Newly Distinguished Cell Punctures Associated with Transmission of the Semipersistent Phloem-Limited Beet Yellows Virus

Abstract

Here we report on plant penetration activities (probing) by the aphid (Sulzer, 1776) in association with the transmission, acquisition, and inoculation of the semipersistent (BYV; ) in sugar beet. During electrical penetration graph (EPG) recording of stylet pathways, standard intracellular stylet punctures occur which are called potential drop (pd) waveforms. In addition to the standard pd, there also appeared to be a unique type of intracellular stylet puncture that always preceded the phloem salivation phase (waveform E1). This type of pd, the phloem-pd, showed properties distinct from those of the standard pds and has never been described before. We manually ended EPG recordings during the acquisition and inoculation tests by removing aphids from the source or test plant after specific waveforms were recorded. Inoculation of BYV occurred at the highest rate when probing was interrupted just after a single or various phloem-pds. In contrast, BYV acquisition showed an intimate association with sustained phloem sap ingestion from phloem sieve elements (SEs) (E2 waveform). Our work shows for the first time that the inoculation of a phloem-limited virus occurs during specific intracellular stylet punctures and before phloem salivation (waveform E1). Further studies are needed to establish in what cells this novel phloem-pd occurs: phloem parenchyma, companion, or SE cells. The role of the different stylet activities in the acquisition and inoculation of BYV by is discussed. We discovered the specific feeding activities of (Sulzer, 1776) associated with the transmission of (BYV; ). Our work strongly suggests that aphids can insert their stylets into the membranes of phloem cells-visualized as a unique type of waveform that is associated with the inoculation of BYV. This intracellular puncture (3 to 5 s) occurs just before the phloem salivation phase and can be distinguished from other nonvascular stylet cell punctures. This is the first time that the transmission of a phloem-limited semipersistent virus has been shown to be associated with a unique type of intracellular puncture. Our work offers novel information and strongly contributes to the existing literature on the transmission of plant viruses. Here we describe a new kind of aphid behavioral pattern that could be key in further works, such as studying the transmission of other phloem-limited viruses (e.g., luteoviruses).

Non-circulative virus transmission by Aphids : new insights into the mechanisms of transmission and the interference for retention sites in the vector

“…In Chapter 6 we described a distinctive type of pd (named phloem-pd) during which the phloem-limited virus BYV was inoculated by M. persicae into sugar beet plants (Jiménez et al, 2018). Inoculation of a phloem-limited virus is a strong indication that these pds are intracellular punctures in phloem cells.…”

“…When a phloem-pd was observed during the series of 10-11 consecutive standard-pds (treatment I) the aphids were not used for cryofixation and discarded. Standard-pds and putative phloem-pds are distinguished by several features that were initially described by Jiménez et al (2018) and are illustrated in Fig. 7.1: potential drop magnitude (difference of the output voltage after 100x gain between the voltage in a stabilized C waveform twothree seconds before the potential drop and the beginning of subphase II-2), number of intervals in subphase II-2 (intervals are the distance between two sequential downward peaks - Fig.…”

“…Myzus persicae occurs during specific intracellular stylet punctures that always precede PSEP (Jiménez et al, 2018). These pds had some distinct differences from the standardpds that are associated with non-persistent virus transmission, so the authors hypothesized these novel pds occurred during penetration of phloem cells, either into phloem parenchyma, companion or sieve element cells.…”

“…Therefore, the study of insect feeding activities in the plant and their relationship to virus acquisition and inoculation enhances our understanding of virus epidemiology and may contribute to developing strategies for controlling the spread of plant viruses. Numerous studies over many years have focused on the mechanisms of transmission of plant pathogens by insect vectors (Watson, 1938;Scheller and Shukle, 1986;Jiménez et al, 2018). Among insecttransmitted pathogens, viruses vectored by aphids have been the most studied.…”

“…Aphid stylet activities and their associated EPG waveforms involved in non-persistent (NP) virus transmission have been well studied. Acquisition and inoculation of NP viral particles occur during brief intracellular stylet punctures in superficial plant tissues, primarily in epidermal or mesophyll cells (Martín et al 1997) and often named "standardpds" (Chen et al, 1997;Jiménez et al, 2018). These intracellular punctures are composed of three different phases (I, II and III).…”