Microscopic Atlas of Citrus Huanglongbing Unravels Its Sequential Disease Development Mechanism

Citrus huanglongbing (HLB), caused by the phloem-colonizing ‘ Candidatus Liberibacter asiaticus’ (CLas), is a devastating disease worldwide. CLas systemically infects all organs containing phloem. How CLas causes pathogenicity in different organs remains poorly understood. Here, we conducted transmission electron microscope and light microscope analyses of different organs in a grove and greenhouse at four time points over a year. Compared with the healthy controls, increased phloem cell death was observed in CLas-positive tissues, including leaf, fruit albedo, pedicel, and columella, as well as bark tissues of the branches and trunk, but not in root and seed coat tissues. CLas infection causes more phloem callose deposition in leaf, fruit albedo, pedicel, and columella, as well as bark tissues of the branches and trunk, but not, or to a lesser extent, in root and seed coat tissues. Our data demonstrate that CLas triggers phloem cell death and callose deposition in photosynthetic tissues but not, or to a lesser extent, in nonphotosynthetic tissues. CLas infection also caused starch accumulation in mature leaves and branch bark and starch depletion in roots. Our data have revealed the sequential order of HLB disease development. CLas triggers phloem cell death and callose deposition, which cause phloem transportation blockage and subsequent starch accumulation in leaves. Starch accumulation leads to chloroplast degeneration and subsequent blotchy mottling and yellowing symptoms. Root decay mainly results from carbon starvation, which further contributes to HLB symptom development, such as nutrient deficiency, tree decline, and death. In sum, CLas causes disease effects on photosynthetic and nonphotosynthetic tissues via distinct mechanisms. [Formula: see text]