Role of a xyloglucan-specific endo-beta-1,4-glucanase inhibitor in the interactions of Nicotiana benthamiana with Colletotrichum destructivum, C. orbiculare or Pseudomonas syringae pv. tabaci.

A xyloglucan-specific endo-beta-1,4-glucanase inhibitor cDNA, NbXEGIP1, was amplified from diseased leaves of Nicotiana benthamiana. The sequence was similar to the tomato xyloglucan-specific endo-beta-1,4-glucanase inhibitor (XEGIP) and tobacco nectarin IV genes that have been described as binding and inactivating fungal Family 12 xyloglucan-specific endo-beta-1,4-glucanases. Expression of NbXEGIP1 was not detected in healthy leaves, but the gene was induced during the later stages of infection by the fungi Colletotrichum destructivum and C. orbiculare. Induction of NbXEGIP1 also occurred during disease development by the bacterium Pseudomonas syringae pv. tabaci and during the hypersensitive response produced by P. syringae pv. tabaci expressing avrPto. A portion of NbXEGIP1 was cloned into a tobacco rattle virus vector for virus-induced gene silencing in N. benthamiana. Silencing NbXEGIP1 did not affect the interactions with either Colletotrichum species but did significantly reduce population levels of P. syringae pv. tabaci in the compatible interaction and P. syringae pv. tabaci expressing avrPto in the incompatible interaction. In the susceptible response to P. syringae pv. tabaci, silencing of NbXEGIP1 also resulted in visibly wilted leaves several hours prior to necrosis, which was not observed in control plants. This was related to a significantly higher level of electrolyte leakage and higher expression of a defensin gene from infected NbXEGIP1-silenced leaves compared with control leaves. Silencing appeared to be specific as it did not affect expression of a related gene, NbXEGIP2. NbXEGIP1 may act as an inhibitor of a bacterial enzyme that degrades the xyloglucan-cellulose plant cell-wall network, and degradation of the cell wall results in host membrane disruption and signalling of defence responses.