All cultivated plant species are subjected to biotic stresses that can endanger crop yields and cause relevant economic losses. Genetically based resistance may be a simple and efficient solution to protect plants against pathogens and pests. Resistance to plant parasitic nematodes has been proved to be an effective management tool that improves crop yields, lowers pest population densities, and favors the developments of effective rotation systems. Most of the studies on resistance and virulence in plant-nematode interactions have been focused on those nematode families that are the most diffused and cause the major economic losses to worldwide agriculture, i.e. the sedentary endoparasites belonging to the cyst (Globodera and Heterodera spp.) and root-knot (Meloidogyne spp.) nematodes. Sedentary endoparasitic nematodes enter the roots as motile second-stage juveniles (J2) that do not kill parasitized cells and induce very specialized and complex relationships with their hosts. Cyst-forming J2 induce the formation of feeding sites, called syncytia, in which few cells merge by dissolving their cell walls, while root-knot nematodes induce the formation of few discrete giant or nurse cells; both types of feeding site have the role of actively transferring solutes and nutrients toward the developing nematode. Once their feeding sites have been arranged, J2 become sedentary and begin to develop into enlarged adult females through subsequent molts to J3 and J4. Several genes conferring resistance to sedentary endoparasitic nematodes (R-genes) have been isolated from plants. They are generally expressed constitutively in proteins that are assigned to the recognition of the presence of nematode avirulent factors. The interaction between R-proteins and avirulent factors induces conformational changes in R-proteins that lead to signaling of defense response. Avirulence to specific R-genes is inherited as a single dominant trait in some plant parasitic nematode species. It seems that the loss of this trait determines the failure of pest recognition by resistant plants and produce the development of virulent populations. Virulent populations are able to develop on resistant crop plants and can be selected in laboratory and in field by repeated exposure to R-genes. This chance to break resistance by field populations can endanger resistance durability and the use of resistance as a valid management against nematodes for farmers. However, R-genes exert high selection pressures on nematode populations, thus leading to fitness costs, such as loss of competitiveness and lower reproduction potentials on susceptible hosts, with respect to wild field populations. Moreover, virulent populations specifically develop only on plants that carry the gene on which selection occurred. In this chapter, the genetic bases of resistance and virulence, along with the most up-to-date studies on the molecular events and interactions leading to incompatible/compatible responses of the plants to nematodes will be described, with particular attention to the role of some important hormones in plants, such as salicylic acid, and genetic variability of nematode populations. eliminare Autore/i: Nematodes
Resistance and Virulence in Plant-Nematode Interactions
Molinari S
2012
Abstract
All cultivated plant species are subjected to biotic stresses that can endanger crop yields and cause relevant economic losses. Genetically based resistance may be a simple and efficient solution to protect plants against pathogens and pests. Resistance to plant parasitic nematodes has been proved to be an effective management tool that improves crop yields, lowers pest population densities, and favors the developments of effective rotation systems. Most of the studies on resistance and virulence in plant-nematode interactions have been focused on those nematode families that are the most diffused and cause the major economic losses to worldwide agriculture, i.e. the sedentary endoparasites belonging to the cyst (Globodera and Heterodera spp.) and root-knot (Meloidogyne spp.) nematodes. Sedentary endoparasitic nematodes enter the roots as motile second-stage juveniles (J2) that do not kill parasitized cells and induce very specialized and complex relationships with their hosts. Cyst-forming J2 induce the formation of feeding sites, called syncytia, in which few cells merge by dissolving their cell walls, while root-knot nematodes induce the formation of few discrete giant or nurse cells; both types of feeding site have the role of actively transferring solutes and nutrients toward the developing nematode. Once their feeding sites have been arranged, J2 become sedentary and begin to develop into enlarged adult females through subsequent molts to J3 and J4. Several genes conferring resistance to sedentary endoparasitic nematodes (R-genes) have been isolated from plants. They are generally expressed constitutively in proteins that are assigned to the recognition of the presence of nematode avirulent factors. The interaction between R-proteins and avirulent factors induces conformational changes in R-proteins that lead to signaling of defense response. Avirulence to specific R-genes is inherited as a single dominant trait in some plant parasitic nematode species. It seems that the loss of this trait determines the failure of pest recognition by resistant plants and produce the development of virulent populations. Virulent populations are able to develop on resistant crop plants and can be selected in laboratory and in field by repeated exposure to R-genes. This chance to break resistance by field populations can endanger resistance durability and the use of resistance as a valid management against nematodes for farmers. However, R-genes exert high selection pressures on nematode populations, thus leading to fitness costs, such as loss of competitiveness and lower reproduction potentials on susceptible hosts, with respect to wild field populations. Moreover, virulent populations specifically develop only on plants that carry the gene on which selection occurred. In this chapter, the genetic bases of resistance and virulence, along with the most up-to-date studies on the molecular events and interactions leading to incompatible/compatible responses of the plants to nematodes will be described, with particular attention to the role of some important hormones in plants, such as salicylic acid, and genetic variability of nematode populations. eliminare Autore/i: NematodesI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
