Agriculture Reference
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by the Na
+
/H
+
exchanger in the PM [100] or its capture by tonoplast [109]; changes of metab‐
olism and signalization by polyamines and Reactive Oxygen Species (ROS) and the antioxidant
activity [110, 111].
4.1.1.2. Reactive oxygen species
ROS fluctuations in time and space can be interpreted as signals to regulate growth, develop‐
ment, cell death and stress responses [112, 113]. Understanding the mechanisms that control
ROS signaling in cells in response to water stress and salinity could therefore provide a
powerful strategy for increasing crop tolerance to these environmental stress conditions [114].
Among the targets of ROS action at the cellular level, there are ion channels that mediate ion
exchange in the PM. In the PM of roots and guard cells H
2
O
2
, stimulates the channels activated
by hyperpolarization that mediate the influx of Ca
2+
and NSCC [112, 115, 116] and inhibit the
K+ outward and inward rectifier currents [117]. The stimulation of the influx of Ca
2+
in guard
cells appears to mediate the induction of stomata closure by ABA [116, 118-120]. At the same
time it was reported that the OH• activates a Ca
2+
inward and K+ outward currents in
epidermal protoplasts derived from mature and growth zone of Arabidopsis roots [115]. A
larger stimulation of the inward current of Ca
2+
in the growth zone may indicate that ROS are
involved in growth regulation via Ca
2+
signaling. Moreover, the OH• produced by NADPH
oxidase in Arabidopsis root hairs activated a Ca
2+
inward rectifier conductance causing an
increase in cytosolic Ca
2+
allowing the root elongation [112]. Recently it has been reported that
under severe water stress autophagy programmed cell death occurs in the region of the root
apical meristem [121]. There is evidence that this defense mechanism is promoted by the
accumulation of ROS in stressed meristematic cells of root tips. Analysis of the expression of
BAX inhibitor-1 (AtBI1, apoptotic inhibitor) and the phenotypic response of the mutant
atbi1-1
under severe water stress indicates that AtBI1 and the pathway of endoplasmic reticulum
stress response modulates the induction of PCD by water stress. As a result, thin and short
roots induce an increase in their tolerance to stress. These authors also propose that under
severe drought stress, plants activate the PCD program in the root apical meristem, removing
the apical dominance; so they can remodel the RSA to adapt to stressful environments [122].
A slight drought stress increases the expression of enzymes associated with root morphology
(Xyloglucan endotransglucosylase) while other structural proteins (actin and tubulin) are
downregulated, these proteins are strongly correlated with root growth since its function is
the vesicular carrying in cells with polarized growth (e.g. root hairs) allowing its growth and
hence an augmentation in the surface of water uptake. However, when there is a greater stress,
these structural proteins increase their expression. It is believed that alterations in the expres‐
sion of these proteins are positively correlated with the of LR development that partially has
an indirect effect on whole plant photosynthetic process [123]. While the decrease of lateral
root development is a well-known response to water stress, none of the mutants that are
resistant to drought stress have a reduced number of LR [124]. Only a few transcription factors
have shown to regulate the formation of roots under drought conditions, among them stands
the MYB96 transcription factor since it plays an important role in LR growth under drought
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