Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2010
Publication Date: 8/1/2010
Publication URL: http://www.apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-100-8-0756
Citation: Hartung, J.S., Paul, C., Achor, D., Brlansky, R. 2010. Colonization of dodder, Cuscuta indecorans, by Ca. Liberibacter asiaticus and Ca. Liberibacter americanus. Phytopathology. 100(8):756-762. Interpretive Summary: Huanglongbing, or citrus greening, threatens the global citrus industry. Citrus trees are difficult hosts for greenhouse experiments, but the pathogen can be transferred from citrus to more easily studied experimental hosts by using parasitic dodder plants. These dodder plants grow with their ‘roots’ in the infected citrus and can be made to produce new ‘roots’ on other plants, which then become hosts for the dodder. The new plant host, such as periwinkle plants, develop disease, but the interaction between the pathogen and the parasitic dodder has not been studied. In this study we characterized the interaction between the huanglongbing pathogen and the parasitic dodder plant. Dodder plants were grown on sweet orange and lemon trees infected with huanglongbing disease. Dodder plants, which do not have leaves and which look like spaghetti, were harvested and cut into uniform pieces. The amount of the pathogen present in each piece was quantified, and sections of tissue were viewed by electron microscopy. We predicted that the concentration of the pathogen in the dodder would be much higher at the end of the dodder directly growing out of the diseased citrus tree and lower in sections further removed from the source plant, but this was not what was observed. Instead, the distribution of the pathogen in dodder plants was very irregular when viewed at the whole plant level as well as at the microscopic level. We observed different shapes of pathogen cells within single plant cells, as well as single pathogen cells changing from one shape to another. This observation is both unusual and important because most bacteria have only one fixed shape. Our work shows that the dodder plant is a very good experimental host for the huanglongbing pathogen. Our work will be of interest to other scientists who use dodder to transmit huanglongbing disease as well as plant pathogens.
Technical Abstract: Huanglongbing, or citrus greening, threatens the global citrus industry. The presumptive pathogen, Candidatus Liberibacter asiaticus and Candidatus Liberibacter americanus can be transferred from citrus to more easily studied experimental hosts by using holoparasitic dodder plants. However the interaction between Candidatus Liberibacter spp. and the dodder has not been studied. We combined quantitative PCR with electron microscopy to show that only 65% of tendrils of Cuscuta indecorans grown on Candidatus Liberibacter spp. infected host plants had detectable levels of the pathogen. Among tendrils that were colonized by Liberibacter in at least one 2-cm segment, most were not colonized in all segments. Furthermore, the estimated population levels of the pathogen present in serial 2- cm segments of dodder tendrils varied widely and without any consistent pattern. Thus, there was generally not a concentration gradient of the pathogen from the source plant towards the recipient and populations of the pathogen were sometimes found in the distal segments of the dodder plant but not in the proximal or middle segments. Populations of the pathogen ranged from 1 x 103 to 5.5 x 109 genome targets per 2 cm segment, demonstrating that Candidatus Liberibacter spp. multiplies well in Cuscuta indecora. However, 55% of individual stem segments did not contain detectable levels of the pathogen, consistent with a pattern of non uniform colonization similar to that observed in the much more anatomically complex citrus tree. Colonization of dodder by the pathogen is also non uniform at the ultra structural level, with adjacent phloem vessel elements being completely full of the pathogen or free of the pathogen. We also observed bacteria in the phloem vessels that belonged to two distinct size classes based on the diameters of cross sections of cells. In other sections from the same tendrils we observed single bacterial cells that were apparently in the process of differentiating between the large and round forms to the long and thin forms (or vice versa). The process controlling this morphological differentiation of the pathogen is not known. The highly reduced and simplified anatomy of the dodder plant as well as its rapid growth rate as compared to citrus and the ability of the plant to support multiplication of the pathogen to high levels makes it an interesting host plant for further studies of host pathogen interactions.