Probing the herbivores responses to plant defenses using plant-mediated RNAi
Nicotiana attenuata (Solanaceae) produces 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) in starch-equivalent concentrations that thwart the growth of its specialist lepidopteran herbivore Manduca sexta (Lepidoptera, Sphingidae). Lyciumoside IV and its malonylated forms, nicotianoside I and II, constitute ~80% of N. attenuata’s HGL-DTG pool. Upon ingestion, the malonylated forms are rapidly and non-enzymatically demalonylated to lyciumoside IV by the alkalinity of larval oral secretion. Ingested lyciumoside IV (44%) is excreted as a novel compound, 3-O-α-L-rhamnopyranosyl-(1-4)-β-D-glucopyranosyl-17-hydroxygeranyllinalool (RGHGL). It differs from lyciumoside IV only in its lack of the C-17-glucose. Of M. sexta's midgut-expressed β-glucosidases, only β-glucosidase1 is upregulated upon lyciumoside IV ingestion, and when silenced by plant-mediated RNAi (PMRi), larvae are impaired in lyciumoside IV deglucosylation and showed molting impairments and higher mortality than control larvae. To examine the consequences of this detoxification process on natural tritrophic interactions, β-glucosidase1-silencing PMRi plants were planted into native habitats; control and β-glucosidase1-silenced larvae’s survival was similar; however, while Camptocosa parallela spiders captured and killed the control and lyciumoside IV-replete β-glucosidase1-silenced larvae similarly, the spiders ingested only 25% of β-glucosidase1-silenced larvae and ingestion resulted in locomotor distress in the spiders. While spiders attacked and ingested RGHGL-coated or -ingested larvae equally, they were deterred by the lyciumoside IV-coated larvae. Although lyciumoside IV deters spiders, it is not defensively co-opted by M. sexta larvae, perhaps to avoid its deleterious effects such as molting impairment and mortality. We further show that these PMRi plants also silence the homologous genes in native M. quinquemaculata larvae, feeding on these transgenic plants in nature.
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