Abstract
The most represented group of resistance genes are those of the nucleotide binding site–leucine-rich repeat (NBS-LRR) class. These genes are very numerous in the plant genome, and they often occur in clusters at specific loci following gene duplication and amplification events. To date, hundreds of resistance genes and relatively few quantitative trait loci for plant resistance to pathogens have been mapped in different species, with some also cloned. When these NBS-LRR genes have been physically or genetically mapped, many cases have shown co-localization between resistance loci and NBS-LRR genes. This has allowed the identification of candidate genes for resistance, and the development of molecular markers linked to R genes. This review is focused on recent genomics studies that have described the abundance, distribution and evolution of NBS-LRR genes in plant genomes. Furthermore, in terms of their expression, NBS-LRR genes are under fine regulation by cis- and trans-acting elements. Recent findings have provided insights into the roles of alternative splicing, the ubiquitin/proteasome system, and miRNAs and secondary siRNAs in the regulation of NBS-LRR gene expression at the post-transcriptional, post-translational and epigenetic levels. The possibility to use this knowledge for genetic improvement of plant resistance to pathogens is discussed.
Highlights
Plant responses to pathogenic microorganisms are based on two main mechanisms
Comparative genomic analyses have indicated that plant genomes can encode several hundreds of nucleotide binding site–leucine-rich repeat (NBS-leucine-reach repeats (LRRs)) genes, and that there is a great diversity in the number and distribution of the subclasses of these genes
A variable number of R pseudogenes have been identified in different plant species; these are highly similar to nucleotide-binding site (NBS)-LRR genes at sequence level, their sequences are partial or they lead to the production of partial proteins
Summary
The first is the basal defense, based on the actions of the basal immune system, which was first described over 30 years ago [1] This system can be activated by the so-called elicitors, which are generic signals of the presence of a pathogen, such as bacterial flagellins, lipopolysaccharides or elongation factors, and fungal chitin or heptaglucosides [2]. The second mechanism is based on the actions of the adaptive immune system, which is composed of resistance (R) genes that can recognize host proteins. These are coded by the pathogen Avr genes and they confer a resistant phenotype to the plant, as postulated by the gene for gene theory [4]. Particular attention is paid to the following aspects: (i) genomic organization of the gene family in plant species in which the genome has been sequenced; (ii) molecular aspects of the actions of NBS-LRR genes in plant responses to pathogens, and the mechanisms of the regulation of their expression, from transcriptional to the post-translational level; (iii) coincidence of NBS-LRR genes with R loci, and the implications of this knowledge of the NBS-LRR gene family for the genetic improvement of crops in terms of their resistance to pathogens, without increasing the metabolic cost of resistance
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