ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 362

induces a rapid activation of mitogen-activated protein kinases (MAPKs), and
of calcium-dependent protein kinases (CDPKs), some of which are also targeted
by SnRK2s for phosphorylation (Jammes et al. 2009 ; Lu et al. 2002 ; Mori et al.
2006 ; Zhu et al. 2007 ; Wang et al. 2013a ). AGAMUS-like MADS-box transcrip-
tion factors were shown to be MPK substrates (Popescu et al. 2009 ). Additionally,
the flower repressor FLC (flowering locus C), the FLC paralog MAF5.2 (MADS
AFFECTING FLOWERING 5variant 2), and REF6 (RELATIVE OF EARLY
FLOWERING) are phosphorylated by MAPKs. Thus MPK signalling may play an
important role in the control of flowering time as well as adding further complex-
ity to the ABA-regulated network of phosphorylation.
The early flowering phenotype of triple snrk2 mutants is much more pro-
nounced under SDs conditions compared to LDs (Wang et al. 2013a ). Also,
despite exhibiting an early flowering phenotype under LDs, triple pp2c mutants
are considerably late under SDs (Riboni et al. 2013 ). These independent findings
are consistent with a negative role of ABA signalling in flowering under SDs.
Because the photoperiodic pathway plays a marginal role in flowering under SDs,
it is highly unlikely that the negative effect of ABA signalling in flowering under
SDs is exerted through the downregulation of FT . Since several pathways promote
flowering under SDs directly at the shoot apex, ABA might counteract other floral
promoting signals directly in the SAM (Porri et al. 2012 ; Galv￣o et al. 2012 ; Wang
et al. 2009 ). Nevertheless ABA biosynthetic mutants do not present altered flower-
ing under SDs (Riboni et al. 2013 ). Thus, ABA signalling, rather than ABA per se,
is likely to be responsible for the floral inhibition observed under SDs. Shedding
some light on this paradox is the recent discovery that a subclass of PYR proteins
can bind and inhibit PP2Cs in the absence of any ligand. This finding supports the
notion that a basal level of ABA signalling is constitutively present in Arabidopsis,
even when ABA is absent (Hao et al. 2011 ). Inhibition of flowering might be an
important role fulfilled by this particular clade of PYR proteins.
18.6 Role of ABA Downstream Effectors in Flowering
ABA plays an important role in gene expression by affecting the activity of several
transcription factors, which, in turn, mediate ABA responses. ABA INSENSITIVE
3 and 5 genes encode B3-type and basic Leucine zipper (bZIP)-type transcription
factors, respectively (Finkelstein et al. 2002 ). ABI3, and ABI5 play a prominent
role during seed germination and early seedling development (when these genes are
highly expressed). However, low levels of expression of these genes can be found in
vegetative tissues (Rohde et al. 1999 ; Brocard et al. 2002 ). Knockout and gain-of-
function alleles in these ABA transcription factors result in alterations in flowering
time suggesting a role for ABA during vegetative development through modula-
tion of these transcription factors. Besides affecting individual transcription factor
activity, ABA is likely to play a more general role in control of gene expression by
affecting chromatin remodelling complexes (Saez et al. 2008 ; Han et al. 2012 ).