An Efficient Intragenic Vector for Generating Intragenic and Cisgenic Plants in Citrus
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analysis. The transformation efficiencies obtained in both
Arabidopsis and “Duncan” grapefruit indicate its great
potential for citrus genetic improvement.
2. Materials and Methods
2.1. Construction of Citrus Intragenic Vector
pUFCI
Citrus-derived T-DNA-like regions were identified by
BLAST searching the C. clementina genome sequence in
Citrus Genome Database (http://www.citrusgenomedb.
org/) using a T-DNA left border (LB) sequence
(GTTTACACCACAATATATCCTGCCA) as a query
[5]. Scaffold_89 (an assembled genomic DNA sequence),
which harbors the first seven nucleotides of the LB se-
quence, prospective cloning sites, and the last seven nu-
cleotides of a right border (RB) sequence, was selected.
The rest of the LB sequence was obtained from Scaf-
fold_2 and fused with the above T-DNA-like fragment
through PCR amplification using primers CcF1 and
CcR1 (Table 1). PCR products were directly ligated into
the pGEM-T Easy vector and verified by sequencing.
The plasmid DNA was digested with PstI and EcoRI to
Table 1. Sequences of the primers used in this study.
Primer Sequence (5’ - 3’)
CcF1 GCTGCAGTTTACCCGCCAATATATCCTGTC
ATATTTTGAAACCAATATCAGAG
CcR1 GGAATTCGGGCTAAGGCGGCAGTTCGGCG
ATGGAGGTGGCAGGATATATTGTGGTGTA
AACGAGATGTTTGTTACTTATAGGAAACG
VecF2 GCTGCAGCCGGAATTCATACAGGCAGCCC
ATCAGTCC
VecR1 GCTGCAGCTAAGAGAAAAGAGCGTTTATT
AGAATAATCG
C-intra-F CAAGAGGACAAGAGTCTATCC
C-intra-R TGAGGATGAAGACCTGAACG
Ct-check-F TTCAGGTCTTCATCCTCACG
Ct-check-F2 ATCGAGCACACACCATCATG
Ct-check-R2 AGCCTTAGGTTGTGACAGTG
Ct-check-R3 CGTTGGAGTGGAGTAATCAG
Ct-check-R4 TTTGTAAAGCGAGGAGCAGG
Ct-check-R5 TCTCTTGCCTCAGTTCAAGG
IntraVecRBF1 GTTTACCCGCCAATATATCCTG
IntraVecRBF2 TCATATTTTGAAACCAATATCAGAG
IntraVecRBR AGGTATCAGCATCTAACATCC
CtF2 ACGGAGTTCGGTTTGTTGTC
CtR2 AGCCTCAAGAGAGTTGCTAG
EF1F AAGCCCATGGTTGTTGAGAC
EF1R CAACAGCAAACTGGTGGAAG
BamHI-BarF GTGGATCCCCCGGGCTGCAGG
SpeI-BarR GACTAGTGGTCGACGGTATCGATAAGC
produce a sticky end fragment. The binary vector
pCB302 was used as the template to generate the origin
of replication and the nptIII expression cassette by PCR
using primers VecF2 and VecR1 (Table 1) [8]. The PCR
products were digested with PstI and EcoRI and ligated
to the above sticky end citrus-derived T-DNA-like frag-
ment to produce an intermediate vector pUFCI-1. To
delete the duplicated SacI restriction site within the
T-DNA-like region, pUFCI-1 was digested with SacI and
self-ligated to further produce pUFCI-2. To avoid inte-
gration of vector backbone sequences beyond the LB into
recipient plant genome during T-DNA transfer, a ~3 kb
fragment from C. clementina genome (scaffold_34) was
inserted into pUFCI-2 between the LB and the vector
backbone through the EcoRI site [9,10]. The direction of
the added fragment was identified by PCR using primers
Ct-check-F and Ct-check-F2 or Ct-check-R2 (Figure 1
and Table 1). The sequence of the C. clementina-derived
T-DNA-like region and the buffering fragment outside of
the LB in the final citrus intragenic vector, pUFCI, was
confirmed by sequencing and shown in Figure 2. Three
restriction sites, SpeI, SacI, and BamHI, in the T-DNA-
like region can be utilized for gene cloning.
2.2. pUFCI-Bar Plasmid Construction,
Arabidopsis Transformation, and Selection
A pair of primers BamHI-BarF and SpeI-BarR (Table 1)
were used to amplify the entire exp ression cassette of th e
Bar gene from the binary vector pCB302 [8]. The PCR
products were digested with BamHI and SpeI, and then
ligated into the BamHI and SpeI sites of pUFCI. The re-
sulting plasmid was introduced into the Agrobacterium
tumefaciens strain GV3101(pMP90) by electroporation
and transformed into the Arabidospsis thaliana (L.)
Heynh. ecotype Columbia (Col-0) following the floral
dip method [11]. Transformants were identified by spray-
ing T1 seedlings with Basta.
2.3. Citrus Transformation and Positive
Regenerant Identification
The empty vector was introduced into the A. tumefaciens
strain EHA105 by electroporation and transformed into
‘Duncan’ grapefruit following the protocol described
previously [12]. No selection pressure was exerted on
explants. Regenerated shoots from individual transfor-
mation events were screened by PCR using primers
C-intra-F and C-intra-R (Table 1). A characteristic ~500
bp PCR product was u sed to iden tify p os itive regenerants.
The LB and RB integration sites were mapped by PCR
amplification using a set of primer pairs (Figure 1).
3. Results and Discussion
To facilitate development of intra-/cisgenic citrus culti-
ars, we have constructed an intragenic vector, pUFCI, v
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