Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 143

orientation of auxin transporters, cell shape, and auxin transport parameters predicts a maximum
auxin concentration in the QC and a steep auxin gradient in the proximal meristem, which drops
according to the cell number from the quiescent center [71, 72]. This agrees with the auxin levels
found in protoplasts derived from different apical cell types, as well as with the expression
patterns of auxin responsive genes, such as members of the PLETHORA (PLT) family, in the
different root tissues [73]. PLT 1 and PLT2 are known to be crucial for interpreting this gradi‐
ent in the terms of root growth and development. They encode for AP2-domain transcription
factors, and losing of their function results in the loss of stem cells, arrest of transit-amplifying
divisions and reduction of cell expansion [74]. PLT pathway has other effects over cell cycle
control. Histone acetyltransferase, a chromatin modifier and required to maintain the divid‐
ing ability in meristem cells, is also required to sustain PLT expression and support both transit-
amplifying divisions and the root stem cell status at the root apex [75]. Moreover, the action of
SUMO E3 ligase is vital to repress endoreplication in shoot and root meristems, and in the root,
this SUMO E3 ligase acts in the PLT pathway [76]. It can be said then that the root tip is charac‐
terized by an auxin maximum, and auxin is required to support transit-amplifying divisions [77].
4. Root system development and abiotic stress
Abiotic cues as water and nutrient availability limit plant productivity in almost all ecosys‐
tems in the world. Typically, RS has to growth in media where the biotic and abiotic
components are distributed heterogeneously. Soils are complex, a broad range of chemical
a physical process occurs due to intrinsic soil characteristics and the action of biotic fac‐
tors. Thus, this complexity presents several challenges to survive. As soon as the root makes
contact with the soil must sense and integrate biotic and abiotic cues in order to adjust their
genetic program of post-embryonic root development (PERD). This capacity to change their
PERD allows them change their architecture to find the supplies of water and nutrients that
could be limited and localized [3, 4, 12]. Environmental cues such as water, salinity and
nutrient can modulate the ARS.
4.1. Regulation of root system architecture by water availability and salinity stress
Water and salinity can indirectly modulate the RSA because they can produce unfavorable
changes in the nutritional composition of the soil, the distribution of said nutrients, the density
and compaction of soil, and the type of soil particles [9]. Those interactions complicate the
dissection of specific transduction pathways involved in root growth and development [78]
The RS is the first to perceive the stress signals for drought and salinity, therefore its devel‐
opment is deeply affected by their availability in soil. In many agriculturally important species,
the whole plant growth is inhibited during water starvation, however, RS is more resistant
than shoots and continues growing under low water potentials that are completely inhibitors
for shoot growth [79]. Notably, while growth of PR is not appreciably affected by water deficit,
the number of LRs and its growth are significantly reduced [80]. It has been suggested that the
reduction of the LR formation may be caused by the suppression of the activation of the lateral
root meristems, not because of the reduction of the initiation in the LR per se, as primordia