Callose deposition in the phloem plasmodesmata and inhibition of phloem transport in citrus leaves infected with “Candidatus Liberibacter asiaticus”
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Huanglongbing (HLB) is a destructive disease of citrus trees caused by phloem-limited bacteria, Candidatus Liberibacter spp. One of the early microscopic manifestations of HLB is excessive starch accumulation in leaf chloroplasts. We hypothesize that the causative bacteria in the phloem may intervene photoassimilate export, causing the starch to over-accumulate. We examined citrus leaf phloem cells by microscopy methods to characterize plant responses to Liberibacter infection and the contribution of these responses to the pathogenicity of HLB. Plasmodesmata pore units (PPUs) connecting companion cells and sieve elements were stained with a callose-specific dye in the Liberibacter-infected leaf phloem cells; callose accumulated around PPUs before starch began to accumulate in the chloroplasts. When examined by transmission electron microscopy, PPUs with abnormally large callose deposits were more abundant in the Liberibacter-infected samples than in the uninfected samples. We demonstrated an impairment of symplastic dye movement into the vascular tissue and delayed photoassimilate export in the Liberibacter-infected leaves. Liberibacter infection was also linked to callose deposition in the sieve plates, which effectively reduced the sizes of sieve pores. Our results indicate that Liberibacter infection is accompanied by callose deposition in PPUs and sieve pores of the sieve tubes and suggest that the phloem plugging by callose inhibits phloem transport, contributing to the development of HLB symptoms.
KeywordsCandidatus Liberibacter asiaticus Huanglongbing Plasmodesmata Plasmodesmata pore unit Callose Phloem
We are grateful to Dr. Karen Koch (University of Florida) and her lab assistants for their help in 14CO2 pulse labeling. We also thank Dr. Rob Ferl (University of Florida) and Dr. Tony Romeo (University of Florida) for allowing us to use their fluorescence stereomicroscope and phosphorimager scanner, respectively. We are indebted to Dr. Dean Gabriel (University of Florida) for the use of his citrus plants in the University of Florida Plant Containment Facility. We thank Dr. Mullendore (Washington State University) for his advice in carrying out scanning electron microscopy of sieve pores. This work was supported by the Florida Citrus Production Research Advisory Council (grant no. 113 to B-H. K.) and by the United States Department of Agriculture, (NIFA Award no. 2010-34446-21694 to B-H. K.)
Conflict of Interest
The authors declare that they have no conflict of interest.
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