Water Use and Drought Response in Cultivated and Wild Apples - Abiotic Stress: Plant Responses and Applications in Agriculture - page 254

7. Genes associated with drought avoidance and escape
An example of the rapid evolution of a drought escape mechanism (early flowering) was
demonstrated in a population of Brassica napa subjected to a multiyear drought [47]. Compar‐
ison of seeds collected from individual plants before the drought with those obtained from the
drought-affected population indicated a significant earlier onset of flowering in the latter
population. This observation was further expanded in a study of quantitave trait loci (QTLs)
in maize associated with flowering time ( Vgt1 ) where a cis -acting region upstream of a
transcription factor was shown to be the link between a QTL and the early flowering trait [48].
The authors speculate that natural genetic variations in flowering time enabled the selection
of maize lines adapted to a range of latitudes and growing seasons, including drought
tolerance. Another study of natural variation in ecotypes of Arabidopsis [49] found a strong
positive genetic correlation (rG = 0.98) between δ 13 C (drought avoidance) and flowering time
(drought escape). They also observed compelling evidence for pleiotropy in lines varying in
FLOWERING LOCUS C, suggesting that correlated evolution of δ 13 C and flowering time could
be partly explained by coordinated allele fixation altering both traits.
In alfalfa a gene encoding a zinc-finger motif is expressed in roots [50, 51]. The protein encoded
by Alfin1 binds DNA at a specific cis -elemtent and is proposed to be a root growth regulator,
as transgenic lines overexpressing the gene show enhanced root growth under both normal
and high salt conditions [52]. These same transgenic lines were significantly more salt tolerant,
and presumably more drought tolerant (high salt concentrations in the media decrease water
uptake), althought drought tolerance per se was not measured.
8. Genes associated with drought tolerance and resistance
Studies of specific genes associated with dehydration responses have been conducted in a
number of plants, and roles for many of these genes have been correlated with specific
morphological or physiological traits known to be involved in drought resistance. Abscisic
acid (ABA) signaling and stomatal function are correlated with WUE and drought resistance,
so it is not surprising that several genes involved in ABA perception and stomata opening/
closing respond to severe dehydration. Two calcium-dependent protein kinases from Arabi‐
dopsis have been implicated in slow-type anion channel activation [53]. In the double mutants,
ABA and Ca 2+ -induced stomatal closing were impaired, but not completely. These genes may
contribute to a rapid Ca 2+ -reactive response resulting in stomatal closure, as opposed to the
slower Ca 2+ -programmed response which maintains long-term stomatal closure. Similar
studies have also implicated a G protein-coupled receptor, GCR1, in ABA signaling perception
in guard cells and during seed germination [54]. To examine stomatal function in more detail,
Klein et al. [55] used a T-DNA insertion disrupting At MRP5 , an ABC transporter in Arabi‐
dopsis. The mutants had reduced transpiration rates and showed increased water use
efficiency. In a similar study using T-DNA insertion disruption of At MRP4 (another type of
ABC transporter), Klein et al. [56] found the mutant lines to be more drought susceptible. In a