Correlation of electronic monitoring and stylet pathways elucidate the role of sclerenchymatous ring as a barrier to phloem feeding on citrus leaves by Asian citrus psyllid

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

Submitted to: International Research Conference on Huanglongbing
Publication Type: Abstract Only
Publication Acceptance Date: 10/31/2016
Publication Date: 3/15/2017
Citation: George, J., Ammar, E., Hall, D.G., Lapointe, S.L. 2017. Correlation of electronic monitoring and stylet pathways elucidate the role of sclerenchymatous ring as a barrier to phloem feeding on citrus leaves by Asian citrus psyllid[abstract]. International Research Conference on Huanglongbing. Abstracts of Presentations. Available: Journal of Citrus Pathology. p.14.

Interpretive Summary: Asian citrus psyllid (ACP, Diaphorina. citri) feeding behaviors play a significant role in the transmission of the phloem-limited Candidatus Liberibacter asiaticus (CLas) bacterium that causes the economically devastating citrus greening disease. Recent studies have shown a fibrous ring of thick-walled sclerenchyma around the phloem in mature citrus leaves that is more prominent on the lower compared with the upper side. We performed Electrical Penetration Graph (EPG) studies on ACP adults placed on lower or upper surfaces of young or mature Valencia orange leaves. Feeding sites on the same leaf tissues were then sectioned and examined by epifluorescence microscopy. Based on the EPG recordings and histological correlations, we found that thick-walled fibrous sclerenchyma located around the phloem of mature citrus leaves significantly reduced phloem ingestion by psyllids placed on the lower leaf surface compared with ingestion from the upper surface of mature leaves or on young leaves. The longest duration of phloem ingestion was observed from the upper side of young flush leaves that had the least developed sclerenchyma. Bouts of phloem salivation (E1 waveform), however, were significantly longer on mature leaves compared with young flush. ACP adults made consecutive phloem feeding attempts (bouts) on the lower side of mature leaves and those bouts resulted in unsuccessful or shorter periods of phloem ingestion. ACP adults also made more frequent and longer bouts of xylem ingestion on mature leaves compared with psyllids placed on young leaves. Our results support the hypothesis that the presence of a thick, well-developed fibrous ring around phloem tissues of mature leaves acts as a barrier to frequent or prolonged 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.

Technical Abstract: Asian citrus psyllid (ACP, Diaphorina. citri) feeding behaviors play a significant role in the transmission of the phloem-limited Candidatus Liberibacter asiaticus (CLas) bacterium that causes the economically devastating citrus greening disease. Recent studies have shown a fibrous ring of thick-walled sclerenchyma around the phloem in mature citrus leaves that is more prominent on the lower compared with the upper side. We performed Electrical Penetration Graph (EPG) studies on ACP adults placed on lower or upper surfaces of young or mature Valencia orange leaves. Feeding sites on the same leaf tissues were then sectioned and examined by epifluorescence microscopy. Based on the EPG recordings and histological correlations, we found that thick-walled fibrous sclerenchyma located around the phloem of mature citrus leaves significantly reduced phloem ingestion by psyllids placed on the lower leaf surface compared with ingestion from the upper surface of mature leaves or on young leaves. The longest duration of phloem ingestion was observed from the upper side of young flush leaves that had the least developed sclerenchyma. Bouts of phloem salivation (E1 waveform), however, were significantly longer on mature leaves compared with young flush. ACP adults made consecutive phloem feeding attempts (bouts) on the lower side of mature leaves and those bouts resulted in unsuccessful or shorter periods of phloem ingestion. ACP adults also made more frequent and longer bouts of xylem ingestion on mature leaves compared with psyllids placed on young leaves. Our results support the hypothesis that the presence of a thick, well-developed fibrous ring around phloem tissues of mature leaves acts as a barrier to frequent or prolonged 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.