PATTERN-TRIGGERED IMMUNITY (PTI) INDUCTION AND TRANSCRIPTIONAL REPROGRAMMING IN PERSISTANT ALLEXIVIRUS INFECTION
In virus-plant interactions, one of the major mechanisms for plant antiviral immunity relies on RNA silencing, which is often suppressed by co-evolving viral suppressors, thus enhancing viral pathogenicity in susceptible hosts. However RNA silencing should not only be viewed as an antiviral mechanism that must be counteracted. In fact, many viruses encode weak or transiently active suppressors and probably do not use these viral proteins for control RNA silencing; for example, Shallot virus X (ShVX) do not code the active silencing suppressor and consequently use the another molecular mechanism to overcome the silencing immune barrier, establish the persistent infection and prevent catastrophic damage to its host. We hypothesized that this “non-suppressor” mechanism is the process of transcriptomic reprogramming (TRP) induced by the PTI (pattern-triggered immunity), the first layer of plant defence, which is triggered by specific recognition of conserved microbe- or pathogen-associated molecular patterns (MAMPs, or PAMPs, respectively) by pattern recognition receptors (PRRs) at the plasma membrane and the induction of defense signaling. Recently a role of PTI in antiviral defence has been demonstrated in Arabidopsis by showing that mutants in the PRR (PRRs, Pattern recognition receptors) coreceptor kinases exhibit increased susceptibility to different RNA viruses. Our preliminary results confirm this hypothesis and show that there is a negative correlation between the ShVX reproduction rates and the levels of RNA-dependent RNA-polymerase (RDR) and DCL proteins in roots and leaves of infected shallot plants. The task of this study is the experimental verification of our PTI-induced TRP hypothesis by quantitative real-time PCR (Comparative CT experiment, delta-delta CT algorithm; calibrator: healthy shallot seedlings; normalizer: 18S RNA; The 7500/7500 Fast Real-Time PCR Systems, Applied Biosystems, USA) to evaluate in vivo expression levels of transcripts coding PTI markers, factors of RNA-silencing, NB-LRR receptors and complex of TCTP-PIRL-GRF6-DBP1 proteins in the healthy and ShVX-infected shallot ( Allium cepa L. var. aggregatum L.G. Don) plants. In this study for the first time we obtained the convincing data about PTI and TRP induction in ShVX-infected shallot plants. As result of TRP, repression of all factors of RNA-silencing, some NB-LRR receptors (e.c., Tm22 ) and some proteins of TCTP-PIRL-GRF6-DBP1 complex take place in this virus-plant system. On the other hand, group of defense genes with high expression levels has been discovered in this system: SOBIR - log10RQ ~ 1.0; ARM (genes encoding armadillo protein family) - log10RQ ~ 2.0; Pathogenesis-related protein 1, PR1 - log10RQ ~ 2.0; Pathogenesis-related protein 5, PR5 - log10RQ ~ 4.0 (!); Pathogenesis-related protein 14 = nsLTP - log10RQ ~ 2.0. So, in leaves and roots of infected plants ShVX programs dynamical and coordinated process of TRP and downregulation of genes coding for core RNAi components and disease resistance proteins might be correlated with successful virus reproduction and persistent virus infection establishment. We are of opinion that plant viral-specific PRRs identification, plant viral PAMP-triggered PTI and PRR (Pattern recognition receptors)-mediated transcriptomic reprogramming mechanisms ascertainment are the main tasks of coming antiviral plant immunity research period. Сloning of plant PRRs involved in plant virus PAMPs recognition, and the inter-species transfer of plant virus-sensing PRRs are the promising future technologies for broad spectrum antiviral resistant plants creation (D. Bao et al., 2017).