Interaction between viral RNA silencing suppressors and host factors in plant immunity

To elucidate events in the molecular arms race between the host and pathogen in evaluating plant immunity, a zigzag model is useful for uncovering aspects common to different host-pathogen interactions. By analogy of the steps in virus-host interactions with the steps in the standard zigzag model outlined in recent papers, we may regard RNA silencing as pattern-triggered immunity (PTI) against viruses, RNA silencing suppressors (RSSs) as effectors to overcome host RNA silencing and resistance gene (R-gene)-mediated defense as effector-triggered immunity (ETI) recognizing RSSs as avirulence proteins. However, because the standard zigzag model does not fully apply to some unique aspects in the interactions between a plant host and virus, we here defined a model especially designed for viruses. Although we simplified the phenomena involved in the virus-host interactions in the model, certain specific interactive steps can be explained by integrating additional host factors into the model. These host factors are thought to play an important role in maintaining the efficacy of the various steps in the main pathway of defense against viruses in this model for virus-plant interactions. For example, we propose candidates that may interact with viral RSSs to induce the resistance response

Hypothesis on particle structure and assembly of rice dwarf phytoreovirus: interactions among multiple structural proteins

To study the morphogenesis and packaging of rice dwarf phytoreovirus (RDV), the interactions among multiple structural proteins were analysed using both the yeast two-hybrid system and far-Western blotting analysis. The following protein-protein interactions were observed. P3 (major core protein) bound to itself as well as to P7 (nucleic acid-binding protein) and P8 (major outer capsid protein). P7 bound to P1 (RNA-dependent RNA polymerase) and P8, in addition to P3. Based on these findings, we hypothesize that the core shell structure is based on P3-P3 interactions and that P7 has the ability to bind to multiple structural proteins as well as to genomic RNAs during viral particle assembly. Based on the observed protein-protein interactions and on computer-aided analysis of the numbers of structural proteins per particle, possible RDV assembly events are proposed

VIGS as a strategy to reverse aphid wing induction by Y‐satellite RNA of cucumber mosaic virus

Y‐satellite RNA (Y‐sat) of cucumber mosaic virus upregulates the expression of the aphid ABCG4 gene, which promotes aphid wing formation. We used ABCG4 virus‐induced gene silencing (VIGS) to prevent the wing‐induction mechanism of Y‐sat and thus inhibited aphid wing formation. Of the aphids on plants with VIGS of ABCG4, only about 30% had wings, and 60–70% of the winged aphids were small and likely impaired in flying ability. In addition, we showed that double‐stranded RNAs (dsRNAs) and small RNAs were transferred from the plant to the aphid to adequately silence aphid genes. Supplying ABCG4 dsRNA by VIGS to aphids is thus a potential strategy to inhibit aphid wing formation

The star-type color pattern in Petunia hybrida 'red Star' flowers is induced by sequence-specific degradation of chalcone synthase RNA

Petunia hybrida ‘Red Star’ is a variety whose flowers exhibit a star-type red and white bicolor pattern. We analyzed the mRNA levels of six genes involved in anthocyanin biosynthesis. Only the level of chalcone synthase (CHS) mRNA was depressed in the unpigmented flower sectors. Both transcriptional activity and the accumulation of short interfering RNA of CHS in the unpigmented sectors were detected. Viral infection blocked the generation of CHS-silenced sectors. These results indicate that sequence-specific degradation of CHS RNA is the primary cause of the formation of white sectors in ‘Red Star’ flowers

Virus induced gene silencing in Antirrhinum majus using the Cucumber mosaic virus vector : Functional analysis of the AINTEGUMENTA (Am-ANT) gene of A. majus

The Arabidopsis gene AINTEGUMENTA (At-ANT) functions in cell proliferation and organ growth. The ANT protein has two copies of the AP2 domains, R1 and R2. Recently, a partial cDNA sequence of the At-ANT homolog in Antirrhinum majus (Am-ANT) was reported (Delgado-Benarroch et al., 2009). Here, we used virus-induced gene silencing (VIGS) to analyze the function of the reported Am-ANT. We then determine the open reading frame (ORF) of Am-ANT and its predicted amino acid sequence. We induced VIGS using Cucumber mosaic virus (CMV-A1) and suppressed the level of Am-ANT mRNA and noted any phenotypic changes. The function of Am-ANT was very similar to that of At-ANT. The A1:ANT-infected Antirrhinum plants had smaller floral organs and leaves, even though cell sizes were unchanged in flowers and larger in leaves. The CMV-based VIGS showed that the cloned Am-ANT gene was indeed functional in cell proliferation and organ growth as observed for At-ANT. In A. majus, CMV vector provide great advantages for analysis of gene functions

Combinations of a host resistance gene and the CI gene of turnip mosaic virus differentially regulate symptom expression in Brassica rapa cultivars

In the pathosystem of Brassica rapa and turnip mosaic virus (TuMV), the type of symptoms expressed by susceptible plants are determined by the gene combinations between the host cultivar and virus strain. In this study, we found that the resistant reaction and symptoms such as systemic lethal necrosis, leaf malformation and mosaic were differentially determined, depending on the combinations of the genotypes for a host locus or two closely linked host loci and the viral CI gene. Systemic necrosis caused by TuMV-UK1 on some B. rapa subsp. pekinensis cultivars is induced in conjunction with a recessive gene, rnt1-2 (resistance and necrosis to tumv 1-2), which is allelic or closely linked to TuMV resistance gene Rnt1-1 on chromosome R6. rnt1-2 is incompletely recessive to rnt1-3, which does not cause any necrotic responses. The genotype rnt1-2/rnt1-3 caused a mild necrosis along leaf veins of severely malformed leaves. A spontaneous mutant, TuMV-UK1 (UK1m), with the amino acid substitution (V1827E) in CI broke Rnt1-1 resistance and altered the systemic necrosis and leaf malformation induced by rnt1-2. This single amino acid in the CI protein of UK1 was also associated with severe mosaic and abnormal leaf development, perhaps interacting with a host unknown factor(s). To clarify the relationship of Rnt1-1 with TuRB01b, which was previously reported as a TuMV-UK1 resistance gene on chromosome R6, the B. rapa cultivar Tropical Delight carrying TuRB01b was inoculated with UK1m or the infectious UK1 clone with the CI V1827E mutation. Because Tropical Delight showed resistance to both mutants, Rnt1-1 might be different from TuRB01b