Cross Talk of Signaling Pathways Between ABA and Other Phytohormones - Abscisic Acid: Metabolism, Transport and Signaling - page 242

and early seedling growth (Cheng et al. 2009 ). Through screening for mutations
that either enhanced or suppressed the ABA-resistant seed germination phenotype
of the Arabidopsis abi1 - 1 mutant, alleles of the constitutive ethylene response
mutant ctr1 and ethylene-insensitive mutant ein2 were recovered as enhancer
and suppressor mutations, respectively (Beaudoin et al. 2000 ). Genetic analy-
ses showed that the ethylene-signaling cascade defined by the ETR1, CTR1, and
EIN2 genes inhibit ABA signaling in seeds. Epistasis analysis between ethylene-
and ABA-insensitive mutations indicated that endogenous ethylene promotes
seed germination by decreasing sensitivity to endogenous ABA (Beaudoin et al.
2000 ). In contrast to the situation in seeds, ein2 and etr1 - 1 roots were resistant
to both ABA and ethylene. This study indicates that ABA and ethylene antago-
nistically or synergistically to regulate seed germination and root growth, respec-
tively (Beaudoin et al. 2000 ). Similarly, Ghassemian et al. ( 2000 ) demonstrated
that ethylene appears to be a negative regulator of ABA action during germination
and positively regulates some aspects of ABA action in regulation of root growth
(Ghassemian et al. 2000 ). For example, the era3 mutant that is allelic to ein2
overaccumulates ABA, suggesting that ethylene signaling negatively regulates
ABA biosynthesis (Ghassemian et al. 2000 ). These findings indicate that interac-
tion between ABA and ethylene is complex and possibly interconnected through
multiple feedback mechanisms. In contrast, the identification of ABA-insensitive
mutants unveiled the LONG HYPOCOTYL5 (HY5) transcription factor as an
important molecular link between ABA and ethylene biosynthesis (Li et al. 2011 ).
A dramatic increase in ethylene levels in the ABA-insensitive hy5 mutant sug-
gested its role in the repression of ethylene biosynthesis (Li et al. 2011 ). Taken
together, these results suggest ABA and ethylene interact to inhibit each other's
biosynthesis.
To examine the cross talk between the ethylene and ABA signal transduction
pathways during ABA-induced stomatal closure, Tanaka et al. ( 2005 ) examined an
ethylene overproducing mutant ( eto1 - 1 ) and the two ethylene-insensitive mutants
( etr1 - 1 and ein3 - 1 ). Their results showed that stomata of wild-type plants were
closed within a few minutes in response to ABA, whereas stomata of the eto1 -
1 mutant showed a similar but less-sensitive ABA response. Moreover, ABA-
induced stomatal closure could be inhibited by application of ethylene or the
ethylene precursor ACC. In contrast, stomata of the etr1 - 1 and ein3 - 1 mutants
were able to close in response to concomitant ABA and ACC application. This
study indicates that the ethylene-signaling pathway delays ABA-induced stomatal
closure (Tanaka et al. 2005 ).
12.6 Cross Talk Between ABA and Jasmonate
The plant hormone jasmonates (JAs) play essential roles in plant defense and
development. Anderson et al. ( 2004 ) revealed a complex interplay between JA-
and ABA-signaling pathways that regulates plant defense gene expression and