Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2015
Publication Date: 9/9/2015
Publication URL: https://handle.nal.usda.gov/10113/62859
Citation: Pitino, M., Armstrong, C.M., Duan, Y. 2015. Rapid Screening for Citrus Canker Resistance Employing Pathogen-Associated Molecular Pattern-Triggered Immunity Responses. Horticulture Research. 2:15042. https://doi.org/10.1038/hortres.2015.42.
Interpretive Summary: Asiatic citrus canker (ACC) is a devastating disease for most citrus species because of the extensive damage it causes to commercial citrus cultivars in subtropical production areas. The disease is extremely persistent once established with current control methods consisting of the application of copper-based bactericides, which are not curative and simply act as a preventive measure for disease management. Host resistance is emerging as an effective alternative practices for ACC control and various techniques are being used to produce ACC resistant plants. This manuscript describes a rapid luminol-based screening method that can identify citrus seedlings less susceptible to infection by the bacterium that causes ACC based on the accumulation of reactive oxygen species (ROS). Here, the production of ROS, triggered by a conserved 22-amino acid sequence from the Xanthomonas citri flagellin, was also correlated to the expression of other defense-related genes and the limitation of disease symptoms, which indicated a role of the plant’s pathogen-associated-molecular-pattern triggered immunity response in the resistance mechanisms to canker. This study also revealed the existence of a genetic factor associated with the response by examining genetic variations both within as well as amongst citrus cultivars. Using the presented method, horticulturists can identify citrus seedlings tolerant to Xcc without the need for live pathogen testing and in a shortened time-frame compared to other available testing methods.
Technical Abstract: Citrus canker, caused by the bacterial pathogen Xanthomonas citri ssp. citri (Xcc), has been attributed to millions of dollars in loss or damage to commercial citrus crops in subtropical production areas of the world. Since identification of resistant plants is one of the most effective methods of disease management, the ability to screen for resistant seedlings plays a key role in the production of a long-term solution to canker. Here, an inverse correlation between reactive oxygen species (ROS) production by the plant and the ability of Xcc to grow and form lesions on infected plants is reported. Based on this information, a novel screening method that can rapidly identify citrus seedlings that are less susceptible to early infection by Xcc was devised by measuring ROS accumulation triggered by a 22-amino acid sequence of the conserved N-terminal part of flagellin (flg22) from X. citri ssp. citri (Xcc-flg22). In addition to limiting disease symptoms, ROS production was also correlated with the expression of basal defense-related genes such as the pattern recognition receptors LRR8 and FLS2, the leucine-rich repeat receptor-like protein RLP12, and the defense-related gene PR1, indicating an important role for pathogen-associated molecular pattern-triggered immunity (PTI) in determining resistance to citrus canker. Moreover, the differential expression patterns observed amongst the citrus seedlings demonstrated the existence of genetic variations in the PTI response among citrus species/varieties.