Ultrastructural changes in sweet orange with symptoms of huanglongbing

Authors: FU, SHIMIN - Southwest University; Hartung, John; ZHOU, C - Southwest University; SU, H - Southwest University; TAN, J - Southwest University; LI, Z - Southwest University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2014
Publication Date: 3/1/2015
Publication URL: http://handle.nal.usda.gov/10113/61525
Citation: Fu, S., Hartung, J.S., Zhou, C.Y., Su, H.N., Tan, J., Li, Z.A. 2015. Ultrastructural changes in sweet orange with symptoms of huanglongbing. Plant Disease. 99:320-324.

Interpretive Summary: Citrus greening (Huanglongbing [HLB]) is the most destructive disease of sweet orange trees. The disease is caused by a bacterium that lives inside the cells of infected trees that the tree uses to move sugar made in the leaves to the rest of the tree. The interaction between the bacterium and the host is unusual because the bacterium lives inside the plant cell rather than attacking the cell from the outside. The visual symptoms observed in an infected plant are consistent with impaired movement of sugar, and we used light and electron microscopy to study this interaction to identify symptoms of infection at the sub microscopic level. We found that the host cells produced large amounts of a polysaccharide which effectively closed the pores between connected cells in the sugar transporting network. Infected cells also became distorted in shape and crushed by adjacent cells that grew in response to the bacterium, thus accounting for impaired movement of sugar in the trees. We also observed accumulation of starch in and the disintegration of the chloroplasts used to make sugar in the leaves. In the course of this work we also identified what appear to be viruses infecting some of the bacteria in the infected cells. Viruses may have a dual nature with respect to this pathogen. They may paradoxically kill the bacterium but increase the severity of disease symptoms. Such viruses have been observed previously in infected experimental plants of other types, but this is the first observation of suspected virus particles in sweet orange trees infected with this pathogen. The information will be of interest to other researchers working on this critically important disease problem.

Technical Abstract: Citrus greening (Huanglongbing [HLB]) is one of the most destructive citrus diseases worldwide. To better understand the ultrastructural changes of sweet orange seedlings in response to infection, anatomical analyses of HLB-infected sweet orange were carried out by light and electron microscopy. As compared to healthy citrus, sieve elements were plugged with starch granules and the plasmodesmata in particular were plugged with callose. In some samples the phloem was collapsed. Chloroplast structures in neighboring cells were deformed and thylakoids had largely disappeared due to the accumulation of starch granules. Many osmiophilic granules were seen in chloroplasts and idioblasts were formed in adjacent cells. Prophage sequences occupy a significant portion of the genome of ‘Ca. Liberibacter asiaticus’ and have been used to distinguish strains from Yunnan and Guangdong provinces in China and Florida USA. Interestingly, a large number of possible phage particles were observed attached on the surface of ‘Ca. Liberibacter asiaticus’ cells in plants inoculated with strain FJ3 from Fujian province, China. Phage particles have been observed previously only in periwinkle plants artificially inoculated in Florida with ’Ca. Liberibacter asiaticus’ which carried the SC1-type prophage. PCR verified the presence of the SC1 type prophage sequences previously described from this bacterium in the Fujian isolate. This is the first time that suspected phage particles have been observed in sweet orange trees infected with ‘Ca. Liberibacter asiaticus’.