Novel Strategies For Controlling Plantparasitic Nematodes

An important long-range goal of basic studies on nematode biology and host interactions is to identify novel control strategies for these often devastating pests. Resistance genes can be very effective for control, but are often not available for a particular host. Attempts to transfer cloned natural resistance genes to new hosts have met with limited success. The tomato gene Mi-1 confers RKN resistance when transferred to eggplant, but not to more distantly related hosts (Goggin et al., 2006). The tomato gene Hero A, which confers resistance to G. rostochiensis, disappointingly did not function when transferred to potato in which this nematode is a serious problem (Sobczak et al, 2005). A variety of novel transgenic approaches have also been carried out. Some strategies have expressed transgenically proteins that are detrimental to the nematode (Fuller et al., 2008). Transgenic expression of cysteine proteinase inhibitors (cystatins) has so far shown the most promise. Field trials have indicated that commercially useful resistance can be obtained in potato (Urwin et al., 2001). Promoters limiting expression of cystatin to specific tissues such as the feeding structures also have resistance to both cyst nematode and RKN (Lilley et al., 2004). The expression of specific crystal (Cry) proteins from Bacillus thuringiensis in plants has been widely successful for controlling targeted insects. Genes encoding Cry proteins with toxicity to nematodes have been identified, extensively modified and expressed in tomato roots; results so far suggest that this strategy has promise for control against PPNs (Li et al., 2008).

There has been considerable interest in using the strategy of silencing nematode genes by expression in planta of dsRNA corresponding to nematode genes in plants (Gheysen and Vanholme, 2007; Lilley et al., 2007). This approach was stimulated by evidence from several independent groups that genes can be silenced in both RKNs and cyst nematodes by soaking infective J2 in solutions of dsRNA corresponding to that gene (Chen et al., 2005; Rosso et al, 2005; Urwin et al., 2002). Because PPNs normally feed only on the cytoplasm of their feeding cells, a chemical stimulus was often used to facilitate the uptake of the dsRNA from the solution. These experiments stimulated efforts to test whether resistance to nematodes could be produced by in planta production of dsRNA. Yadav et al. (2006) found that expression of dsRNA corresponding to specific RKN genes could confer resistance in tobacco plants. Similarly, reduced reproduction of root-knot species was found in Arabidopsis and of H. glycines on transgenic soybean with other constructs designed to express dsRNA corresponding to specific nematode genes (Huang et al., 2006a; Steeves et al., 2006). Further studies have indicated that often resistance generated by this method is partial and durability of resistance generated using this strategy is yet to be investigated (Sindhu et al. 2009). The now available genome sequences are resources for the identification of pathways unique to nematode development and parasitism that could potentially serve as new targets for nematicides. For example, searching the RNAi experiment repository in WormBase for C. elegans genes for which RNAi led to a lethal phenotype identifies more than 340 M. incognita genes as potential nematode targets for antiparasitic drug design.

Past experience with pest control has shown that there is not likely to be one simple, permanent cure for these persistent pests. Combining or pyramiding different resistance types is likely to be of value. Importantly, continued studies on the biology of PPNs and their complex interactions will be necessary, but these are fascinating creatures and in many ways it is an exciting adventure.

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