Cross Talk Between Light and ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 251

coauthors ( 2013 ) further found that ABA is required for full expression of differ-
ent LHCB members likely via the WRKY40 transcription factor. ABA may be an
inducer to fine-tune LHCB expression under stressful conditions in cooperation
with light that allows plants to adapt to environmental changes. Moreover, a signal
transduction chain consisting of GCR1 (a potential G-protein-coupled receptor),
GPA1 (the sole Ga subunit), RPN1 (one of four members of an iron-containing
subgroup of the cupin superfamily), and a nuclear factor Y convergences blue light
and ABA signals to regulate LHCB expression in etiolated Arabidopsis seedlings
(Warpeha et al. 2007 ).
In addition, ABA regulates genes involved in the light signal transduction
pathway. For instance, the transcript levels of FHY3 and FAR1 , encoding two key
positive transcription factors in the phyA pathway, are induced in Arabidopsis
seedlings after ABA treatment (Tang et al. 2013 ).
13.4 Light and ABA Coregulate Plant Responses
Light and phytohormone ABA coordinately regulate many plant developmental
processes, including seed germination, seedling growth, stomatal movement, and
hydrotropic response, as reviewed below in detail. We focus on the function of
signaling factors that were genetically identified in recent studies and their regula-
tory mechanisms on each distinct response.
13.4.1 Seed Germination
Seed germination is an adaptive trait of higher plants that is controlled by both
environmental cues and internal growth regulators, including light, GA, and ABA.
GA is known to break seed dormancy and promote germination, whereas ABA is
involved in maintaining seed dormancy and inhibiting germination (Koornneef et al.
2002 ; Finch-Savage and Leubner-Metzger 2006 ). It is now much clear that GA pro-
motes germination by promoting destruction of DELLA repressors, whereas ABA
prevents germination by stimulating the expression of ABI repressors. Endogenous
ABA biosynthesis in imbibed seeds is required for the maintenance of seed dor-
mancy in Arabidopsis and tobacco (Ali-Rachedi et al. 2004 ; Grappin et al. 2000 ).
Light is a critical determinant environmental factor for seed germination in some
small-seeded plants, such as Arabidopsis and lettuce (Shinomura 1997 ). In the mid-
dle of twentieth century, it was discovered that red light promotes, whereas far-red
light inhibits lettuce seed germination, and the process is reversible by red and far
red (Borthwick et al. 1952 ). The photoreceptor responsible for the reversible photo-
reaction was discovered from etiolated Brassica rapa and Zea mays and was named
phytochrome (Butler et al. 1959 ). It has been well established that phyA and phyB
play curical role in the light-mediated seed germination (Shinomura et al. 1994 , 1996 ;