Evidence for seed transmission of Xylella fastidiosa in pecan (Carya illinoinensis)

Authors: CERVANTES, KIMBERLY - New Mexico State University; Hilton, Angelyn; STAMLER, RIO - New Mexico State University; HEERMA, RICHARD - New Mexico State University; Bock, Clive; Wang, Xinwang; JO, YOUNG-KI - Texas A&M University; Grauke, Larry; RANDALL, JENNIFER - New Mexico State University

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2022
Publication Date: 4/8/2022
Citation: Cervantes, K., Hilton, A.E., Stamler, R.A., Heerma, R., Bock, C.H., Wang, X., Jo, Y., Grauke, L.J., Randall, J.J. 2022. Evidence for seed transmission of Xylella fastidiosa in pecan (Carya illinoinensis). Frontiers in Microbiology. Article e13:780335. https://doi.org/10.3389/fpls.2022.780335.
DOI: https://doi.org/10.3389/fpls.2022.780335

Interpretive Summary: Xylella fastidiosa is a bacterial pathogen that is widespread throughout the U.S. pecan industry and causes the disease, Pecan Bacterial Leaf Scorch (PBLS). The bacterium has a wide host range and can be transmitted via xylem-feeding insect vectors and from grafting scions to rootstocks. Previous modes of transmission were unknown. Recent research of Pecan Bacterial Leaf Scorch revealed the presence of Xylella fastidiosa in developing nuts. This study followed up by performing molecular diagnostics and DNA sequencing to determine the presence of the bacterium in developing nuts, mature nuts, and seedlings. Evidence was provided to show Xylella fastidiosa is seed transmitted. These results provide a greater understanding of the host-pathogen interaction and can have implications for the pecan industry, such as cultivation and trade.

Technical Abstract: Pecan bacterial leaf scorch (PBLS), caused by Xylella fastidiosa subsp. multiplex, is an economically significant disease with known detrimental impacts on nut yield in susceptible cultivars. The presence of the bacterium in seed endosperm points to new evidence for seed-to-seedling transmission in pecan. In this study, endosperm was harvested from developing pecan seed and X. fastidiosa was detected by direct PCR and confirmed by sequencing. DNA was isolated from mature seed that were collected from seven trees, revealing a positivity rate up to 90%, and transmission of X. fastidiosa from infected seed to seedlings was found to be over 80%. Further epidemiological analyses were performed to determine where X. fastidiosa localizes in mature seed and seedlings. The highest concentrations of X. fastidiosa DNA were found in the hilum and petioles, respectively. High-, medium-, and low-quality seed, as determined by nut density, were harvested to determine the impact of the presence of the bacterium on seed quality and seedling growth rate. The growth rate of seedlings originating from low-quality nuts was significantly reduced in comparison to the medium- and high-quality nuts, which were not significantly different. Despite the increased growth and germination rates, the high-quality seed group had a greater proportion of samples that tested positive for the presence of X. fastidiosa by qPCR. These results demonstrate the ability of X. fastidiosa to colonize developing seed, and to be efficiently transmitted from well-developed seeds to germinated seedlings. Continued research is needed to understand the plant-microbe interactions involved in the colonization of X. fastidiosa in pecan seed, as well as for the development of effective phytosanitary approaches to reduce the risks posed by seed transmission of X. fastidiosa.