The immune repressor BIR1 contributes to antiviral defense and undergoes transcriptional and post‐transcriptional regulation during viral infections

Abstract

Summary BIR1 is a receptor‐like kinase that functions as a negative regulator of basal immunity and cell death in Arabidopsis. Using Arabidopsis thaliana and Tobacco rattle virus (TRV), we investigate the antiviral role of BIR1, the molecular mechanisms of BIR1 gene expression regulation during viral infections, and the effects of BIR1 overexpression on plant immunity and development. We found that SA acts as a signal molecule for BIR1 activation during infection. Inactivating mutations of BIR1 in the bir1‐1 mutant cause strong antiviral resistance independently of constitutive cell death or SA defense priming. BIR1 overexpression leads to severe developmental defects, cell death and premature death, which correlate with the constitutive activation of plant immune responses. Our findings suggest that BIR1 acts as a negative regulator of antiviral defense in plants, and indicate that RNA silencing contributes, alone or in conjunction with other regulatory mechanisms, to define a threshold expression for proper BIR1 function beyond which an autoimmune response may occur. This work provides novel mechanistic insights into the regulation of BIR1 homeostasis that may be common for other plant immune components.

Turnip mosaic virus in oilseed rape activates networks of sRNA-mediated interactions between viral and host genomes

“…For example, the strong virus-induced accumulation of miR1885b was accompanied by a subtle increment of its predicted target transcripts in both cultivars. This scenario may illustrate a fine-tuned miRNA-driven regulation that prevents the excessive accumulation of target transcripts that are upregulated during infection, as recently described for several plant immune receptors 2 , 56 , 57 . In other instances, negative correlation was seen in only one of the two cultivars, which could be caused by cultivar-specific differences in the expression of the target gene.…”

“…The production of vasiRNAs from the coding region of genes has been previously noted in Arabidopsis plants systemically infected with several unrelated viruses 2,28 . vasiRNA biogenesis is DCL4-and RNA-DIRECTED RNA POLYMERASE 1-dependent and it is suppressed by the VSR of Cucumber mosaic virus, but not by the VSR of TuMV.…”

“…Red index letters indicate vsiRNAs and corresponding mRNA target cleavage signatures found redundantly in both cultivars. The vsiRNA labeled with "g [1][2][3][4] " is predicted to guide the cleavage of conserved sequences in eight different mRNAs encoding four different ethylene-responsive factors (ERFs). c The vsiRNA labeled with "g [1][2][3][4] " mimics miR172 by causing ERF mRNA cleavage at exactly the same nucleotide position as the miRNA.…”

“…We propose that miR403 in conjunction with other AGO2derived vasiRNAs contributes to maintain AGO2 under functional levels when it is transcriptionally activated in response to virus infection. This regulatory scenario recalls the posttranscriptional control of many other pathogen-responsive genes and serves to avoiding potential deleterious effects resulting from protein overexpression 2,56,57,73 . NBR1 genes illustrate how trans-acting vasiRNAs are competent to target even other unrelated, secondary mRNA targets in TuMV-infected cells.…”

“…Viral infection in susceptible plant hosts is associated with the altered expression of genes and the activation of pattern-triggered immunity (PTI) 1 , 2 and RNA silencing 3 . Both defense responses are triggered upon recognition of double-stranded RNA (dsRNA) produced during viral replication.…”