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Microscopy analysis was performed to compare the phloem of HLB infected citrus and healthy control. It was observed that 1) there was significantly greater accumulation of starch in phloem parenchyma cells of infected leaves, 2) callose deposition occurred more frequently in infected midribs. With the use of transmission electron microscopy, it was observed that sieve pores of the midribs of infected plant were plugged with an amorphous substance. Collapse of sieve tubes and companion cells were also observed in HLB-infected midribs but not in healthy midribs. TEM observations also indicated that the HLB bacterium can pass through the sieve plate pore, which suggests that it is unlikely that the HLB bacterium physically caused phloem blockage because multiple bacterial cells were not aggregating. It is likely that the host response results in sieve pore plugging.


FCPRAC Progress Report (January, 2009).


Microarray analysis (based on 33,879 expressed sequence tag sequences from several citrus species and hybrids) indicated that HLB infection significantly affected expression of 624 genes whose encoded proteins were categorized according to function. The upregulation of three key starch biosynthetic genes including ADPglucose pyrophosphorylase, starch synthase, granule-bound starch synthase and starch debranching enzyme likely contributed to accumulation of starch in HLB-affected leaves.


FCPRAC Progress Report (January, 2009). Co-author of Kim in Phytopathology article (2009)

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