Root Parasitic Nematodes: An Overview

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Abstract

This overview emphasizes the present state of knowledge on the feeding behaviour of root parasitic nematodes and associated cell responses. Several unresolved questions related to this topic could recently be answered with the aid of high resolution video-enhanced contrast light microscopy, which enables direct observations of live nematodes within roots. Unlike in other plant pathogens, such as fungi, where cell wall surface molecules elicit a cascade in plant gene regulation, nematodes inject copious amounts of salivary secretions from oesophageal gland cells to transform root cells and tissues for their own reproductive success. Hence this chapter covers several examples of root cell responses to salivary secretions. As shown, Xiphinema index may be a good model. It has a huge dorsal gland cell and the effects of its secretions can be studied in detail. However, from an economic point of view, nearly all efforts in understanding the molecular bases for feeding structure establishment are currently devoted to the economically most important cyst and root-knot nematodes. In order to compare their feeding structures with those of other nematodes with different parasitic strategies, this introductory chapter gives also a general overview on the wide array of food cell modifications induced and maintained. These modifications are entirely nematode specific, irrespective of the host plant parasitized. Each is successful in its own to support development and reproduction of the nematode species involved. From an evolutionary point of view the uninucleate giant cells of some non-cyst-forming nematodes are for instance more primitive than the syncytia of cyst-forming nematodes, which ensure a higher reproductive capacity. Root-knot nematodes are generally regarded to have evolved the most advanced form of root parasitism. Multinucleate giant cells may therefore be considered torepresent the most advanced form of nurse cell systems. Hopefully, with rapid advances in molecular biology, we will finally find an answer to the intriguing question how nematode secretions regulate plant gene expression for their own benefit and how the specificity of the nurse cells or nurse cell systems induced by them can be explained.