Sclerenchymatous ring as a barrier to phloem feeding by Asian citrus psyllid: Evidence from electrical penetration graph and visualization of stylet pathways

Authors: George, Justin; AMMAR, EL-DESOUKY - Former ARS Employee; Hall, David; Lapointe, Stephen

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2017
Publication Date: 3/9/2017
Citation: George, J., Ammar, E-D., Hall, D.G., Lapointe, S.L. 2017. Sclerenchymatous ring as a barrier to phloem feeding by Asian citrus psyllid: Evidence from electrical penetration graph and visualization of stylet pathways. PLoS One. doi:10.1371/PO.2017-0173520

Interpretive Summary: Asian citrus psyllid (ACP) feeding behavior is complex. ACP mouthparts enter plant tissue and must reach the vascular tissues (xylem or phloem) to ingest the liquid contents. Transmission of pathogens between plants occurs when the disease agent is picked up from an infected plant and injected into a healthy plant’s phloem. ACP feeding behavior and the particular components of mouthpart movement within the plant play a significant role in the transmission of citrus greening disease, a bacterial pathogen. Many researchers have observed that ACP require young citrus leaves for their survival. We show, through histology, that there is a fibrous ring of thick-walled cells (sclerenchyma) around the phloem especially in mature citrus leaves. This ring is more prominent on the lower side of leaves compared to the upper side. We showed, by electronically recording ACP feeding behavior, that the presence and thickness of this fibrous ring influences the selection of feeding sites by ACP, which in turn can affect pathogen acquisition and transmission. We found that the presence of the ring on the lower surface of mature leaves reduced ingestion from the phloem. ACP adults were made consecutive phloem feeding attempts on the lower surface of mature leaves, and these bouts resulted in unsuccessful or shorter periods of phloem ingestion. ACP adults made more frequent and longer bouts of xylem ingestion on mature compared with young leaves. Our results support the hypothesis that a thick, well-developed, fibrous ring around phloem tissues on mature compared with young leaves acts as a barrier to successful feeding by ACP on citrus leaves. This may have an important role in limiting or preventing CLas acquisition and/or transmission by ACP, and could be used for identification and development of resistant citrus cultivars.

Technical Abstract: Asian citrus psyllid (ACP) feeding behaviors play a significant role in the transmission of the phloem-limited Candidatus Liberibacter asiaticus (CLas) bacteria that cause citrus greening disease. Sustained phloem ingestion by ACP on CLas infected plants is very important in pathogen acquisition and transmission. Recent studies have shown that there is a fibrous ring of thick-walled sclerenchyma around the phloem especially in old/mature citrus leaves, and are more prominent on the lower side of leaves compared to the upper side. The presence and thickness of this fibrous ring may have a role in selection of feeding sites by ACP based on leaf age and leaf surface, which in turn can affect pathogen acquisition and transmission. We performed Electrical Penetration Graph (EPG) studies on ACP adults feeding/probing on the upper or lower surfaces of young and old Valencia orange leaves to study the role of sclerenchymatous fibrous ring in modifying feeding behaviors. Histological correlation studies were performed on the same leaf tissues to identify the occurrence and termini of ACP salivary sheaths during these feeding probes. Based on the EPG recordings and histological correlations, we found that the presence of a thick-walled fibrous ring on the lower surface of old mature leaves significantly reduces phloem ingestion (E2 waveform). Longest duration of phloem ingestion activities was observed on the upper surface of young flush leaves that had the least developed fibrous ring. Phloem salivation (E1 waveform), on the other hand, was significantly longer on mature leaves compared to young flushes. ACP adults apparently were observed to make consecutive phloem feeding attempts (bouts) on the lower surface of mature leaves, and these bouts resulted in unsuccessful or shorter periods of phloem ingestion. ACP adults made more frequent and longer bouts of xylem ingestion on mature compared with young leaves. Our results support the hypothesis that a thick, well-developed, fibrous ring around phloem tissues on mature compared with young leaves acts as a barrier to frequent or sustained phloem ingestion by ACP from citrus leaves. This may have an important role in limiting or preventing CLas acquisition and/or transmission by ACP, and could be used for identification and development of resistant citrus cultivars.