Dissecting the temporal genetic networks programming soybean embryo development from embryonic morphogenesis to post-germination.

KEY MESSAGE: Desiccation-stage transcription factors perform similar functions, with early ones focused on desiccation tolerance and later ones on development. Gene networks governing late embryo development diverge between soybean and Arabidopsis. To understand gene activities programming seed embryo development, we profiled the soybean embryo transcriptome across embryonic morphogenesis through post-germination. Transcriptomic landscapes across embryo development feature highly prevalent transcripts, categorized into early and late groups, with shared and distinct functions. During the mid-storage reserve accumulation stage, the upregulated genes are enriched with regulatory tasks at both the transcriptional and chromatin levels, including DNA methylation and chromatin remodeling. The epigenetic-related functions also dominate in the upregulated genes during germination, involving core histone variants and histone chaperones. Gene network analysis reveals both stage-specific modules and modules active across multiple stages. The desiccation-associated gene module integrates diverse transcription factors (TFs) that are sequentially active during different desiccation stages, transitioning from abiotic stress functions early on to developmental functions later. Two TFs, active during the early and mid-desiccation stages were functionally assessed in Arabidopsis overexpression lines to uncover their potential roles in desiccation processes. Interestingly, nearly half of the Arabidopsis orthologs of soybean TFs active in the desiccation-associated module are inactive during Arabidopsis desiccation. Our results reveal that chromatin and transcriptional regulation coordinate during mid-storage reserve accumulation, while distinct epigenetic mechanisms drive germination. Additionally, gene modules either perform stage-specific functions or are required across multiple stages, and gene networks during late embryogenesis diverge between soybean and Arabidopsis. Our studies provide new information on the biological processes and gene networks underlying development from embryonic morphogenesis to post-germination.

Abscisic Acid Induces Resistance against Bamboo Mosaic Virus through Argonaute2 and 3.

Plant resistance to pathogens is tuned by defense-related hormones. Of these, abscisic acid (ABA) is well documented to moderate resistance against fungi and bacteria. However, ABA's contribution to resistance against viruses is pleiotropic. ABA affects callose deposition at plasmodesmata (therefore hindering the viral cell-to-cell movement), but here, we show that when callose synthase is down-regulated, ABA still induces resistance against infection with Bamboo mosaic virus (BaMV). By examining the potential connections between the ABA and RNA-silencing pathways in Arabidopsis (Arabidopsis thaliana), we showed that ABA regulates the expression of almost the whole ARGONAUTE (AGO) gene family, of which some are required for plant resistance against BaMV Our data show that BaMV infection and ABA treatment regulate the same set of AGOs, with positive effects on AGO1, AGO2, and AGO3, no effect on AGO7, and negative effects on AGO4 and AGO10 The BaMV-mediated regulation of AGO1, AGO2, and AGO3 is ABA dependent, because the accumulation of these AGOs in BaMV-infected ABA mutants did not reach the levels observed in infected wild-type plants. In addition, the AGO1-miR168a complex is dispensable for BaMV resistance, while AGO2 and AGO3 were important for ABA-mediated resistance. While most ago mutants showed increased susceptibility to BaMV infection (except ago10), ago1-27 showed reduced BaMV titers, which was attributed to the up-regulated levels of AGO2, AGO3, and AGO4 We have established that ABA regulates the expression of several members of the AGO family, and this regulation partially contributes to ABA-mediated resistance against BaMV These findings reveal another role for ABA in plants.

Exploring the Multifunctional Roles of Odontoglossum Ringspot Virus P126 in Facilitating Cymbidium Mosaic Virus Cell-to-Cell Movement during Mixed Infection.

Synergistic interactions among viruses, hosts and/or transmission vectors during mixed infection can alter viral titers, symptom severity or host range. Viral suppressors of RNA silencing (VSRs) are considered one of such factors contributing to synergistic responses. Odontoglossum ringspot virus (ORSV) and cymbidium mosaic virus (CymMV), which are two of the most significant orchid viruses, exhibit synergistic symptom intensification in Phalaenopsis orchids with unilaterally enhanced CymMV movement by ORSV. In order to reveal the underlying mechanisms, we generated infectious cDNA clones of ORSV and CymMV isolated from Phalaenopsis that exerted similar unilateral synergism in both Phalaenopsis orchid and Nicotiana benthamiana. Moreover, we show that the ORSV replicase P126 is a VSR. Mutagenesis analysis revealed that mutation of the methionine in the carboxyl terminus of ORSV P126 abolished ORSV replication even though some P126 mutants preserved VSR activity, indicating that the VSR function of P126 alone is not sufficient for viral replication. Thus, P126 functions in both ORSV replication and as a VSR. Furthermore, P126 expression enhanced cell-to-cell movement and viral titers of CymMV in infected Phalaenopsis flowers and N. benthamiana leaves. Taking together, both the VSR and protein function of P126 might be prerequisites for unilaterally enhancing CymMV cell-to-cell movement by ORSV.

Convenient Auto-Processing Vector Based on Bamboo Mosaic Virus for Presentation of Antigens Through Enzymatic Coupling.

We have developed a new binary epitope-presenting CVP platform based on bamboo mosaic virus (BaMV) by using the sortase A (SrtA)-mediated ligation technology. The reconstructed BaMV genome harbors two modifications: 1) a coat protein (CP) with N-terminal extension of the tobacco etch virus (TEV) protease recognition site plus 4 extra glycine (G) residues as the SrtA acceptor; and 2) a TEV protease coding region replacing that of the triple-gene-block proteins. Inoculation of such construct, pKB5G, on Nicotiana benthamiana resulted in the efficient production of filamentous CVPs ready for SrtA-mediated ligation with desired proteins. The second part of the binary platform includes an expression vector for the bacterial production of donor proteins. We demonstrated the applicability of the platform by using the recombinant envelope protein domain III (rEDIII) of Japanese encephalitis virus (JEV) as the antigen. Up to 40% of the BaMV CP subunits in each CVP were loaded with rEDIII proteins in 1 min. The rEDIII-presenting BaMV CVPs (BJLPET5G) could be purified using affinity chromatography. Immunization assays confirmed that BJLPET5G could induce the production of neutralizing antibodies against JEV infections. The binary platform could be adapted as a useful alternative for the development and mass production of vaccine candidates.

Disrupting the Homeostasis of High Mobility Group Protein Promotes the Systemic Movement of Bamboo mosaic virus.

Viruses hijack various organelles and machineries for their replication and movement. Ever more lines of evidence indicate that specific nuclear factors are involved in systemic trafficking of several viruses. However, how such factors regulate viral systemic movement remains unclear. Here, we identify a novel role for Nicotiana benthamiana high mobility group nucleoprotein (NbHMG1/2a) in virus movement. Although infection of N. benthamiana with Bamboo mosaic virus (BaMV) decreased NbHMG1/2a expression levels, nuclear-localized NbHMG1/2a protein was shuttled out of the nucleus into cytoplasm upon BaMV infection. NbHMG1/2a knockdown or even overexpression did not affect BaMV accumulation in inoculated leaves, but it did enhance systemic movement of the virus. Interestingly, the positive regulator Rap-GTPase activation protein 1 was highly upregulated upon infection with BaMV, whereas the negative regulator thioredoxin h protein was greatly reduced, no matter if NbHMG1a/2a was silenced or overexpressed. Our findings indicate that NbHMG1/2a may have a role in plant defense responses. Once its homeostasis is disrupted, expression of relevant host factors may be perturbed that, in turn, facilitates BaMV systemic movement.