Abiotic Stress in Plants and Metabolic Responses - Abiotic Stress: Plant Responses and Applications in Agriculture - page 27

activation of regulatory proteins ( e.g. , transcription factors, protein phosphatases, and kinases)
and signaling molecules are essential in the concomitant regulation of signal transduction and
stress-responsive gene expression [12, 13]. Early plant response mechanisms prevent or
alleviate cellular damage caused by the stress and re-establish homeostatic conditions and
allow continuation of growth [14]. Equilibrium recovery of the energetic, osmotic, and redox
imbalances imposed by the stressor are the first targets of plant immediate responses.
Observed tolerance responses towards abiotic stress in plants are generally composed of stress-
specific response mechanisms and also more general adaptive responses that confer strategic
advantages in adverse conditions. General response mechanisms related to central pathways
are involved in energy maintenance and include calcium signal cascades [15, 16], reactive
oxygen species scavenging/signaling elements [17, 18], and energy deprivation (energy sensor
protein kinase, SnRK1) signaling [19]. Induction of these central pathways is observed during
plant acclimation towards different types of stress. For example, protein kinase SnRK1is a
central metabolic regulator of the expression of genes related to energy-depleting conditions,
but this kinase also becomes active when plants face different types of abiotic stress such as
drought, salt, flooding, or nutrient depravation [20-24]. SnRK1 kinases modify the expression
of over 1000 stress-responsive genes allowing the re-establishment of homeostasis by repres‐
sing energy consuming processes, thus promoting stress tolerance[24, 25]. The optimization
of cellular energy resources during stress is essential for plant acclimation; energetically
expensive processes are partially arrested, such as reproductive activities, translation, and
some biosynthetic pathways. For example, nitrogen and carbon assimilation are impaired in
maize during salt stress and potassium-deficiency stress; the synthesis of free amino acids,
chlorophyll, and protein are also affected [26-28]. Once energy-expensive processes are
curtailed, energy resources can be redirected to activate protective mechanisms. This is
exemplified by the decrease in de novo protein synthesis in Brassica napus seedlings , Glycine
max, Lotus japonicas , and Medicago truncatula during heat stress accompanied by an increased
translation of heat shock proteins [29, 30].
4. Metabolic adjustments during stressing conditions: Osmolyte
accumulation
A common defensive mechanism activated in plants exposed to stressing conditions is the
production and accumulation of compatible solutes. The chemical nature of these small
molecular weight organic osmoprotectants is diverse; these molecules include amino acids
(asparagine, proline, serine), amines (polyamines and glycinebetaine), and γ -amino-N-butyric
acid (GABA). Furthermore, carbohydrates, including fructose, sucrose, trehalose, raffinose,
and polyols (myo-inositol, D-pinitol) [12, 31], as well as pools of anti-oxidants such as gluta‐
thione (GSH) and ascorbate [32, 33], accumulate in response to osmotic stress. Common
characteristics of these diverse solutes are a high level of solubility in the cellular milieu and
lack of inhibition of enzyme activities even at high concentrations. Accumulation of compatible
solutes in response to stress is not only observed in plants, it is a defense mechanism triggered
in animal cells, bacteria, and marine algae, indicative of an evolutionarily conserved trait [34,