Academic journal article Genetics

Low Levels of Polymorphism in Genes That Control the Activation of Defense Response in Arabidopsis Thaliana

Academic journal article Genetics

Low Levels of Polymorphism in Genes That Control the Activation of Defense Response in Arabidopsis Thaliana

Article excerpt

ABSTRACT

Plants use signaling pathways involving salicylic acid, jasmonic acid, and ethylene to defend against pathogen and herbivore attack. Many defense response genes involved in these signaling pathways have been characterized, but little is known about the selective pressures they experience. A representative set of 27 defense response genes were resequenced in a worldwide set of 96 Arabidopsis thaliana accessions, and patterns of single nucleotide polymorphisms (SNPs) were evaluated in relation to an empirical distribution of SNPs generated from either 876 fragments or 236 fragments with >400 bp coding sequence (this latter set was selected for comparisons with coding sequences) distributed across the genomes of the same set of accessions. Defense response genes have significantly fewer protein variants, display lower levels of nonsynonymous nucleotide diversity, and have fewer nonsynonymous segregating sites. The majority of defense response genes appear to be experiencing purifying selection, given the dearth of protein variation in this set of genes. Eight genes exhibit some evidence of partial selective sweeps or transient balancing selection. These results therefore provide a strong contrast to the high levels of balancing selection exhibited by genes at the upstream positions in these signaling pathways.

PLANTS defend themselves in response to pathogens and parasites, ranging from viruses to insect herbivores, using both physical and chemical defenses. Chemical defenses include the synthesisof antimicrobial compounds, such as phytoalexins and defensins, and the production of pathogenesis-related (PR) proteins, comprising enzymes capable of degrading pathogen cell walls. Defense responses often begin with a gene-forgene recognition of an avirulence (avr) gene of the pathogen by a resistance (R) gene of the host (Van Der Biezen and Jones 1998; Dangl and Jones 2001; Staskawicz et al. 2001). R gene-mediated resistance is usually accompanied by reactive oxygen species (ROS) and hypersensitive cell death (HR) (Veronese et al. 2003), which can be followed by activation of a salicylic acid (SA)-dependent signaling pathway (Figure 1) and the consequent expression of a characteristic set of defense genes (Glazebrook 2001). As a result, plants becomemoreresistant to subsequent attack by otherwise virulent-in the sense of "ability to infect"-pathogens, a phenomenoncalledsystemic acquiredresistance (SAR) (Sticher et al. 1997; Durrant and Dong 2004). The SA signaling pathway is activated not only as a result of R gene action but also as a result of pathogen-derived signals. Additional plant defense responses include those controlled by signal transduction networks requiring jasmonic acid ( JA) and ethylene (ET). The JA/ ET-dependent signaling pathways mediate resistance responses to herbivores or necrotrophic pathogens (Thomma et al. 2001), whereas the SA-dependent signaling pathway mediates resistance to biotrophic pathogens (Ryals et al. 1996). The SA, JA, and ET signaling pathways interact extensively (Glazebrook 2001; Traw et al. 2003). Finally, gene-for-gene interaction between an R gene and its corresponding avr gene in the pathogen (Flor 1956) can trigger downstream pathways in addition to SAR, of which three have been identified (Aarts et al. 1998; McDowell et al. 2000; Glazebrook 2001).

Defense response signal transduction pathways include many different genes that together control the activation of defense response as a reaction to pathogen attack and will therefore be referred to as "defense response genes." There is presently no information regarding the selective pressures that act on this group of genes in plants. Evolutionary analyses on receptor molecules in the innate immune system of insects have produced diverse results. In general it appears that balancing selection as observed for MHC genes and plant R genes is uncommon for innate immunity genes, although in some instances there are records of transient forms of balancing selection (Lazzaro 2005). …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.