Belowground Defence Strategies Against Migratory Nematodes

Abstract

The biology of migratory plant parasitic nematodes has been less studied than that of the sedentary endoparasites. The damage they cause is less obvious, their presence and number are more difficult to quantify and they are difficult organisms to study. Nevertheless, they are economically serious pests of many crops, from wheat and barley grown in low rainfall areas to horticultural crops (e.g. Lilium longiflorum) and tropical crops such as coffee, banana and sugarcane. The most studied migratory nematodes are the root lesion nematodes, Pratylenchus spp., the burrowing nematode Radopholus similis and the rice root nematode Hirschmanniella oryzae. In the life cycle of migratory nematodes apart from the egg, all stages of juveniles and adults are motile and can enter and leave host roots. They do not induce the formation of a permanent feeding site, but feed from individual host cells. They create pathways for entry of other root pathogens, often resulting in lesions, stunted roots, yellowing of leaves and plants showing symptoms of water stress, leading to yield loss and decreased quality of produce. In terms of genetic plant defences, no major genes for resistance to migratory nematodes have been found, and resistance breeding is usually based on QTL analysis and marker-assisted selection to combine the best minor resistance genes. Feeding damage reduces root function, and root damage and necrotic lesions the nematodes cause can then make them leave the root and seek others to parasitise. Infestation induces classical plant defence responses and changes in host metabolism which reflects the damage they cause, although detailed studies are lacking. New genomic resources are becoming available to study migratory endoparasites, and the knowledge gained can contribute to improved understanding of their interactions with hosts. Notably transcriptomes of Pratylenchus coffeae, Pratylenchus thornei, Pratylenchus zeae, R. similis and H. oryzae and the first genomic sequence, for P. coffeae, are now available. From these data, some candidate effector genes required for parasitism have been identified: many effectors similar to those found in sedentary endoparasites are present, with the exception of those thought to be involved in formation of feeding sites induced by the sedentary parasites. Belowground defence, in the form of enhanced resistance to migratory parasites, may also be achieved by transgenic expression of modified cysteine protease inhibitors (cystatins), anti-root invasion peptides and host-induced gene silencing (RNAi) strategies, demonstrating that migratory nematodes are amenable to control by these technologies. New more environmentally friendly nematicides, combined with better biological control agents, can be applied or used in seed coatings in integrated pest management approaches to defend roots from attack by migratory nematodes.