Cross Talk of Signaling Pathways Between ABA and Other Phytohormones - Abscisic Acid: Metabolism, Transport and Signaling - page 237

12.2 Cross Talk Between ABA and Auxin
Auxin and ABA modulate many aspects of plant development together, mostly in
opposite directions, suggesting that active cross talk occurs between the signaling
pathways of the two hormones. For example, several components of auxin or ABA
signaling have been identified as the interaction nodes between ABA- and auxin-
signaling pathways. Wang et al. ( 2011a ) recently found that the expression of
AUXIN RESPONSE FACTOR 2 ( ARF2 ) could be induced by ABA and the ARF2
mutants showed enhanced ABA sensitivity in seed germination and primary root
growth. In contrast, the primary root growth and seed germination of transgenic
plants overexpressing ARF2 were less inhibited by ABA than that of the wild type.
Further results showed that ABA treatment altered auxin distribution in the pri-
mary root tips and made the relative auxin accumulation or auxin signal around
quiescent center (QC) cells and their surrounding columella stem cells to other
cells stronger in ARF2 - 101 than in the wild type. Together, this study indicates that
ARF2 is a novel integrative hub between ABA- and auxin-signaling pathways in
regulating plant growth (Wang et al. 2011a ).
ABA is known to be the sole plant hormone to maintain seed dormancy, which
acts through a gene expression network involving the transcription factor absci-
sic acid insensitive 3 (ABI3). However, whether the plant growth hormone auxin
plays a role in the regulation of seed dormancy in response to environmental and
internal signals remains unclear. Liu et al. ( 2013 ) recently showed that auxin also
plays a critical role in seed dormancy in Arabidopsis . Disruptions in auxin sign-
aling in MIR160 -overexpressing plants, auxin receptor mutants, or auxin biosyn-
thesis mutants dramatically released seed dormancy, whereas increases in auxin
signaling or biosynthesis greatly enhanced seed dormancy (Liu et al. 2013 ).
Further results showed that auxin action in seed dormancy requires the ABA-
signaling pathway (and vice versa), indicating that the roles of auxin and ABA
in seed dormancy are interdependent (Liu et al. 2013 ). Furthermore, auxin acts
upstream of the major regulator of seed dormancy, ABI3, by the auxin response
factors ARF10 and ARF16 to control the expression of ABI3 during seed ger-
mination. This study uncovers a novel mechanism underlying the cross talk of
auxin- and ABA-signaling pathways that auxin controls seed dormancy through
stimulation of ABA signaling by inducing ARF-mediated ABI3 activation in
Arabidopsis (Liu et al. 2013 ).
It is generally believed that auxin plays a dominant role in LR formation.
However, ABA negatively affects LR formation. Recently, Shkolnik-Inbar and
Bar-Zvi ( 2010 ) found that mutation of ABSCISIC ACID INSENSITIVE 4 ( ABI4 ),
which encodes an ABA-regulated AP2 domain transcription factor, results in an
increased number of LRs. They further showed that ABI4 overexpression impairs
LR development. The expression of ABI4 in roots is enhanced by ABA and is
repressed by auxin. Expression of the auxin efflux carrier protein PIN1 is reduced
in ABI4 overexpressors, whereas enhanced in abi4 mutants. Transport levels of
exogenously applied auxin were elevated in abi4 mutants and reduced in ABI4