Architecture of virulence in herbivorous insects or the evolutionary interactions between plants and their insect pests

While there is little data on systems involving plants and herbivorous insects, those involving plants and microbes have been studied extensively, leading to the development of a four phase ‘zigzag’ model. It is now understood that molecules known as pathogen-associated molecular patterns and effectors play key roles in the virulence of pathogens towards plants. The first phase of plant resistance is the basal defense, in which the pathogen’s PAMPs are recognized by the plant’s pattern recognition receptors, initiating PAMP-triggered immunity. This prompted the evolution of virulence effectors in pathogens to suppress PTI. As a result, plants evolved R genes that mediate specific pathogen resistance mechanisms in which the R protein acts as a receptor that recognizes the pathogen’s effectors, inducing effector-triggered immunity. Effector proteins are usually the products of virulence genes and can be regarded as virulence factors in pathogens and aphids. The relationship between the R and Avr genes is consistent with the gene-for-gene model. All of these findings are likely to be relevant in studies on the interactions of plants and insect herbivores because plants may well use similar defensive strategies to cope with the effects of herbivore attacks. Brown planthopper resistance was first reported in the Mudgo rice variety in 1969, and the first resistance gene, Bph1, was detected in the same variety in 1971. A total of 28 different brown planthopper resistance genes have since been identified in cultivated and wild rice species, all of which are located on specific regions of the rice chromosomes. Of these resistant genes, only the Bph14 gene has been isolated and characterized via a map-based cloning strategy. The Bph14 gene encodes a coiled-coil, nucleotide-binding, and leucine-rich repeat protein of the NB–LRR family that resembles the R proteins that contribute to plant resistance against disease-causing pathogens. Two other plant insect resistance genes, Mi-1.2 and Vat, have since been cloned; both likewise encode NB–LRR proteins. These three insect resistance genes originate from different plants but reveal similarities between the molecular mechanisms of insect and disease resistance. A receptor-like kinase gene OsLecRK was recently shown to be involved in basal defense response to brown planthopper attacks and may be a PRR that recognizes molecules secreted by these insects. Despite decades of investigation, the genetic basis of virulence in N. lugens remains to be identified. Genetic studies on this agriculturally important insect have been hindered by a lack of genome-wide linkage resources. However, it is known to exhibit considerable individual variation within biotypes. Crossing experiments using selected biotypes indicate that the virulence is continuously distributed in the offsprings and cannot be predicted using simple Mendelian models.

Leave a Reply