Water Use and Drought Response in Cultivated and Wild Apples - Abiotic Stress: Plant Responses and Applications in Agriculture - page 255

recent study of the effects of overexpressing or silencing early response to dehydration
( ERD15 ) in Arabidopsis, Kariola et al. [57] observed decreased drought tolerance in the
overexpressing lines, whereas the silenced lines were hypersensitive to ABA and showed
enhanced tolerance to drought. This study also suggests a negative role for ERD15 in mediating
stress-related ABA signaling.
The cuticle is an important barrier to moisture loss in plants, therefore genes associated with
cuticle synthesis and turnover are expected to contribute to plant water status. A recent study
in alfalfa demonstrated that increased wax production activated by a putative TF (transcription
factor) [ WXP1 ] also enhanced drought tolerance in transgenic plants [58]. Similar studies with
other transcription associated factors have also demonstrated a correlation between increased
wax synthesis and drought tolerance [59, 60].
Constitutive expression of a barley Group III LEA (late embryogenesis abundant) protein
placed in wheat under control of the maize ubi1 promoter resulted in improved water use
efficiency and higher total dry mass in the majority of transgenic lines [61]. Overexpressing a
small molecular weight heat shock protein ( HSP17.6 ) conferred drought tolerance in Arabi‐
dopsis transgenic lines [62]. The authors also demonstrated that HSP17.6 had chaperone-like
activity and could protect citrate synthase from chemical denaturation. Taken together these
studies emphasize the different mechanisms so far discovered that affect water use and
drought resistance in plants and indicate that different genes in the same pathway may be
used by plants to control WD responses.
9. Regulation of pathways/signaling networks associated with dehydration
Most studies of dehydration responsive signaling pathways implicate ABA directly in altering
specific gene expression [63]. In fact genes that respond to ABA usually have multiple copies
of an ABA response element (ABRE) or a combination of an ABRE with other motifs such as
Myb, Myc or coupling elements [for example, 64]. A second pathway involves drought
response element binding (DREB) proteins, particularly DREB2 -encoding genes, and may also
involve ABA indirectly [65]. A recent report on the isolation of a fourth CBF ( CBF4 ) from
Arabidopsis suggests that this TF only responds to drought, in contrast to observations
reported for CBFs1-3 which respond to both cold and drought stress [66].
Compelling evidence indicates that the ABA pathway likely involves Ca 2+ signal transduction
as an early step and important relay system for dehydration responses. ABA can also increase
reactive oxygen species through higher levels of H 2 O 2 [67, reviewed in 68]. Other studies have
suggested stress-responsive pathways that operate through osmotic sensing independently of
ABA [69]. An osmotic sensor similar to bacterial two-component receptors has been identified
in Arabidopsis [70]. The gene was able to complement several mutations in yeast osmosensors
and activated the HOG1 response pathway through a mitogen-activated protein kinase. No
doubt other signaling components, both ABA-dependent and independent, will be identified
in the near future.