Differential screening of the cDNA library with
a polymerase chain reaction (PCR)-amplified control
and subtracted probe was carried out. The
preparation of the probes was performed basically
according to Gamas et al. [23]. For preparation of
the subtracted probe, 3 mg poly(A)+ RNA from 24
h F. eumartii inoculated tubers were reversed-transcribed
using a mixture of oligodT12–18 and random
hexamers to prime the cDNA synthesis. The
poly(A)+ RNA (10 mg) obtained from control
tubers (tubers in which PDA sterile disks were
kept for 24 h) was photobiotinylated. Next, a
subtracted hybridization step between biotinylated
poly(A)+ RNA prepared from control tubers and
the single-stranded cDNA prepared from fungalinfected
tubers was performed. Finally, the subtracted
cDNA was converted to double strand
molecules, ligated to linkers [24] and amplified by
PCR. A control cDNA probe was also prepared
following the same procedure, starting from 3 mg
poly(A)+ RNA from 24 h PDA sterile diskstreated
tubers, and avoiding the subtractive step.
Both cDNA probes were labeled with [a-
32P]dCTP
by random priming [25], and used directly for the
screening of the potato tuber cDNA library.
About 500 000 plaques at a density of approximately
50 000 plaques/15 cm plate were screened.
Hybridizations were carried out according to
Gamas et al. [23]. Positive recombinant clones
were isolated by plaque purification using the
same hybridization conditions. In vivo excision
was carried out from the selected Uni-ZAP XR
cDNA clones, following the manufacturer’s instructions
(Stratagene) and recombinant pBluescript
SK(−) containing colonies were obtained
from phagemid infections of XL1-Blue strain
(Stratagene).