The phloem-pd: a distinctive brief sieve element stylet puncture prior to sieve element phase of aphid feeding behavior

A recent electrical penetration graph (EPG) study identified a unique intracellular puncture that is associated with inoculation of the semipersistently transmitted, phloem-limited Beet yellows virus (BYV, Closterovirus) by the aphid Myzus persicae. This new aphid EPG pattern (named phloem-pd) always occurs shortly before phloem sieve element phase (PSEP) and has a similar voltage drop as PSEP, both of which are less than the voltage drop of standard-pds. Structure of pd subphase II-2 differs between phloem-pds and standard-pds. The objective of this study was to determine the type of phloem cell penetrated during phloem-pds. Stylets of M. persicae feeding on sugar beet (Beta vulgaris) were fixed in situ by cryofixation during phloem-pds, standard-pds and PSEP waveform E1. The cell penetrated by the stylet tips was then identified by confocal laser-scanning microscopy and micro-computed tomography. Sieve elements (SEs) or companion cells (CCs) were penetrated during phloem-pds, whereas cells other than SEs or CCs (mesophyll, bundle sheath cells and possibly phloem parenchyma) were penetrated during standard-pds. SEs were penetrated during waveform E1. The implications of these findings for inoculation of other phloem-limited viruses that currently are thought to be inoculated primarily during waveform E1 are discussed.