Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 149

nutrient-induced plasticity of PERDP. This stimulation could be dependent on NRT1.1 (Nitrate
Transporter 1). This is partially due to the fact that NRT1.1 represses LRP emergence and growth
of young LRs in the absence of nitrate. NRT1.1 transports nitrate and facilitates auxin trans‐
port in a concentration-dependent manner. NRT1.1 represses LR growth at low nitrate availabil‐
ity by promoting basipetal auxin transport out of the LRP, towards the parental root [149]. MADS-
box transcription factor NITRATE REGULATED (ANR1) and Auxin signaling F-box protein 3
(AFB3) are key regulators of RSA in response to nitrate availability. The Chlorate-resistant 1 mutant
( chl1) is ANR1 affected, and is less responsive to the localized NO 3− -rich patches similarly to
transgenic plants in which ANR1 expression is down-regulated. In the tips of LR and LRP, ANR1
is expressed and is localize with NRT1.1 [150]. The afb3-1 mutant shows altered root develop‐
ment response to nitrate. AFB3 is an auxin receptor gene induced by nitrate in the primary root
tip and pericycle; its mRNA is the target of miR393 that is induced by the products of NO 3−
assimilation.
4.2.3. Potasium and iron
Contrasting with physiological and molecular responses to low K and Fe, changes in RSA have
been scarcely described. Potassium deficiencies arrest LR and PR development in Arabidopsis
(Figure 2) [129]. K + transporters play a crucial role in SRA changes in response to K + availability.
Disruption of the root-specific K + -channel AKT1 in the akt1-1 Arabidopsis mutant causes
reduced ability of plants to grow in low potassium media (100 μM)[151]. In Arabidopsis,
changes in the gravitropic behavior of RS were also observed in low potassium media. The
genes of the KUP/HAK/KT family are homologous to bacterial KUP (TrkD) potassium porters.
The trh1 (tiny root-hair 1) mutant, which is disrupted in AtKUP4/TRH1 gene shows agravi‐
tropic behavior in its roots independently of K + concentration in the media when grown on
vertical agar plates, and also, ProTRH1:GUS expression is limited to the root cap where gravity
is sensed. Interestingly, agravitropic responses in trh1 are complemented by exogenous auxin.
This mutation is associated with the loss of auxin pattern in the root apex. Thus, TRH1 is an
important part of auxin transport system in Arabidopsis roots [151-153].
Typically, the root architectural changes in response to low availability of Fe include ectopic
formation of root hair due to modulation in their position and abundance [154]. Recently, Giehl
et al. (2012) analyzed the changes in LR architecture in response to localized Fe supply in wild-
type and Fe acquisition and translocation- defective mutant plants. They found that lateral
root elongation is highly responsive to local Fe and that the symplastic Fe pool in LR favors
local auxin accumulation. They identified the auxin transporter AUX1 as a major Fe-sensitive
component in the auxin signaling pathway that mainly directs the rootward auxin stream into
LRs that have access to Fe.
4.3. Meristematic activity regulation by abiotic stress
To cope with environmental changes, plants have to adapt their growth timing and pattern by
altering rates of cell proliferation and differentiation. The expression of several cell cycle genes
is increased or decreased upon external cues (Figure 3) [155] but it is poorly understood the
full molecular basis supporting these transcriptional controls, and if the cell cycle control
modifications happen to fall into the post-translational category, the current knowledge is also