Publications - Faculty - Recently Submitted Faculty Publications 2009

Albrigo - Plant Pathology

Anatomical evolution of symptoms in citrus from infection with the huanglongbing (HLB) bacterium

D. S. Achor, K-R. Chung, E. Etxeberria, S. Y. Folimonova, N. Wang and L. G. Albrigo

Abstract. The most characteristic symptom of huanglongbing (HLB) is the non-symmetrical mottled chlorosis of leaf blades.  Starch accumulation and phloem collapse have been associated with symptom development in this disease presumed to be caused by Candidatus Liberibacter asiaticus.  Several hypotheses regarding phloem disruption to starch accumulation to chlorosis evolved concerning symptom development. These were tested using light and transmission electron microscopy (TEM).  Samples collected and fixed for TEM from various stages of HLB symptom development revealed the following:  starch accumulation occurred after phloem plugging and cell collapse and, therefore, localized carbohydrate deficiency may be a factor.  Starch packing of chloroplasts did not rupture the outer membranes, but the inner grana structure was disrupted thus leading to chlorosis, but only in parts of the leaf where phloem plugging occurred.  Sieve elements were obstructed by both amorphous and filamentous materials and both occurred in readily observed amounts, while bacteria were insufficient to directly cause plugging.  The amorphous material was positively identified as callose by immunoassay with gold labeling.  Phloem protein 2 lectin was identified in the filamentous plugging material using immunoassay with gold labeling.  The data support the development of HLB symptoms in the following sequence: phloem plugging and necrosis, cell wall swelling of sieve elements and companion cells followed by some phloem cell collapse, presumed sugar backup in localized leaf blade areas leading to starch accumulation until chloroplast structure is disrupted with resulting chlorosis.