Effectors Involved In Plant Defence Suppression

A brief, low-amplitude oxidative burst is observed at the onset of RKN parasitism, but host defences are subsequently suppressed (Jammes et al., 2005; Melillo et al., 2006). This suppression may result from the active modulation of plant defences by the nematode. Effectors synthesized in the oesophageal glands and potentially responsible for regulating plant defence responses have been identified. For example, it has been suggested that an oesophageal gland-specific chorismate mutase plays an active role in suppressing plant defences (Doyle and Lambert, 2003; Huang et al., 2005b). Plant chorismate mutases are key enzymes in the shikimic acid pathway directing the synthesis of cellular aromatic amino acids and several secondary metabolites, including phytohormones and plant defence compounds. The release of the nematode chorismate mutase into the plant cell cytoplasm would decrease the synthesis of flavonoids, salicylic acid, phytoalex-ins and auxin. A second RKN effector potentially able to modulate plant defences is calreticulin, which accumulates in the walls of giant cells. Calreticulin was the first molecule shown to be secreted into the feeding site via the nematode stylet (Jaubert et al., 2005). The function of nematode calreticulin in the modulation of plant responses remains unclear, but calreticulins secreted by eukaryotic animal parasites have been identified as key modulators of host immune defences (Kasper et al., 2001; Suchitra and Joshi, 2005). RKN calreticulin may also be involved in calcium signalling and cell cycle regulation (Ghiran et al., 2003) during giant cell formation.

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