Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 145

in lateral root mutant 2 ( lrd2 ) nor in two ABA deficient [80, 82]. Abscisic acid and a recently
identified gen LRD2 are linked to repression of LR formation in response to osmotic stress. It
is very interesting to note that these regulators are also related to the establishment of RSA
without apparent effect of osmotic stress. The mutant lrd2 presents an altered response to
exogenous application of ABA, while ABA-deficient mutants and lrd2 show an altered
response to inhibitors of polar auxin transport [95-97] suggesting a joint interaction of the
hormonal signaling pathway in the regulation of LR formation. Some authors propose a model
where the promotion or suppression of hormonal signaling pathway and regulators as LRD2
determine the type of LR primordium (LRP) and coordinate the RAS in response to environ‐
mental stimuli [87]. In contrast, under drought stress conditions or osmotic stress, activation
of the LR meristem is suppressed by ABA-mediated signals, producing few small LRs [80,
98]. While auxins seem to be the main initialization hormone, pattern and emergence of LRs;
ABA is the main hormone that controls the environmental effect (like drought and salt stress)
over the RSA [99].
4.1.1. Cellular responses
4.1.1.1. Epidermis
Root epidermis is the first tissue that makes contact with salt; hence, it is the first to perceive
osmotic and ionic changes in cells and the first one that triggers rescue mechanisms. The
accumulation of sodium in the cells and the resulting ionic imbalance is the main cause of
inhibition of plant growth and yield decrease [100]. Therefore, maintaining low intracellular
sodium levels is critical for plant adaptation to water and salinity stress. Plants use different
strategies to fight against salinity damage in every organizational level, from cellular, bio‐
chemical, molecular to anatomic, morphological and phenological level. At cellular and
molecular level, plants cells keep a low cytosolic sodium (Na + ) content by means of compart‐
mentalization and ionic transport regulation [100, 101]. During salinity stress, processes of
membrane transport play a very special role. Some transport mechanisms implied in the
perception of salt stress are: water output of the cell by osmotic gradient, the decrease of the
availability of potassium (K + ) in roots due to the reduced activity of this cation in soil solution,
where sodium competes for binding sites for K + transporters in PM (plasma membrane)
including low and high affinity, also the increased efflux of K + by selective and non-selective
channels [102] and finally that these ionic events initially evoked in the PM of epidermal root
cells are propagated to intracellular organelles (mainly vacuoles) and other plant tissues such
as leaves. Considering the entry of Na + and K + loss, preventing worsening of the K + /Na +
cytosolic relation is a key criterion for resistance to salt stress. Once the stress is perceived, the
respective signalization triggers and changes in metabolism and genetic expression take place;
all these are related with defense mechanisms [102, 103]. For the response to osmotic changes
in metabolic compartments, it occurs an immediate osmotic adjustment by synthesizing
compatible osmolytes and inorganic ions capture [104], for the toxic component of stress is
performed a compartmentalization of harmful ions and ion transport [105]; and it generally
occurs a restriction of unidirectional Na + entry via non-selective cation channels (NSCC) [105,
106] and high affinity potassium transporters (HKT) [107, 108], the Na + efflux from the cytosol