| The silverleaf whitefly (Bemisia tabaci, type B) is a phloem-feeding agricultural and horticultural pest that causes extensive, economically devastating effects on plants in temperate climates world-wide. The whitefly has a prolonged and intimate interaction with the plant host and causes developmental disorders in many crop species. Currently little is understood about plant defense signal transduction products induced in response to this pest. To this end, the model plant organism Arabidopsis thaliana served as an efficient tool to examine plant-insect interactions at the molecular level. As observed in crop plants, whitefly infestation resulted in a marked "yellowing" phenotype and a cost-of-fitness in vegetative and reproductive tissues. Transcriptome analysis was used to examine global RNA transcript levels and found that primarily the salicylic acid (SA)- regulated defense pathway was induced by 2nd and 3rd instar feeding. This was confirmed by measuring transcript levels in local and systemic tissues with RT-PCR. No-choice assays with SA, ET and JA pathway mutants (SA: NahG, npr1, cim10; JA: coi1, cev1; ET: ein2) found that mutants with increased JA-defense gene transcripts delayed whitefly nymph development. These results suggested that Arabidopsis induced the ineffectual SA defense pathway while suppressing JA-regulated whitefly defense genes. Antixenotic mechanisms did not contribute to defense as choice assays found that whitefly adults do not preferentially deposit eggs on any mutant or wild-type line. A biphasic plant response was observed in response to SLWF-adult vs. nymph feeding. Unlike late defense signaling, quantitative-PCR showed that JA genes (LOX2, PDF1.2) RNAs accumulated to higher levels than SA-response genes (ANK, PR1 ) during early phases (2, 4, 8, 12, and 24 hpi) of whitefly adult feeding further suggesting that whitefly-nymph effectors may suppress effectual plant defense signaling. Lastly, in order to examine novel defense genes, the results from the transcriptome analysis were used to identify three genes repressed by whitefly nymphal feeding including a trypsin inhibitor ( ATTI6), phosphorylase (PFS1), and nucleoside phosphatase (NPTL1) and one induced gene, a plant defensin ( PDF1.4). The transcript levels of these RNAs from noninfested and whitefly infested leaves from wild-type and defense mutant plants were compared. In addition, the accumulation of ATTI6, PFS1, NTPL1 and PDF1.4 RNAs after JA and SA treatments and wounding were measured at 2, 6 and 24 hrs. All gene transcripts were found to be JA- and wound-inducible. |
