Breaching the Sieve Element—The Role of Saliva as the Molecular Interface between Aphids and the Phloem

This chapter discusses the potential role of aphid saliva within the sieve element, focusing largely on the watery saliva because this component is known to be actively secreted both prior to and during ingestion of phloem sap. It begins with a discussion on production, secretion, and composition of aphid saliva. The occlusion mechanisms that prevent leakage of sieve tube contents are described next. Secretion of watery saliva into sieve elements directly after penetration by aphid stylets, but prior to sieve‐tube sap ingestion, is a common behavior observed in all aphid species studied to date. Detailed analysis of aphid feeding behavior has demonstrated that aphids respond to artificial induction of occlusion with prolonged periods of watery saliva secretion. Factors that trigger the occlusion mechanisms include calcium fluxes, turgor loss, and variations in redox potential. The chapter finally discusses salivary proteins, which are modifiers of ingestion and the sieve element environment