Cross Talk of Signaling Pathways Between ABA and Other Phytohormones - Abscisic Acid: Metabolism, Transport and Signaling - page 240

this action is independent of the ethylene signal transduction pathway (Ding et al.
2008 ). These results showed that ABA can suppress Nod factor signal transduction
in the epidermis and can regulate cytokinin induction of the nodule primordium in
the root cortex. Therefore, this study suggests that ABA is capable of coordinately
regulating the nodule formation and cytokinin signaling (Ding et al. 2008 ).
12.4 Cross Talk Between ABA and Gibberellin
The phytohormones gibberellin (GA) and ABA play essential and often antago-
nistic roles in regulating plant growth, development, and stress responses. For
example, GA is associated with the promotion of germination, growth, and flow-
ering. However, ABA inhibits these processes. Moreover, the antagonistic rela-
tionship of the two hormones regulates the transition from embryogenesis to seed
germination (Razem et al. 2006 ). Several different mechanisms have been shown
to underlie this antagonistic interaction in different developmental processes.
During cereal seed germination, the developing embryo releases GAs to the aleu-
rone cells where they induce the transcription of several genes encoding hydrolytic
enzymes, including a-amylase. These enzymes are then secreted to the endosperm
and hydrolyze starch and proteins, supplying nutrients to the developing embryo
(Weiss and Ori 2007 ). In contrast, ABA suppresses a-amylase expression. The
GA-induced, ABA-suppressed transcription of a-amylase in the aleurone layer
of cereal seeds was classically used as an experimental system to study the inter-
action between GA and ABA. The a-amylase promoter contains a GA response
element, required for both its activation by GA and suppression by ABA (Rogers
and Rogers 1992 ). Gubler et al. ( 1995 ) identified a GA-induced Myb-like protein
(GAMyb) that binds to the GA response element box of the a-amylase promoter
(Gubler et al. 1995 ). Induction of GAMyb and a-amylase transcription was shown
to be mediated by the DELLA protein SLR1. How does ABA affect this pathway?
The induction of GAMyb and a-amylase by GA is suppressed by an ABA-induced
Ser/Thr protein kinase, PKABA1. ABA and PKABA1 inhibited the upregulation
of GAMyb and a-amylase in slr1 mutants as well, suggesting that the inhibition
of GAMyb and a-amylase by PKABA1 occurs downstream of DELLA (Gomez-
Cadenas et al. 1999 , 2001 ). However, a more recent study has shown that when
PKABA1 is suppressed by RNAi, ABA still inhibits the GA-induced a-amylase
expression. This finding indicates that ABA affects this process through an addi-
tional, PKABA1-independent pathway (Zentella et al. 2002 ). A candidate alterna-
tive ABA-signaling pathway to suppress GA responses in rice may involve two
ABA-induced WRKY transcriptional regulators (Xie et al. 2006 ).
A different mechanism of interaction between GA and ABA in the regulation of
root growth was proposed (Achard et al. 2006 ). In Arabidopsis , GA promotes and
ABA suppresses root growth, and both effects seem to be mediated by the DELLA
proteins (Achard et al. 2006 ). ABA application increased the stability of RGA and
blocked its GA-induced degradation. Moreover, the quadruple-DELLA mutant