Cross Talk of Signaling Pathways Between ABA and Other Phytohormones - Abscisic Acid: Metabolism, Transport and Signaling - page 238

overexpressors. Accordingly, the abi4 mutant and ABI4 overexpressor exhibit
enhanced and reduced root acropetal auxin transport, respectively (Shkolnik-Inbar
and Bar-Zvi 2010 ). This study therefore demonstrates that ABI4 integrates ABA
with auxin pathways probably through the regulation of polar auxin transport.
Although auxin is a major morphogenesis hormone in plants, little is known
about ABA-auxin interactions during early seedling growth. Under unfavora-
ble environmental conditions, ABA inhibits the developmental transition from
an embryo into a young seedling. Belin et al. ( 2009 ) developed a genetic screen
to isolate Arabidopsis mutants whose early seedling development was resistant
to ABA. Through this approach, they identified a recessive mutation in AUXIN
RESISTANT1 ( AUX1 ), encoding an auxin influx carrier. They showed that the aux1
and pin2 mutants are insensitive to ABA-dependent repression of embryonic axis
elongation. Genetic and physiological experiments showed that this involved auxin
transport to the embryonic axis elongation zone, where ABA enhanced the activity
of an auxin-responsive promoter. Together, this study suggests that ABA represses
embryonic axis elongation by potentiating auxin signaling in its elongation zone
(Belin et al. 2009 ).
On the other hand, several key components of ABA signaling have also been
identified in regulating the auxin responses by the transcriptional regulation
of downstream genes in Arabidopsis . For example, Yang et al. ( 2011 ) recently
reported the functional identification of rice ABI5-like1 (ABL1), which regu-
lates both ABA and auxin responses. ABL1 is expressed in various tissues and
is induced by both ABA and auxin. The ABL1 deficiency mutant, abl1 , showed
reduced ABA responses, and ABL1 expression in the Arabidopsis abi5 mutant
was able to rescue the ABA sensitivity (Yang et al. 2011 ). The ABL1 protein is
localized to the nucleus and can directly bind ABRE (G-box) elements in vitro.
The gene expression analysis confirms that the large proportion of downregulated
genes of abl1 is involved in stress responses, consistent with the transcriptional
activating effects of ABL1. Most importantly, the abl1 mutant is hypersensitive to
exogenous IAA, suggesting that ABL1 modulates both ABA and auxin responses
in rice (Yang et al. 2011 ).
Rac-like GTPases or Rho-related GTPases from plants (RAC/ROPs) are impor-
tant components of hormone-signaling pathways in plants. Nibau et al. ( 2013 )
recently revealed that AtRAC7/ROP9 functions as a modulator of both auxin and
ABA signaling. Plants with reduced levels of AtRAC7/ROP9 had increased sen-
sitivity to auxin and were less sensitive to ABA (Nibau et al. 2013 ). On the other
hand, overexpressing AtRAC7/ROP9 activated ABA-induced gene expression but
repressed auxin-induced gene expression. In addition, both hormones regulated
the activity of the AtRAC7/ROP9 promoter, suggesting a feedback mechanism to
modulate the AtRAC7/ROP9-mediated signaling (Nibau et al. 2013 ). These results
place AtRAC7/ROP9 as an important signal transducer that integrates auxin and
ABA signaling in the plant. By contrast, Choi et al. ( 2013 ) recently demonstrated
that ROP-interactive CRIB motif-containing protein 1 (RIC1) is also involved
in the interaction between auxin- and ABA-regulated root development. RIC1
expression is highly induced by both hormones and expressed in the roots of