Biology Reference
In-Depth Information
gene, encodes a basic leucine zipper transcription factor. Expression of ABI5 de-
fines a narrow developmental checkpoint following germination, during which
Arabidopsis plants sense the water status in the environment. ABI5 is a rate-
limiting factor conferring ABA-mediated postgermination developmental growth
arrest [5].ABI3 is also reactivated by ABA during a short development window.
Like ABI5, ABI3 is also required for the ABA-dependent postgermination growth
arrest [15]. However, ABI3 acts upstream of ABI5 and is essential for ABI5 gene
expression [15]. In arrested, germinated embryos, ABA can activate de novo late
embryogenesis programs to confer osmotic tolerance. During a short develop-
ment window, ABI3, ABI5 and late embryogenesis genes are reactivated by ABA.
ABA can activate ABI5 occupancy on the promoter of several late embryogenesis-
abundant genes, including Em1 and Em6 and induce their expression [15,16].
On the other hand, ABA-induced miR159 accumulation requires ABI3 but is
only partially dependent on ABI5. MYB33 and MYB101, two miR159 targets,
are positive regulators of ABA responses during germination and are subject to
ABA-dependent miR159 regulation [17].
The family of plant-specific WRKY transcription factors contains over 70
members in Arabidopsis thaliana [18-20]. WRKY proteins typically contain one
or two domains composed of about 60 amino acids with the conserved amino
acid sequence WRKYGQK, together with a novel zinc-finger motif. WRKY do-
main shows a high binding affinity to the TTGACC/T W-box sequence [21].
Based on the number of WRKY domains and the pattern of the zinc-finger motif,
WRKY proteins can be divided into 3 different groups in Arabidopsis [20].
A growing body of studies has shown that WRKY genes are involved in regu-
lating plant responses to biotic stresses. A majority of reported studies on WRKY
genes address their involvement in disease responses and salicylic acid (SA)-me-
diated defense [20,22-25]. In addition, WRKY genes are involved in plant re-
sponses to wounding [26]. Although most WRKY proteins studied thus far have
been implicated in regulating biotic stress responses, some WRKY genes regulate
plant responses to freezing [27], oxidative stress [28], drought, salinity, cold, and
heat [29-31].
There is also increasing evidence indicating that WRKY proteins are key regu-
lators in certain developmental programs. Some WRKY genes regulate biosynthe-
sis of anthocyanin [32], starch [33], and sesquiterpene [34]. Other WRKY genes
may regulate embryogenesis [35], seed size [36], seed coat and trichome develop-
ment [32,37], and senescence [38-40].
A number of studies have suggested that WRKY genes may mediate seed
germination and postgermination growth. For example, wild oat WRKY pro-
teins (ABF1 and ABF2) bind to the box2/W-box of the GA-regulated
α
-Amy2
promoter [41]. A barley WRKY gene, HvWRKY38, and its rice (Oryza sativa)
