Abstract
Genes conferring resistance to plant viruses fall in two categories; the dominant genes that mostly code for proteins with a nucleotide binding site and leucine rich repeats (NBS-LRR), and that directly or indirectly, recognize viral avirulence factors (Avr), and the recessive genes. The latter provide a so-called recessive resistance. They represent roughly half of the known resistance genes and are alleles of genes that play an important role in the virus life cycle. Conversely, all cellular genes critical for the viral infection virtually represent recessive resistance genes. Based on the well-documented case of recessive resistance mediated by eukaryotic translation initiation factors of the 4E/4G family, this review is intended to summarize the possible approaches to control viruses via their host interactors. Classically, resistant crops have been developed through introgression of natural variants of the susceptibility factor from compatible relatives or by random mutagenesis and screening. Transgenic methods have also been applied to engineer improved crops by overexpressing the translation factor either in its natural form or after directed mutagenesis. More recently, innovative approaches like silencing or genome editing have proven their great potential in model and crop plants. The advantages and limits of these different strategies are discussed. This example illustrates the need to identify and characterize more host factors involved in virus multiplication and to assess their application potential in the control of viral diseases.
Highlights
Plants have evolved sophisticated processes to evade pathogen attacks
Experiments in tomato highlighted the difficulty of generating loss-of-function mutations without affecting resistance spectrum or plant growth even in the case of gene redundancy like for eIF4 factors (Gauffier et al, 2016). In this tomato/potyvirus pathosystem a knock out engineered eIF4E1 mutant appeared to present a narrower resistance spectrum than the natural resistance allele that encodes several amino acid substitutions within the eIF4E1 protein, probably because eIF4E1 regulates the availability of eIF4E2 for the virus (Gauffier et al, 2016)
Techniques to precisely target sequences to be modified in plant genomes have recently been developed, based on DNA repair after double strand breaks provoked by sequence-specific endonucleases like zinc-finger nucleases (ZFNs), transcription activator-like effectors nucleases (TALENs), and CRISPR associated (Cas) endonucleases
Summary
Plants have evolved sophisticated processes to evade pathogen attacks. The main antiviral defense responses are considered to be RNA silencing, R gene mediated resistance and recessively inherited resistance, other cellular mechanisms like autophagy (Hafren et al, 2017; Haxim et al, 2017; Li et al, 2018), RNA methylation (Martínez-Pérez et al, 2017) or ubiquitination (AlcaideLoridan and Jupin, 2012) are important to counter viruses. R proteins are mainly of the NLR family (or NBS-LRR as a reference to their nucleotide binding site and leucine-rich repeat domain) They interact with viral effector proteins displaying various functions in the virus life cycle. HR is characterized by programmed cell death and generally results in virus containment at its site of entry This resistance response referred to as effector triggered immunity is genetically conditioned by both the plant and the pathogen and any allelic variation that impairs this recognition changes the response to a compatible reaction. Various elements of the translational initiation complex have been described to interact with viral RNAs This simplified scheme depicts the eIF4F complex composed of eIF4E (4E) which interacts with the cap (m7Gppp) of the mRNA, eIF4G (4G) which interacts with both eIF4E and the polyA bound PolyA binding protein (PAB), and eIF4A (4A). The mechanisms of the recessive resistance conferred by eIF4 factors are not all understood, it has been widely tested mostly because it can target a large variety of viruses in a wide range of plants from both the monocotyledon and dicotyledon clades, in model or crop plants (barley, rice, pea, tomato. . .)
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