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Appl. Environ. Microbiol., Aug 1997, 3261-3267, Vol 63, No. 8
S Li, RN Spear and JH Andrews
A 21-mer oligonucleotide probe designated Ap665, directed at the 18S rRNA
of Aureobasidium pullulans and labelled with five molecules of fluorescein
isothiocyanate, was applied by fluorescence in situ hybridization (FISH) to
populations of the fungus on slides and apple leaves from growth chamber
seedlings and orchard trees. In specificity tests that included Ap665 and a
similarly labelled universal probe and the respective complementary probes
as controls, the hybridization signal was strong for Ap665 reactions with
12 A. pullulans strains but at or below background level for 98 other fungi
including 82 phylloplane isolates. Scanning confocal laser microscopy was
used to confirm that the fluorescence originated from the cytoplasmic
matrix and to overcome limitations imposed on conventional microscopy by
leaf topography. Images were recorded with a cooled charge-coupled device
video camera and digitized for storage and manipulation. Image analysis was
used to verify semiquantitative fluorescence ratings and to demonstrate how
the distribution of the fluorescence signal in specific interactions (e.g.,
Ap665 with A. pullulans cells) could be separated at a given probability
level from nonspecific fluorescence (e.g., in interactions of Ap665 with
Cryptococcus laurentii cells) of an overlapping population. Image analysis
methods were used also to quantify epiphytic A. pullulans populations based
on cell number or percent coverage of the leaf surface. Under some
conditions, leaf autofluorescence and the release of fluorescent compounds
by leaves during the processing for hybridization decreased the
signal-to-noise ratio. These effects were reduced by the use of appropriate
excitation filter sets and fixation conditions. We conclude that FISH can
be used to detect and quantify A. pullulans cells in the phyllosphere.
Copyright © 1997, American Society for Microbiology
Quantitative fluorescence in situ hybridization of Aureobasidium pullulans on microscope slides and leaf surfaces
Department of Plant Pathology, University of Wisconsin, Madison 53706, USA.
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