Cross Talk Between Light and ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 256

13.4.4 Hydrotropic Response
Plant roots undergo hydrotropic growth in response to moisture gradient that
helps plants acquire water and nutrients. ABA is involved in hydrotropism as the
hydrotropic response was reduced in the ABA-deficient mutant aba1 and ABA-
responsive genes were induced upon hydro-treatment (Takahashi et al. 2002 ;
Moriwaki et al. 2010 ). Recent study found that hydrotropism is less pronounced
in dark-grown seedling than in light-grown seedling and pointing out that a light
signal is required for the hydrotropic response (Moriwaki et al. 2012 ). A genetic
study identified MIZUKUSSEI1 (MIZ1) as an essential factor for hydrotropism
(Kobayashi et al. 2007 ). Blue light, but not red light, induces the localization of
MIZ1-GFP fusion protein in the root tip. Light and ABA induce the expression
of MIZ1 (Moriwaki et al. 2012 ). MIZ1 transcript level was down-regulated in the
phyAphyB double and hy5 mutants. Consistently, the hydrotropic curvature was
reduced in these mutants compared to the wild type (Moriwaki et al. 2012 ). Thus,
phyA and phyB photoreceptors and HY5 transcription factor play important roles
in blue light-mediated induction of MIZ1 and hydrotropism. Moreover, applica-
tion of ABA to hy5 restored its hydrotropic defect, whereas abamine SG (ABA
synthesis inhibitor) treatment further reduced the hydrotropic response of hy5
(Moriwaki et al. 2012 ).
13.5 Future Perspectives
The last decade has made promising progress in our understanding of the coregu-
lation of light and ABA in plant developmental programs, especially in regulating
seed germination. Although a number of signaling factors in the pathway were iden-
tified, the molecular and biochemical functions of some components are less well
understood. Some of the known proteins belong to transcription factors and play
roles through modulating gene expression. Other regulatory levels, including post-
transcription, translation, post-translational modification, and epigenetic regulation,
are likely involved as well. Furthermore, are there more components involved in
the cross talk between light and ABA? If any, how do they function? Future studies
using the combination of genetic and molecular approaches are deserved to answer
these questions. Elucidating the model and underlying mechanism between light
and ABA will certainly contribute to our better understanding of plants' adaptability
and plasticity to changing environments and help to design stress-tolerant crops in
agriculture.
Acknowledgments The work in the authors' laboratory was supported by grants from the
National Natural Science Foundation of China (31170221, 31325002) and the Chinese Academy
of Sciences.