Agriculture Reference
In-Depth Information
generation is unaffected [9, 80-82]. Mutants with alterations in the development of LRs respond
differently to drought stress [80, 83]. Suppression of the growth of LR by drought has been
widely accepted as an adaptive response to ensure the plant survival under unfavorable
growing conditions [83]. Another factor that plays an important role in growing and devel‐
opment of plants to tolerate the drought stress is the hydrotropism [84, 85]. A recent study
showed that a gradient of moisture generated by water stress causes an immediate degradation
of amyloplasts in the columella cells of plant roots, producing a minor response to gravity and
an increase of hydrotropism [86]. However, it is unknown how the gravity signals interact
with other environmental signals to modulate the direction of root growth. Less known are
the adaptations in root morphology and its relevance to salinity tolerance. Many halophytes
have developed morphological adaptations, like the formation of specialized organs to expel
salt out of their leaves, which allows them to keep the water and take out the salt in an active
manner. Glycophytes have not developed permanent changes on its morphology to deal with
salt, but they can adjust the root growth and its architecture in response to salinity, like in the
case of Arabidopsis [87]. Also it has been observed that Arabidopsis RS exhibit a reduced
gravitropism under salt stress, growing against the gravity vector [88]. Arabidopsis RS
exposed to a simultaneous salinity and gravity stimuli responded to salinity with a change in
growing direction in a way that apparently represents an adaptive arrangement between
gravitropic and saline simulation. Control of the relation between gravitropism and hydro‐
tropism allows plants to direct the root growing for a better water uptake, giving an advantage
during development of the radical system under stress conditions. It is known that the salt
stress inhibits the growth of the PRs in Arabidopsis seedlings, although it has been reported
that salt stress also modulates root gravitropism of PR in young seedlings. In vertical position,
five day seedlings germinate normally in MS medium (Murashige and Skoog) containing
different concentrations of sodium chloride (NaCl), however the direction of root growth
changes according to the increase of NaCl concentrations, and the root curves in stressed plants
with 150 mM NaCl in the medium [88].These results suggest that the salt stress and the
induction of signal translations by stress modulate the direction of the root, despite of the
gravity. Some reports suggest that the gravitropic signal and the answers in root apex are
controlled, at least partially by Salt Overly Sensitive (SOS) signaling pathway. Therefore, this
pathway might interact with the gravity sensor system in the cells of the columella to direct
root growth in a coordinated way[88]. Abscisic acid (ABA) and auxins participate in a complex
signal system that plays a very important role in the development of the RSA under drought
conditions. These hormonal effects (levels) even though are considered as intrinsic [82] can
change in response to environmental cues. Cytokinins, gibberellins and abscisic acid are
produced in roots to be transported to other tissues, where they play their roles in development
and growth. Although auxins are the major determinants of root growth [89], cytokinin and
especially abscisic acid [90-92] have been proposed as potential chemical signals in response
to water stress to modulate RSA. The decrease in water potential of roots caused by salinity is
the factor that triggers the production of ABA in different species [93]. A condition of mild
osmotic stress also inhibits the LR formation in a dependent way of ABA [80, 82, 83, 94]. In
Arabidopsis, the reduced water availability dramatically inhibits the formation of LR, but not
by the suppressing of initiation of LR at the lateral primordia. This inhibition does not occur
Search WWH ::
Custom Search