In feeding, aphids inject saliva into plant tissues, gaining access to phloem sap and eliciting (and sometimes overcoming) plant responses. We are examining the involvement, in this aphid-plant interaction, of individual aphid proteins and enzymes, as identified in a salivary gland cDNA library. Here, we focus on a salivary protein we have arbitrarily designated Protein C002. We have shown, by using RNAi-based transcript knockdown, that this protein is important in the survival of the pea aphid (Acyrthosiphon pisum) on fava bean, a host plant. Here, we further characterize the protein, its transcript, and its gene, and we study the feeding process of knockdown aphids. The encoded protein fails to match any protein outside of the family Aphididae. By using in situ hybridization and immunohistochemistry, the transcript and the protein were localized to a subset of secretory cells in principal salivary glands. Protein C002, whose sequence contains an Nterminal secretion signal, is injected into the host plant during aphid feeding. By using the electrical penetration graph method on c002-knockdown aphids, we find that the knockdown affects several aspects of foraging and feeding, with the result that the c002-knockdown aphids spend very little time in contact with phloem sap in sieve elements. Thus, we infer that Protein C002 is crucial in the feeding of the pea aphid on fava bean.aphid-plant interaction ͉ saliva ͉ RNAi ͉ electrical penetration graph ͉ immunohistochemistry T he ability, or inability, of an aphid to feed on a plant results from a multifaceted interplay between the feeding systems of the insect and the defense systems of the plant (for recent reviews, from several perspectives, of aphid-plant interactions, see refs.

SummaryGreen peach aphid (GPA) Myzus persicae (Sü lzer) is a phloem-feeding insect with an exceptionally wide host range. Previously, it has been shown that Arabidopsis thaliana PHYTOALEXIN DEFICIENT4 (PAD4), which is expressed at elevated levels in response to GPA infestation, is required for resistance to GPA in the Arabidopsis accession Columbia. We demonstrate here that the role of PAD4 in the response to GPA is conserved in Arabidopsis accessions Wassilewskija and Landsberg erecta. Electrical monitoring of aphid feeding behavior revealed that PAD4 modulates a phloem-based defense mechanism against GPA. GPA spends more time actively feeding from the sieve elements of pad4 mutants than from wild-type plants, and less time feeding on transgenic plants in which PAD4 is ectopically expressed. The activity of PAD4 in limiting phloem sap uptake serves as a deterrent in host-plant choice, and restricts aphid population size. In Arabidopsis defense against pathogens, all known PAD4 functions require its signaling and stabilizing partner EDS1 (ENHANCED DISEASE SUSCEPTIBILITY1). Bioassays with eds1 mutants alone or in combination with pad4 and with plants conditionally expressing PAD4 under the control of a dexamethasone-inducible promoter reveal that PAD4-modulated defense against GPA does not involve EDS1. Thus, a PAD4 mode of action that is uncoupled from EDS1 determines the extent of aphid feeding in the phloem.

SUMMARYAgricultural productivity is limited by the removal of sap, alterations in source-sink patterns, and viral diseases vectored by aphids, which are phloem-feeding pests. Here we show that TREHALOSE PHOSPHATE SYNTHASE11 (TPS11) gene-dependent trehalose metabolism regulates Arabidopsis thaliana defense against Myzus persicae (Sü lzer), commonly known as the green peach aphid (GPA). GPA infestation of Arabidopsis resulted in a transient increase in trehalose and expression of the TPS11 gene, which encodes a trehalose-6-phosphate synthase/phosphatase. Knockout of TPS11 function abolished trehalose increases in GPA-infested leaves of the tps11 mutant plant and attenuated defense against GPA. Trehalose application restored resistance in the tps11 mutant, confirming that the lack of trehalose accumulation is associated with the inability of the tps11 mutant to control GPA infestation. Resistance against GPA was also higher in the trehalose hyper-accumulating tre1 mutant and in bacterial otsB gene-expressing plants, further supporting the conclusion that trehalose plays a role in Arabidopsis defense against GPA. Evidence presented here indicates that TPS11-dependent trehalose regulates expression of the PHYTOALEXIN DEFICIENT4 gene, which is a key modulator of defenses against GPA. TPS11 also promotes the re-allocation of carbon into starch at the expense of sucrose, the primary plant-derived carbon and energy source for the insect. Our results provide a framework for the signaling function of TPS11-dependent trehalose in plant stress responses, and also reveal an important contribution of starch in controlling the severity of aphid infestation.

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Plant tolerance to insect pests has been indicated to be a unique category of resistance, however, very little information is available on the mechanism of tolerance against insect pests. Tolerance is distinctive in terms of the plant’s ability to withstand or recover from herbivore injury through growth and compensatory physiological processes. Because plant tolerance involves plant compensatory characteristics, the plant is able to harbor large numbers of herbivores without interfering with the insect pest’s physiology or behavior. Some studies have observed that tolerant plants can compensate photosynthetically by avoiding feedback inhibition and impaired electron flow through photosystem II that occurs as a result of insect feeding. Similarly, the up-regulation of peroxidases and other oxidative enzymes during insect feeding, in conjunction with elevated levels of phytohormones can play an important role in providing plant tolerance to insect pests. Hemipteran insects comprise some of the most economically important plant pests (e.g., aphids, whiteflies), due to their ability to achieve high population growth and their potential to transmit plant viruses. In this review, results from studies on plant tolerance to hemipterans are summarized, and potential models to understand tolerance are presented.