Penetration of faba bean sieve elements by pea aphid does not trigger forisome dispersal


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Immediately after their stylets penetrate a phloem sieve element, aphids inject saliva into the sieve element for approximately 30–60 s before they begin to ingest phloem sap. This salivation period is recorded as waveform E1 in electrical penetration graph (EPG) monitoring of aphid feeding behavior. It has been hypothesized that the function of this initial period of phloem salivation is to reverse or prevent plugging of the sieve element by one of the plant's phloem defenses: formation of P-protein plugs or callose synthesis in the sieve pores that connect adjacent sieve elements. This hypothesis was tested using the pea aphid, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae), and faba bean, Vicia faba L. (Fabaceae), as a model system, and the results do not support the hypothesis. In legumes, such as faba bean, P-protein plugs in sieve elements are formed by dispersal of proteinaceous bodies called forisomes. Contrary to the hypothesis, the great majority of sieve element penetrations by pea aphid stylets do not trigger forisome dispersal. Thirteen sieve elements were cryofixed early in phloem phase before the aphids could complete their salivation period and the forisomes were not dispersed in any of the 13 samples. However, in these samples, the aphids completed on average a little over half of their normal E1 salivation period before they were cryofixed. Thus, it is possible that sieve element penetration triggered forisome dispersal in these samples but the abbreviated period of salivation was still sufficient to reverse dispersal. To rule out this possibility, 17 sieve elements were cryofixed during R-pds, which are an EPG waveform associated with sieve element penetration but without the characteristic E1 salivation that occurs during phloem phase. In 16 of the 17 samples, the forisomes were not dispersed. Thus, faba bean sieve elements usually do not form P-protein plugs in response to penetration by pea aphid stylets. Consequently, the characteristic E1 salivation that occurs at the start of each phloem phase does not seem to be necessary to prevent a plugging response because penetration of sieve elements during R-pds does not trigger forisome dispersal despite the absence of E1 salivation. Furthermore, as P-protein plugs do not normally form in response to sieve element penetration, E1 salivation that occurs at the start of each phloem phase is not a response to development of a P-protein plug. Thus, the E1 salivation period at the beginning of the phloem phase appears to have function(s) unrelated to phloem sealing.