Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 34

were initially responsive [94]. In tobacco plants treated with various doses of salt, 1 day of
treatment with 50 mM NaCl induced accumulation of sucrose, and to a lesser extent glucose
and fructose, through gluconeogenesis. Further stress (500 mM NaCl for another day) led to
elevation of proline and even higher elevation in sucrose levels compared to the lower dose;
at the same time, glucose and fructose levels decreased as transamination-related metabolites
(asparagine, glutamine, and GABA) did. These data suggest that sugar and proline biosyn‐
thesis pathways are metabolic mechanisms for control of salt stress over one- to two-day
periods (short-term). Proline continues to be observed at high levels at later stages (3 to 7 days
under highly stressing concentrations of 500 mM NaCl) and sucrose decreases (although it
remains at high levels compared to control). There are also significant elevations in levels of
asparagine, valine, isoleucine, tryptophan, myo-inositol, uracil, and allantoin, and reductions
in glucose, fructose, glutamine, GABA, malate, fumarate, choline, uridine, hypoxantine,
nicotine, N-methylnicotinamide, and formate [95]. Similarly, in maize plants stressed with salt
solutions ranging in concentration from 50 to 150 mM NaCl, the metabolic profile of the shoot
extracts changes most dramatically compared to controls in the plants exposed to the highest
salt concentration [96].
Another complexity in the metabolic perturbations in salt-stressed plants consists of tissue-
specific response differences. In maize plants exposed to 50-150 mM NaCl saline solution,
levels of sucrose and alanine were increased and levels of glucose decreased in roots and
shoots. Other osmoprotectants exhibited differentiated behavior: GABA, malic acid, and
succinate levels increased in roots, while glutamate, asparagine and glycine betaine were at
higher concentrations in shoots. There were decreased levels of acetoacetate in roots and of
malic acid and trans -aconitic acid in shoots. A progressive metabolic response was more
evident in shoots than in roots [96].
In comparative ionomics and metabolite profiling of related Lotus species ( Lotus corniculatus ,
L. tenuis , and L. creticus ) under salt stress, the extremophile L . creticus (adapted to highly saline
coastal regions) exhibits better survival after long-term exposure to salinity and is more
efficient at excluding Cl - from shoot tissue than the two cultivated glycophytes L. cornicula‐
tus and L. tenuis (grassland forage species). Sodium ion levels are higher in the extremophile
than the cultivars under both control conditions and salt stress. In L. creticus , a differential
homeostasis of Cl - , Na + , and K + is accompanied by distinct nutritional changes compared to
the glycophytes L. corniculatus and L. tenuis . Magnesium and iron levels increase in L. creti‐
cus after salt treatment, but levels of potassium, manganese, zinc, and calcium do not. In non-
stressed control plants, 41 metabolites are found at lower levels in L. creticus than in the two
glycophytes, and 10 metabolites are at higher levels in L. creticus . These data demonstrate that
each of these species has a distinct basal metabolic profile and that these profiles do not show
a concordance with salt stress or salt tolerance. In salt stress conditions, 48 metabolites show
similar changes in all species, either increasing or decreasing, with increased levels the amino
acids proline, serine, threonine, glycine, and phenylalanine; the sugars sucrose and fructose,
myo-inositol and other unidentified metabolites; and with decreased levels of organic acids
such as citric, succinic, fumaric, erythronic, glycolic, and aconitic acid, including ethanolamine
and putrescine, among others. Of note is that more than half of the metabolites affected by salt