BIP and the unfolded protein response are important for potyvirus and potexvirus infection

A vicinal oxygen chelate protein facilitates viral infection by triggering the unfolded protein response in Nicotiana benthamiana

Vicinal oxygen chelate (VOC) proteins are members of an enzyme superfamily with dioxygenase or non-dioxygenase activities. However, the biological functions of VOC proteins in plants are poorly understood. Here, we show that a VOC in Nicotiana benthamiana (NbVOC1) facilitates viral infection. NbVOC1 was significantly induced by infection by beet necrotic yellow vein virus (BNYVV). Transient overexpression of NbVOC1 or its homolog from Beta vulgaris (BvVOC1) enhanced BNYVV infection in N. benthamiana, which required the nuclear localization of VOC1. Consistent with this result, overexpressing NbVOC1 facilitated BNYVV infection, whereas, knockdown and knockout of NbVOC1 inhibited BNYVV infection in transgenic N. benthamiana plants. NbVOC1 interacts with the basic leucine zipper transcription factors bZIP17/28, which enhances their self-interaction and DNA binding to the promoters of unfolded protein response (UPR)-related genes. We propose that bZIP17/28 directly binds to the NbVOC1 promoter and induces its transcription, forming a positive feedback loop to induce the UPR and facilitating BNYVV infection. Collectively, our results demonstrate that NbVOC1 positively regulates the UPR that enhances viral infection in plants.

A viral movement protein co-opts endoplasmic reticulum luminal-binding protein and calreticulin to promote intracellular movement

Intracellular movement is an important step for the initial spread of virus in plants during infection. This process requires virus-encoded movement proteins (MPs) and their interaction with host factors. Despite the large number of known host factors involved in the movement of different viruses, little is known about host proteins that interact with one of the MPs encoded by potexviruses, the triple-gene-block protein 3 (TGBp3). The main obstacle lies in the relatively low expression level of potexviral TGBp3 in hosts and the weak or transient nature of interactions. Here, we used TurboID-based proximity labeling to identify the network of proteins directly or indirectly interacting with the TGBp3 of a potexvirus, Bamboo mosaic virus (BaMV). Endoplasmic reticulum (ER) luminal-binding protein 4 and calreticulin 3 of Nicotiana benthamiana (NbBiP4 and NbCRT3, respectively) associated with the functional TGBp3-containing BaMV movement complexes, but not the movement-defective mutant, TGBp3M. Fluorescent microscopy revealed that TGBp3 colocalizes with NbBiP4 or NbCRT3 and the complexes move together along ER networks to cell periphery in N. benthamiana. Loss- and gain-of-function experiments revealed that NbBiP4 or NbCRT3 is required for the efficient spread and accumulation of BaMV in infected leaves. In addition, overexpression of NbBiP4 or NbCRT3 enhanced the targeting of BaMV TGBp1 to plasmodesmata (PD), indicating that NbBiP4 and NbCRT3 interact with TGBp3 to promote the intracellular transport of virion cargo to PD that facilitates virus cell-to-cell movement. Our findings revealed additional roles for NbBiP4 and NbCRT3 in BaMV intracellular movement through ER networks or ER-derived vesicles to PD, which enhances the spread of BaMV in N. benthamiana.

P3/P3N-PIPO of PVY interacting with BI-1 inhibits the degradation of NIb by ATG6 to facilitate virus replication in N. benthamiana

Introduction

Autophagy not only plays an antiviral role but also can be utilized by viruses to facilitate virus infection. However, the underlying mechanism of potato virus Y (PVY) infection against plant autophagy remains unclear. BI-1, localizing to the endoplasmic reticulum (ER), is a multifunctional protein and may affect the virus infection.

Methods

In this study, Y2H, BiFC, qRT-PCR, RNA-Seq, WB and so on were used for research.

Results

P3 and P3N-PIPO of PVY can interact with the Bax inhibitor 1 (BI-1) of N. benthamiana. However, BI-1 knockout mutant showed better growth and development ability. In addition, when the BI-1 gene was knocked out or knocked down in N. benthamiana, the PVY-infected mutant showed milder symptoms and lower virus accumulation. Analysis of transcriptome data showed that the deletion of NbBI-1 weakened the gene expression regulation induced by PVY infection and NbBI-1 may reduce the mRNA level of NbATG6 by regulated IRE1-dependent decay (RIDD) in PVY-infected N. benthamiana. The expression level of the ATG6 gene of PVY-infected WT was significantly down-regulated, relative to the PVY-infected mutant. Further results showed that ATG6 of N. benthamiana can degrade NIb, the RNA-dependent RNA polymerase (RdRp) of PVY. NbATG6 has a higher mRNA level in PVY-infected BI-1 knockout mutants than in PVY-infected WT.

Conclussion

The interaction of P3 and/or P3N-PIPO of PVY with BI-1 decrease the expression of the ATG6 gene might be mediated by RIDD, which inhibits the degradation of viral NIb and enhances viral replication.

Plantago asiatica mosaic virus: An emerging plant virus causing necrosis in lilies and a new model RNA virus for molecular research

TAXONOMY

Plantago asiatica mosaic virus belongs to the genus Potexvirus in the family Alphaflexiviridae of the order Tymovirales.

VIRION AND GENOME PROPERTIES

Plantago asiatica mosaic virus (PlAMV) has flexuous virions of approximately 490-530 nm in length and 10-15 nm in width. The genome of PlAMV consists of a single-stranded, positive-sense RNA of approximately 6.13 kb. It contains five open reading frames (ORFs 1-5), encoding a putative viral polymerase (RdRp), movement proteins (triple gene block proteins, TGBp1-3), and coat protein (CP), respectively.

HOST RANGE

PlAMV has an exceptionally wide host range and has been isolated from various wild plants, including Plantago asiatica, Nandina domestica, Rehmannia glutinosa, and other weed plants. Experimentally PlAMV can infect many plant species including Nicotiana benthamiana and Arabidopsis thaliana. It also infects ornamental lilies and frequently causes severe necrotic symptoms. However, host range varies depending on isolates, which show significant biological diversity within the species.

GENOME DIVERSITY

PlAMV can be separated into five clades based on phylogenetic analyses; nucleotide identities are significantly low between isolates in the different clades.

TRANSMISSION

PlAMV is not reported to be transmitted by biological vectors. Virions of PlAMV are quite stable and it can be transmitted efficiently by mechanical contact.

DISEASE SYMPTOMS

PlAMV causes red-rusted systemic necrosis in ornamental lilies, but it shows much weaker, if any, symptoms in wild plants such as P. asiatica.

CONTROL

Control of the disease caused by PlAMV is based mainly on rapid diagnosis and elimination of the infected bulbs or plants.

UPR signaling at the nexus of plant viral, bacterial, and fungal defenses

In recent years there have been significant advances in our understanding of the ER stress responses in plants that are associated with virus infection, as well as bacterial and fungal diseases. In plants, ER stress induced by virus infection includes several signaling pathways that include the unfolded protein response (UPR) to promote the expression of chaperone proteins for proper protein folding. Understanding how facets of ER stress signaling broadly engage in pathogen responses, as well as those that are specific to virus infection is important to distinguishing features essential for broad cellular defenses and processes that may be specifically linked to viral infectivity and disease.