New insights into the relationship between phloem occlusion mechanisms and phloem sap-feeders

Phloem, the main transport system for photosynthates in plants, is comprised of different cell types.  Sieve elements are elongate phloem cells that abut end-to-end to form sieve tubes, which are the conduits for sap transport.   The abutting cell walls of adjacent sieve elements are referred to as sieve plates and are perforated with a high density of modified plasmodesmata, called sieve pores, providing cytoplasmic continuity from one sieve element to the next in the sieve tube..  Phloem occlusion is a ubiquitous mechanism in plants to prevent loss of sap in response to damage.  There are two well-known occlusion mechanisms in plants, callose deposition and formation of phloem protein (P-protein) plugs that clog the sieve pores.  In response to damage, callose (β-1,3- glucan) deposition requires de novo synthesis and takes several minutes to plug sieve pores, whereas P-proteins are constitutively present in sieve elements and can instantaneously transform from a physical state that allows the flow of sap to a physical state that plugs the sieve element..   Phloem sap-feeding insects successfully manage to feed for prolonged periods of time from sieve elements, and are assumed to be able to prevent and/or reverse phloem occlusion.   A species-specific relationship between phloem occlusion mechanisms and phloem feeders is hypothesized.  Pea aphid feeding on its preferred host, Vicia faba, does not trigger phloem occlusion and readily ingests phloem sap.  In contrast, when the generalist green peach aphid feeds on faba bean, a plant to which it is poorly adapted, P-protein phloem occlusion is triggered and phloem sap is greatly curtailed. Studies are being conducted to elucidate if green peach aphid triggers P-protein occlusion before penetrating a sieve element, which would suggest that P-protein occlusion is triggered by saliva secreted along the pathway to the phloem and not saliva directly injected into the sieve element upon penetration.