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Abstract
Little is known about plant responses to phloem-feeding insects, which inflict minimal tissue damage, yet cause agronomic losses totaling in the billions of dollars per year. Current control methods against the whitefly are limited, and with emergence of insecticide-resistant whiteflies, there is clear need for new and effective control mechanisms against these pests. In an effort to develop management strategies and/or plant-based defense mechanisms against these pests, an understanding of the defenses activated and effective against these insects is required. To this end, a forward and reverse genetics approach was taken to understand plant-silverleaf whitefly (Bemisia tabaci type B; SLWF) interactions. The model plant Arabidopsis thaliana was used for these studies.
Local and systemic accumulation of SA-dependent defense gene and free and conjugated salicylic acid levels in SLWF-infested versus control plants revealed that Arabidopsis perceived SLWFs as pathogens. To determine the role of SA-dependent responses in defense against this herbivore, mutant and transgenic plants that constitutively activate or impair the SA or JA-dependent defense-signaling pathways were utilized in a bioassay measuring insect performance. These studies showed that JA- or SA- dependent defenses delayed SLWF development. Examination of defense gene transcript accumulation in these lines demonstrated that JA-defenses were effective against the SLWF. SLWF may have manipulated the crosstalk between the two pathways or introduced effectors to suppress effective JA-dependent responses.
Local SA-dependent defenses were ineffective against SLWF, additionally systemic immunity against secondary challenge by pathogens, was not observed. Bioassays measuring growth of Pseudomonas syringae in SLWF-infested and uninfested plants revealed that although all the hallmarks of systemic acquired resistance were induced by whitefly feeding, systemic immunity was not. Likely, components not being measured required for SAR were not provided during SLWF infestation or components acting to suppress the induction of SAR were introduced into the plant by SLWF feeding.
Finally, a novel gene WHITEFLY REGULATED (WFR) was identified by screening the UCR Arabidopsis gene-trap collection. WFR was induced by SLWF infestation and infection by biotrophic pathogens and repressed after glucose/glucose oxidase treatments. Although basal levels of WFR were unaffected after treatment with known defense chemicals, preliminary experiments with defense response mutant lines suggested that SA and JA may have a role in modulating transcripts after infestation by SLWF. Although repressed JA-dependent defenses were effective against SLWF nymph development, no-choice bioassays suggest that induced responses may be important in defense against the SLWF.
