Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 33

sis leaves. More interesting is the recent report of a new compound, annotated as
EITTMS_N12C_ATHR_2988.6_1135EC44, with no previously mass spectra matching record,
accumulated specifically in response to drought stress in this moss [90].
7. Plant metabolomics and salinity stress
A current problem for crop plants worldwide, which will become more critical in the fu‐
ture, is salt stress imposed by salinity in soils due to poor practices in irrigation and
over-fertilization, among other causes. Salt stress induces abscisic acid synthesis; abscisic
acid transported to guard cells closes stomata, resulting in decreased photosynthesis, pho‐
to-inhibition, and oxidative stress. This causes an immediate inhibition of cell expansion,
visible as general plant growth inhibition, accelerated development, and senescence [91].
To cope with salt stress plants implement strategies that include lowering of rates of pho‐
tosynthesis, stomatal conductance, and transpiration [92]. Sodium ion, by its similar
chemical nature to potassium ion, competes with and inhibits the potassium uptake by
the root. Potassium deficiency results in growth inhibition because this ion is involved in
the capacitance of a plethora of enzyme activities in addition to its participation in main‐
taining membrane potential and cell turgor [91].
The metabolic perturbation in plants exposed to salinity involves a broad spectrum of
metabolic pathways and both primary and secondary metabolism. For example, in a proteomic
study in foxtail millet (cv. Prasad), 29 proteins were significantly up- or down-regulated due
to NaCl stress, with great impact on primary metabolism. These proteins were classified into
nine functional categories: cell wall biogenesis (lignin biosynthesis), among these were caffeic
acid 3-O-methyltransferase and caffeoyl CoA 3-O-methyltransferase; photosynthesis and
energy metabolism, which included proteins like cytochrome P450 71D9, phytochrome 1,
photosystem I reaction center subunit IV B, and ATP synthase F1 sector subunit beta, among
others; nitrogen metabolism, proteins like glutamine synthetase root isozyme 4, ferredoxin-
dependent glutamate synthase, chloroplast precursor (Fd-GOGAT), and urease; carbohydrate
metabolism, proteins such as UDP-glucose 4-epimerase GEPI42 (galactowaldenase) and beta-
amylase; and lipid metabolism including isovaleryl-CoA dehydrogenase 2 and aldehyde
dehydrogenase [93].
Studies using metabolomic tools in plant models and plant crops have shown that the
physiology in salt stress courses through a complex metabolic response including different
systematic mechanisms, time-course changes, and salt-dose dependence. The biochemical
changes involve metabolic pathways that fulfill crucial functions in the plant adaptation to salt
stressing conditions. Time-course metabolite profiling in cell cultures of A. thaliana exposed to
salt stress demonstrates that glycerol and inositol are abundant 24 h after salt stress exposure,
whereas lactate and sucrose accumulate 48 h later. The methylation cycle, the phenylpropanoid
pathway, and glycine betaine biosynthesis exhibit induction as a short-term response to
salinity stress, whereas glycolysis and sucrose metabolism and reduction in methylation are
long-term responses. Long-term salt exposure also causes a reduction in the metabolites that