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Subgroup 4
Subgroup 3
Subgroup 21
99
100
At MYB85
74
Subgroup
At MYB85
0.5
Subgroup
At MYB46
Fig. 1. Phylogenetic tree was constructed using the R2R3-MYB genes described as
regulators of SW biosynthesis and/or involved in the regulation of phenylpropanoid/
lignin genes ( Table 1 ). Sequences were aligned using MAFFT with the FFT-NS-i
algorithm ( Katoh et al., 2002 ) and the neighbour-joining phylogenetic tree with 1000
bootstraps was constructed using MEGA5 ( Tamura et al., 2011 ), where branches
corresponding to partitions reproduced in < 50% bootstrap replicates are collapsed.
The evolutionary distances were computed using the Jones-Taylor-Thornton substitu-
tion model and the rate variation among sites was modelled with a gamma distribution
of 1. To ensure contribution of the variable C-terminal domain of the R2R3-MYB genes,
the pairwise gap deletion method was used. Repressor sequences from different species
related with Arabidopsis subgroup 4 are phylogenetically close and grouped together
making a clade including Arabidopsis AtMYB4 and AtMYB32, maize ZmMYB42 and
ZmMYB31, switchgrass PvMYB4a, wheat TaMYB4, Eucalyptus EgMYB1, and Anthir-
rinum AmMYB308 and AmMYB330 sequences. Master switch regulators orthologs of
Arabidopsis AtMYB46 and AtMYB83 are grouped together including Pinus PtMYB4,
Eucalyptus EgMYB2, and Populus PtrMYB20 and PtrMYB3. On the other hand,
Populus PttMYB21a is grouped with subgroup 21 Arabidopsis AtMYB69, AtMYB52
and AtMYB54 sequences. Interestingly, Pinus PtMYB1 is grouped with the Arabidopsis-
related genes AtMYB20, AtMYB85, AtMYB42 and AtMYB43, whereas PtMYB8 does
not seem to be related to any other gene. Moreover, Arabidopsis AtMYB58 and
AtMYB63 (subgroup 3), AtMYB75 (subgroup 6), AtMYB26 and AtMYB103 do not
contain known homolog sequences from other species with a demonstrated role in lignin
or phenylpropanoid biosynthesis.
AtMYB7, AtMYB32 and AtMYB4 negatively regulate their own transcrip-
tion as well as that of AtMYB52 ( Ko et al., 2009 ).
In E. gunnii, EgMYB1 a member of subgroup 4 is highly and preferentially
expressed in xylem. EgMYB1 is able to specifically bind the MYB-binding
sites present in the cis-regulatory regions of the E. gunnii CCR and CAD2
promoters ( Legay et al., 2007 ). In vivo transient coexpression in tobacco leaves
showed that EgMYB1 can repress both EgCAD2 and EgCCR promoter
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