Does aphid salivation affect phloem sieve element occlusion in vivo?

Abstract

To protect against loss of photo-assimilate-rich phloem sap, plants have evolved several mechanisms to plug phloem sieve tubes in response to damage. In many Fabaceae, each sieve element contains a discrete proteinaceous body called a forisome, which, in response to damage, rapidly transforms from a condensed configuration that does not impede the flow of sap to a dispersed configuration that plugs the sieve element. Aphids and other specialized phloem sap feeders can ingest phloem sap from a single sieve element for hours or days, and to do this, they must be able to suppress or reverse phloem plugging. A recent study provided in vitro evidence that aphid saliva can reverse forisome plugs. The present study tested this hypothesis in vivo by inducing forisome plugs which triggered aphids to switch behaviour from phloem sap ingestion to salivation into the sieve element. After salivating into the sieve element for various periods of time, the aphids were instantaneously cryofixed (freeze fixed) in situ on their leaf. The state of the forisome was then determined in the penetrated sieve element and in nearby non-penetrated sieve elements which served as controls for sieve elements not subjected to direct aphid salivation. Forisomes were almost always in close contact with the stylet tips and thus came into direct contact with the saliva. Nonetheless, forisome plugs in the penetrated sieve element did not revert back to a non-plugging state any faster than those in neighbouring sieve elements that were not subjected to direct aphid salivation.

Protein elicitor PeaT1 enhanced resistance against aphid (Sitobion avenae) in wheat

“…The E1 (saliva injection during phloem feeding activities) wave indicated saliva injection into sieve elements by aphids, while the E2 (sap sucking during phloem feeding activities) wave showed phloem sap injection with concurrent salivation, which would reflect a resistance factor in the mesophyll or vascular elements . The extended E1 suggested more plugging or defense compounds in the sieve element . However, there was no significant difference in the E2 period between the control and PeaT1 treatments, indicating no or insufficient variation in compounds in phloem to confer resistance against S. avenae .…”

Faba bean forisomes can function in defence against generalist aphids

“…This has important practical implications for the potential of breeding aphid resistance in crops based on phloem occlusion defences. Medina‐Ortega & Walker () found no evidence that A. pisum saliva can reverse forisome occlusion in vivo , so the focus is on triggering, not reversing forisome occlusion. We propose three hypotheses that address this question: (1) My.…”

Sieve element occlusion provides resistance against Aphis gossypii in TGR‐1551 melons

“…The burn stimulus initiates an electropotential wave that propagates basipetally through the phloem, triggering sieve element occlusion as the wave passes (Furch et al ., , ; Walker, unpublished data). At least seven aphid species feeding on the veins of four plant species respond to a burn stimulus at the distal end of a vein by switching from phloem sap ingestion (waveform E2) to salivation (waveform E1) (Will et al ., , ; Furch et al ., ; Medina‐Ortega & Walker, ) (additionally, an eighth aphid species [ A. gossypii ] on a fifth plant species [ C. melo ] and a whitefly [ Bemisia argentifolii Bellows & Perring] on Vicia faba also respond in the same way to experimentally induced phloem occlusion—Walker, unpublished observations). Consequently, the ability of phloem occlusion to interfere with phloem sap ingestion is well established.…”