Programmed Cell Death as a Response to High Light, UV and Drought Stress in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 229

and NO in guard cells. These signals evoke ion efflux through plasma membrane ion chan‐
nels, resulting in the reduction of guard cell turgor pressure and stomata closure to reduce
water loss through the transpiration [194]. Mutants with the perturbation of ABA synthesis
or signaling display drought hypersensitivity, manifested in significant growth reduction
which suggests that ABA is needed for the proper response to drought [177]. In Arabidopsis ,
stomata closure has been shown to be regulated by ABI1 and ABI3 ( ABA - insensitive 1 and 3 )
belonging to a group of genes identified through mutant screens and being associated with
ABA-mediated metabolic responses to stress. Under drought, ABI1 transcription is up-regu‐
lated while ABI3 is usually down-regulated. Recently, ABI3 has been hypothesized to be es‐
sential for the successful drought recovery [195]. The cell-surface ABA receptors have not
been recognized yet. However, a recent study has proposed the flowering time control pro‐
tein A (FCA) and the chloroplastic magnesium protoporphyrin-IX chelatase H subunit
(CHLH) as candidates for ABA receptors, both of which have been shown to bind ABA in
vitro [196]. Two genes have been found to play a crucial role in the prevention of stomata
opening - GPA1 ( G PROTEIN ALPHA SUBUNIT 1 ) and PLDα1 ( PHOSPHOLIPASE D AL‐
PHA 1 ) [197]. PLD-produced phosphatidic acid has been also shown to play an important
role in the plant response to drought stress [198].
Similarly to ABA, JA also triggers stomata closure and such response is conserved among
various plant species [199]. At the early stage of moderate drought, plants accumulate high
concentrations of ABA and induce ABA-responsive genes. At this stage, no significant dif‐
ferences in JA-responsive genes are observed. At later stage of drought stress, ABA level re‐
turns to normal, while JA synthesis and JA signaling genes are significantly down-
regulated. This suggests the negative correlation between ABA and JA pathways [177]. The
high concentration of JA is probably undesirable during drought stress, as it inhibits the cell
expansion and results in stunted growth [200]. Therefore, plants down-regulate JA synthesis
and signaling pathways to minimize the inhibitory effect of JA on growth, establishing a
new hormone homeostasis.
Downstream of early stress perception events, signaling molecules are activated. Such sec‐
ondary messengers include Ca 2+ ions and ROS. They induce further genes that are needed to
establish a new cellular homeostasis leading to drought resistance and tolerance [180]. Re‐
cent studies have strongly proven that drought response progresses through mitogen-acti‐
vated protein kinase (MAPK) pathways [201]. In yeast and animals, MAPK-regulated
pathways take part in the production of osmolytes and antioxidants. These MAPK path‐
ways are activated by receptors/sensors such as protein tyrosine kinases, G-protein-coupled
receptors and histidine kinases. Among these, G-protein-associated receptors have been
proposed to serve as one kind of membrane-bounded receptors for ABA. A family of histi‐
dine kinases (HK) have been also identified in plants [199]. An Arabidopsis AtHK1 has been
suggested to be involved in the osmotic stress signal transduction [202]. Other members of
this family, ATHB7 and ATHB12 have been proposed to maintain the reduction of plant
growth under drought, which is an acclimation response to survive prolonged drought
stress [177].