LIBRO-CONGRESO-CITRUS

amplify different geographical Huanglongbing affected citrus samples. The PCR products were sequenced and
the diversity of different strains was studied. The results showed that the amplified size is 1654 bp including
5S rRNA gene for six strains from Guangxi, Fujian, Hunan, Taiwan, 1648-1650 bp including 5S rRNA gene
for four strains from South Africa, 1635 bp including glpK gene and 5S rRNA gene for six strains from Brazil,
respectively. The strains from Brazil shared 94.1%-94.3% of sequence identity with Las, 94.0-94.2% with Laf
and 99.8%-100% with Lam. The strains from South Africa shared 97.4%-97.5% of sequence identity with Las,
99.9-100% with Laf and 94.1%-94.2% with Lam. The strains from China mainland, Taiwan and Florida shared
99.8%-100% of sequence identity with Las, 97.3-97.5% with Laf and 94.2%-94.4% with Lam. Total 4 bps of
glpK sequence were mutant among three strains from Brazil, but only two amino acids were changed. A
phylogenetic tree was constructed for the 16 strains and other proteobacterias based on the 23S/5S rDNA
sequences. The results showed that the strains from Brazil were clustered with Lam, those from South Africa
clustered with Laf, and those from China mainland, Taiwan and Florida clustered with Las.
S12P06
Phloem anatomy of citrus trees: healthy vs HLB affected
Etxeberria E., and Narciso C.
University of Florida, Citrus Research and Education Center, USA. eetxeber@ufl.edu
Phloem cells of HLB-affected trees become obstructed with callose, P-protein plugs, and in some instances,
starch grains. The presence of these plugs is believed to hinder the transport of photoassimilates (nitrogenous
and reduced carbon compounds) to the root system. However, even with a seemingly collapsed phloem tissue,
citrus trees remain viable and produce fruit for some time, suggesting either incomplete plugging of phloem
elements or the existence of alternative routes for photoassimilate transport. In this study, we examined
the basic structure of phloem tissue from HLB-unaffected and HLB-affected trees under light and scanning
electron microscopy. To avoid any possible interference with callose induced by injury during sampling, we
employed freeze substitution technique. Sieve elements from HLB-unaffected trees show sizable lateral pit
fields to phloem and ray parenchyma. The cells have very angular sieve plates and appear relatively clean
from cellular components. Sieve pores are of approximately 0.1-0.5 μm in diameter. HLB-affected phloem
cells contain massive amounts of amorphous material clearly traversing sieve plates and lateral pit fields.
Eventually, the walls thicken and cells totally collapse into an almost solid cell wall barrier. Occasionally,
wound phloem appears along the petiole cortex in HLB-affected trees. Most notable is the large number
of wall perforations all along the cortex parenchyma with abundant pit fields. These anatomical features
of the parenchyma cells provide an alternative route that would allow movement of photassimilates from
photosynthetic cells through cortex and ray parenchyma.
S12P07
Anatomical comparison of HLB-affected sweet orange and rough lemon
Fan J. 1 , Chen C. 2 , Gmitter Jr. F.G. 2 , Achor D.S. 2 , Brlansky R.H. 2 , and Li Z.G. 1
1 Chongqing University, Key Laboratory of Biorheological Science and Technology, China; and 2 University of Florida - Citrus Research
and Education Center, USA. fgmitter@ufl.edu
Citrus Huanglongbing (HLB) is considered the most serious problem for citrus production. There is no known
resistant germplasm although some citrus types appear to be more tolerant and/or insensitive to the disease,
exhibiting slow decline and rejuvenating capability. To understand the underlying mechanisms, symptomatic,
asymptomatic, and healthy leaf, stem and root tissues from sensitive sweet orange and tolerant rough lemon
were compared using light and transmission electron microscopy. Phloem collapse, plugged sieve elements
and accumulation of starch were observed in leaf petioles of symptomatic leaves from both HLB-diseased
rough lemon and sweet orange, compared with their own mock-inoculated controls. In symptomless leaves,
significant anatomical changes (e.g. phloem cell collapse and starch accumulation) were found in HLBdiseased
sweet orange, but not in rough lemon. Furthermore, starch depletion and phloem fiber degradation
were observed in secondary roots of diseased sweet orange, but not in rough lemon. In young green stems, a
few plugged sieve elements were seen in both diseased rough lemon and sweet orange, but starch deposition
XII INTERNATIONAL CITRUS CONGRESS 2012 - 201
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