Plant Mol Biol (2014) 85:179–191
Distinct functions of COAR and B3 domains of maize VP1
in induction of ectopic gene expression and plant developmental
phenotypes in Arabidopsis
Masaharu Suzuki · Shan Wu · Qinbao Li ·
Donald R. McCarty
Received: 29 October 2013 / Accepted: 18 January 2014 / Published online: 29 January 2014
© Springer Science+Business Media Dordrecht 2014
Transcriptional regulation is a central mechanism in devel-
opmental control of embryogenesis and seed maturation.
Key transcription factors that comprise the so-called LAFL
network, which include the LEC1-related HAP3 domain
proteins and the ABI3, FUS3, LEC2 B3 domain proteins,
regulate embryo maturation as well as the transition between
seed and seedling developmental phases (Jia et al. 2013a). A
unique characteristic of LAFL genes is that loss-of-function
mutants are unable to complete seed maturation resulting
in precocious initiation of seed germination prior to seed
desiccation (Giraudat et al. 1992; West et al. 1994; Keith
et al. 1994; Meinke et al. 1994; Koornneef et al. 1984a, b).
Hence, seed of LAFL network mutants typically have phe-
notypes that include characteristics of germinating seeds.
Upon seed germination, expression of LEC1 and three B3
genes is strictly repressed by the VAL/HSL family of B3
proteins that form a sister clade to the ABI3/FUS3/LEC2
transcription factors (Tsukagoshi et al. 2007; Suzuki et al.
2007; Jia et al. 2013b). Recruitment of chromatin remode-
ling complexes is implicated in VAL/HSL-mediated repres-
sion of the LAFL network (Suzuki et al. 2007).
The fact that abi3, fus3, lec2 single mutants show simi-
lar seed phenotypes indicates that ABI3, FUS3, and LEC2
genes have non-redundant functions in regulation of mat-
uration. This may be due in part to differences in tempo-
ral and spatial regulation of the ABI3, FUS3, LEC2 genes
and in part to functional differences between the proteins.
For example, expression of FUS3 or ABI3 driven by 35S
promoter is sufﬁcient to rescue the lec2 mutant (To et al.
2006), suggesting that LEC2 may act principally through
activation of FUS3 and ABI3. However, an alternative
possibility is that biochemical function of LEC2 can be
replaced by activity of FUS3 and ABI3 proteins. B3 factors
Abstract Arabidopsis ABI3 and maize VP1 are ortholo-
gous transcription factors that regulate seed maturation.
ABI3 and VP1 have a C-terminal B3 DNA binding domain
and a conserved N-terminal co-activator/co-repressor
(COAR) domain consisting of A1, B1, B2 sub-domains.
The COAR domain mediates abscisic acid signaling via a
physical interaction with ABI5-related bZIP proteins. In
order to delineate the COAR and B3 domain dependent
functions of VP1, we created site directed mutations in the
B3 domain that disrupted DNA binding activity and char-
acterized gene regulation by the mutant proteins in trans-
genic abi3 mutant Arabidopsis plants. In seeds, COAR
domain function of VP1 mutants that lacked B3 DNA
binding activity was sufﬁcient for complementation of the
desiccation intolerant seed phenotype of abi3. Similarly in
seedlings, the B3 domain was dispensable for most VP1
induced gene expression and ectopic developmental pheno-
types, except for a small subset of the genes that showed B3
dependent regulation. Unexpectedly, over-expression of the
DNA-binding deﬁcient VP1-K519R mutant protein caused
quantitative changes in ﬂoral organ size including elonga-
tion of pistils and shortened stamen ﬁlaments that resulted
in a self-incompatible longistyly ﬂower morphology, a key
component of heterostyly type self-incompatibility.
Keywords Seed development · Self-incompatibility ·
Heterostyly · B3 domain · Abscisic acid · VP1
Electronic supplementary material The online version of this
article (doi:10.1007/s11103-014-0177-x) contains supplementary
material, which is available to authorized users.
M. Suzuki (*) · S. Wu · Q. Li · D. R. McCarty
PMCB Program, Horticultural Sciences Department, University
of Florida, Gainesville, FL 32611, USA