Arabidopsis increased AtMYB15 expression and suppressed the expression of stress-responsive genes AtRAB18, AtCBF1 and AtCBF3. These findings are helpful in understanding the roles of ethylene biosynthesis in plant salt-stress response.

Overexpression of the anaphase-promoting complex (APC) genes in Nicotiana tabacum promotes increasing biomass accumulation

The anaphase-promoting complex (APC) plays pivotal roles in cell cycle pathways related to plant development. In this study, we present evidence that overproduction of APC10 from Arabidopsis thaliana in tobacco (Nicotiana tabacum) plants promotes significant increases in biomass. Analyzes of plant's fresh and dried weight, root length, number of days to flower and number of seeds of plants overexpressing AtAPC10 verified an improved agronomic performance of the transgenic plants. Detailed analyzes of the leaf growth at the cellular level, and measurements of leaf cell number, showed that AtAPC10 also produce more cells, showing an enhancement of proliferation in these plants. In addition, crossing of plants overexpressing AtAPC10 and AtCDC27a resulted in a synergistic accumulation of biomass and these transgenic plants exhibited superior characteristics compared to the parental lines. The results of the present study suggest that transgenic plants expressing AtAPC10 and AtAPC10/AtCDC27a concomitantly are promising leads to develop plants with higher biomass.

Tomato plants transformed with the inhibitor-of-virus-replication gene are partially resistant to Botrytis cinerea

Tomato plants transformed with a cDNA clone encoding the inhibitor-of-virus-replication (IVR) gene were partially resistant to Botrytis cinerea. This resistance was observed as a significant reduction in the size of lesions induced by the fungus in transgenic plants compared with the lesions on the nontransgenic control plants. This resistance was weakened when plants were kept at an elevated temperature, 32 degrees C, before inoculation with B. cinerea compared with plants kept at 17 to 22 degrees C prior to inoculation. Resistance correlated with the presence of IVR transcripts, as detected by reverse transcription-polymerase chain reaction. This is one of the few cases in which a gene associated with resistance to a virus also seems to be involved in resistance to a fungal disease.

The influence of RNA-dependent RNA polymerase 1 on potato virus Y infection and on other antiviral response genes

The gene encoding RNA-dependent RNA polymerase 1 (RDR1) is involved in basal resistance to several viruses. Expression of the RDR1 gene also is induced in resistance to Tobacco mosaic virus (TMV) mediated by the N gene in tobacco (Nicotiana tabacum cv. Samsun NN) in an incompatible hypersensitive response, as well as in a compatible response against Potato virus Y (PVY). Reducing the accumulation of NtRDR1 transcripts by RNA inhibition mediated by transgenic expression of a double-stranded RNA hairpin corresponding to part of the RDR1 gene resulted in little or no induction of accumulation of RDR1 transcripts after infection by PVY. Plants with lower accumulation of RDR1 transcripts showed much higher accumulation levels of PVY. Reduced accumulation of NtRDR1 transcripts also resulted in lower or no induced expression of three other antiviral, defense-related genes after infection by PVY. These genes encoded a mitochondrial alternative oxidase, an inhibitor of virus replication (IVR), and a transcription factor, ERF5, all involved in resistance to infection by TMV, as well as RDR6, involved in RNA silencing. The extent of the effect on the induced NtIVR and NtERF5 genes correlated with the extent of suppression of the NtRDR1 gene.

Overexpression of NtERF5, a new member of the tobacco ethylene response transcription factor family enhances resistance to tobacco mosaic virus

A new member of the tobacco (Nicotiana tabacum) AP2/ERF (ethylene response factor) transcription factor family, designated NtERF5, has been isolated by yeast one-hybrid screening. In vitro, recombinant NtERF5 protein weakly binds GCC box cis-elements, which mediate pathogen-regulated transcription of several PR (pathogenesis related) genes. NtERF5 transcription is transiently activated by wounding, by infection with the bacterial pathogen Pseudomonas syringae, as well as by inoculation with Tobacco mosaic virus (TMV). In contrast, NtERF5 transcription is not enhanced after application of salicylic acid, jasmonic acid, or ethylene. Constitutive overexpression of NtERF5 (ERF5-Oex) under control of the 35S promoter results in no visible alterations in plant growth or enhanced resistance to Pseudomonas infection. Furthermore, no constitutive expression of PR genes has been observed. In contrast, ERF5-Oex plants show enhanced resistance to TMV with reference to reduced size of local hypersensitive-response lesions and impaired systemic spread of the virus. Since, in TMV-infected ERF5-Oex plants, the viral RNA accumulates only up to 10 to 30% of the wild-type level, we suggest that NtERF5-regulated gene expression is controlling resistance to viral propagation. Previous research has demonstrated that overexpression of ERF genes enhances resistance to bacterial and fungal pathogens. Here, we provide further evidence that resistance to viral infection can be engineered by overexpression of ERF transcription factors.

Cucumoviruses

Research on the molecular biology of cucumoviruses and their plant-virus interactions has been very extensive in the last decade. Cucumovirus genome structures have been analyzed, giving new insights into their genetic variability, evolution, and taxonomy. A new viral gene has been discovered, and its role in promoting virus infection has been delineated. The localization and various functions of each viral-encoded gene product have been established. The particle structures of Cucumber mosaic virus (CMV) and Tomato aspermy virus have been determined. Pathogenicity domains have been mapped, and barriers to virus infection have been localized. The movement pathways of the viruses in some hosts have been discerned, and viral mutants affecting the movement processes have been identified. Host responses to viral infection have been characterized, both temporally and spatially. Progress has been made in determining the mechanisms of replication, gene expression, and transmission of CMV. The pathogenicity determinants of various satellite RNAs have been characterized, and the importance of secondary structure in satellite RNA-mediated interactions has been recognized. Novel plant genes specifying resistance to infection by CMV have been identified. In some cases, these genes have been mapped, and one resistance gene to CMV has been isolated and characterized. Pathogen-derived resistance has been demonstrated against CMV using various segments of the CMV genome, and the mechanisms of some of these forms of resistances have been analyzed. Finally, the nature of synergistic interactions between CMV and other viruses has been characterized. This review highlights these various achievements in the context of the previous work on the biology of cucumoviruses and their interactions with plants.