Summary
Previous studies have found a correlation between the abilities of PVX vector‐expressed HCPro variants to bind small RNAs (sRNAs), and to suppress silencing. Moreover, HCPro preferred to bind viral sRNAs of 21–22 nucleotides (nt) containing 5′‐terminal adenines. This would require such viral sRNAs to have either different access to the suppressor than those of plant sequences, or different molecular properties.
To investigate this preference further, we have used suppressor‐competent or suppressor‐deficient HCPro variants, expressed from either T‐DNAs or potyvirus constructs. Then, the sRNAs generated in plants and associated with the purified HCPro variants were characterized.
Marked differences were observed in the ratios of sRNAs of plant vs nonplant origin that bound to suppressor‐competent HCPro, depending on the mode of its expression. Regardless of the means of expression, HCPro retained the same preference among the nonplant sRNAs of 21–22 nt for those with 5′‐terminal adenines. Relative methylation levels of individual sRNAs were assessed, and the nonplant sRNAs were found to be significantly less methylated in the presence of the suppressor.
Targeted binding of sRNAs based on size, 5′‐terminal sequence and origin, together with affecting their methylation, could explain how HCPro counteracts silencing.
The receptor-like kinase BIR1 functions as a negative regulator of cell death and defense in Arabidopsis. Previous studies showed that BIR1 expression is up-regulated during infections with microbes and viruses. However, the biological consequences of BIR1 induction remain unknown. Here, we use a DEX-inducible expression system in Arabidopsis to investigate the outputs associated with physiological and non-physiological levels of BIR1 expression. We show that BIR1 induction at physiological levels significantly interferes with gene expression and plasmodesmata callose deposition triggered by canonical PTI elicitors. We found that plants that accumulated non-physiological doses of BIR1 displayed morphological defects that concur with transcriptomic changes in multiple plant defense genes. We provide experimental evidence that EDS1 and SOBIR1 are required for the ETI-type cell death phenotypes associated to non-physiological levels of BIR1. Here we propose that BIR1 induction may represent a pathogen-triggered mechanism to modulate plant defenses during infection. Our model posits that when BIR1 regulation is lost, BIR1 integrity is sensed by one or several guarding resistance (R) proteins to initiate an ETI-like response, in which SOBIR1 cooperates with EDS1 to transduce signals downstream of R proteins.
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