Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 36

developed efficient mechanisms for removal of toxic concentrations of ROS. The antioxidant
system is composed of protective enzymes ( e.g ., superoxide dismutase, catalase, peroxidase,
reductase, and redoxin) and radical scavenger metabolites (mainly GSH and ascorbate). GSH
is an essential component of the antioxidant system that donates an electron to unstable
molecules such as ROS to make them less reactive and also can acts as a redox buffer in the
recycling of ascorbic acid from its oxidized form to its reduced form by the enzyme dehy‐
droascorbate reductase [103]. Organized remodeling of metabolic networks is a crucial
response that gives the cells the best chance of surviving the oxidative challenge.
In A. thaliana , oxidative treatment with methyl viologen causes the down-regulation of
photosynthesis-related genes and concomitant cessation of starch and sucrose synthesis
pathways, meanwhile catabolic pathways are activated. These metabolic adjustments avoid
the waste of energy used in non-defensive processes and mobilize carbon reserves towards
actions of emergency relief such as the accumulation of maltose, a protein structure-stabilizer
molecule [104]. A GC-MS metabolomic study, together with an analysis of key metabolic fluxes
of cell cultures and roots of A. thaliana treated with the oxidative stressor menadione, revealed
the similarities and divergences in the metabolic adjustments triggered in both culture systems.
Inhibition of the tricarboxylic acid cycle (TCA) by accumulation of pyruvate and citrate is
accompanied by a decrement of malate, succinate, and fumarate pools. This early (0.5 h)
response was observed in both systems. Inhibition of TCA cycle concomitantly causes a
decrement in the pools of glutamate and aspartate due to the inhibition of the synthesis of
TCA-linked precursors 2-oxoglutarate and oxaloacetate, respectively. Another mutual early
metabolic redistribution is the redirection of the carbon flux from glycolysis to the oxidative
pentose phosphate (OPP) pathway. This is also reflected by the decrement in the glycolytic
pools of glucose-6 phosphate and fructose 6-P, and the increment in the OPP pathway
intermediates ribulose 5-phosphate and ribose 5-phosphate. Increased carbon flux through the
OPP pathway might supply reducing power (via nicotinamide adenine dinucleotide phos‐
phate, NADPH) for antioxidant activity, since oxidative stress decreases the levels of the
reductants GSH, ascorbate, and NADPH. After 2 and 6 h of stress progression, metabolic
adjustments in response to oxidative stress are different in roots than in cell suspension
cultures. In roots, pools of TCA cycle intermediates and amino acids are recovered. In contrast,
in cell cultures, the concentrations of these metabolites remains depressed throughout the time
course, indicating higher basal levels of oxidative stress in cell cultures. At the end of the
treatment time (6 h), 39 metabolites, including GABA, aromatic amino acids (tryptophan,
phenylalanine, and tyrosine), proline, and other amino acids, were significantly altered in
roots. These results showed the broad spectrum of metabolic modifications elicited in response
to oxidative stress and the influence of the biological system analyzed [105].
Redirection of carbon flux from glycolysis through the OPP pathway and subsequent increase
in the levels of NADPH was also reported in rice cell cultures treated with menadione. CE-MS
analysis of these rice cultures showed the depletion of most sugar phosphates resulting from
glycolysis (pyruvate, 3-phosphoglyceric acid, dihydroxyacetone phosphate, fructose-6-
phosphate, glucose-1-phosphate (G1P), G6P, G3P, phosphoenolpyruvate) and TCA-organic
acids (2-oxoglutarate, aconitate, citrate, fumarate, isocitrate, malate, succinate) and increases