The Molecular Basis of ABA-Mediated Plant Response to Drought - Abiotic Stress: Plant Responses and Applications in Agriculture - page 109

various species, including Arabidopsis and barley [74-76]. Low endogenous ABA content in
the ABA-deficient mutant aba2 impairs MeJA (methyl-jasmonate)-stimulated Ca 2+ elevation,
which is, in turn, important metal closure. Furthermore, MeJA stimulates the expression of
the ABA biosynthetic gene, NCED3 . MeJA signaling in guard cells requires the presence of
endogenous ABA [77]. Another example of cross talk between ABA and jasmonates during
stress response is the up-regulation by JA of AtPYL4 ( PYRABACTINE LIKE 4 ), AtPYL5 ( PYR‐
ABACTINE LIKE 5 ) and AtPYL6 ( PYRABACTINE LIKE 6 ), which are members of the
PYR/PYL/RCAR ABA receptor family [78]. These studies showed the importance and con‐
servation across the species of the role of JA in ABA-dependent response to drought.
Cytokinins (CKs) are another group of hormones involved in stress responses [79-80].
Cytokinins regulate cell proliferation and differentiation [81]. Abiotic stresses, such as
drought, decrease the biosynthesis and transport of CKs from roots to shoots [82]. An in‐
creased concentration of CKs in xylem has been shown to decrease stomatal sensitivity
to ABA [83]. The same effect was observed when exogenous CKs were applied [84-85].
When a plant encounters mild drought conditions, it is not necessary to close the stoma‐
ta and further limit its photosynthetic rate. Since the decline in CK content increases the
stomatal sensitivity to ABA, avoidance of this phenomenon might help in obtaining a
better yield from plants that experience mild drought. CK up-regulation can be achieved
by reduced expression of a gene that encodes cytokinin oxidase, an enzyme that de‐
grades CKs. In addition to maintaining a better photosynthetic rate, increased levels of
CKs lead to enhanced activity of the cell-cycle genes, and the consequent, increase in cell
number may result in improved grain filling [86]. The process of grain filling is actually
an increase in cell number and cell filling in the endosperm [87]. There is a generally
positive relationship between endosperm cell number and grain weight in wheat [88],
barley [89], maize [90] and rice [91]. Thus, endosperm cell number is one important fac‐
tor determining grain weight [87]. Taking into account that endosperm cell number in
cereal crops is established during an early phase of development, it is assumed that this
step can be regulated by cytokinins [87]. Another manipulation of the CK level in plant
tissues was achieved by seed inoculation with CK-producing bacteria, gradually releas‐
ing CKs within the physiological concentration range [92]. Wheat plants in which seeds
were treated with such bacteria and grown under mild drought condition gave a 30-60%
higher yield than non-treated controls. Since a high level of CKs improves grain quality
and photosynthesis rate, and a high level of ABA increases root extension rate, osmopro‐
tectant activity, and solute biosynthesis, another aim of breeders is to obtain a high con‐
tent of both ABA and CKs under mild drought conditions Wilkinson et al. [6].
Ethylene, a gaseous plant hormone that inhibits root growth and development, is involved
in stress-induced leaf senescence and can contribute to reducing the rate of photosynthesis
[93-95]. ABA can modulate the influence of ethylene on stomatal conductance. Contradicto‐
ry results have been published regarding the role of ethylene in stomatal action. Desikan et
al. [96] showed that ethylene induces stomatal closure, whereas Tanaka et al. [97] and Wil‐
kinson and Davies [98] proved that ethylene can antagonize ABA action in the stomata. This