Abiotic Stress Responses in Plants: Unraveling the Complexity of Genes and Networks to Survive - Abiotic Stress: Plant Responses and Applications in Agriculture - page 51

or chilling / freezing) do not involve any chemical ligand. The same applies to radiation stress,
although in this case an analogy between “ligand - receptor” and “photon - receptor” could
be made. Even when molecules are involved, the universal character of the ligand - receptor
model is debatable. In fact, in what concerns the rooting system, it is unclear if cells can sense
the water concentration in the soil [16]. In contrast, experimental evidences point to the
possibility of sensing cell water homeostasis. The isolation of a transmembrane hybride-type
histidine kinase from Arabidopsis thaliana provides experimental evidence for osmosensors in
higher plants [17]. Also sugars generated by photosynthesis and carbon metabolism in source
and sink tissues play an important role in sensing and signaling, modulating growth, devel‐
opment, and stress responses [18].
Following sensing, one or more signaling and signaling transduction cascades are activated,
preparing restitution counter reactions which will lead to the phase of resistance to stress.
Meanwhile, functional declines are generally observed, including the photosynthetic per‐
formance, transport or accumulation of metabolites and/or uptake and translocation of ions,
as described later in section 2.3. If these declines are not counteracted, acute damage and death
may occur. The importance of restitution counter reactions is highlighted in experiments where
different rates of stress imposition are compared: a more pronounced decline of physiological
functions (photosynthesis, photosynthetic capacity and electron transport rate) was observed
when higher plants were rapidly dehydrated than when the rate of water loss was slower [19].
In desiccation resistant bryophytes there is a threshold of water loss rate behind which no
physiological restoration is observed [20]. Increased damage with more rapidly imposed stress
is due, at least in part, to increased production of active oxygen species (AOS) [21]. Significant
differences in the physiological behavior between the phase of alarm and the phase of
resistance were highlighted by Marques da Silva and Arrabaça in [22], who found in the C4
grass Setaria sphacelata a decrease on the activity of the enzyme phosphoenolpyruvate carbox‐
ylase after several days of water stress, in sharp contrast with the several-fold increase of its
activity observed after a short period of acute stress.
2.2. Common and distinctive features of salinity, cold and drought stress
Salinity, cold and drought stress are all osmotic stresses: they cause a primary loss of cell
water, and, therefore, a decrease of cell osmotic potential. However, the elicitor of cell
water loss differs between stresses: i) salinity stress decreases cell water content due to
the decrease of external water potential, caused by the increased ion concentration (main‐
ly Na + and Cl - ), turning more difficult water uptake by roots and water translocation to
metabolically active cells; ii) cold stress decreases cell water content due to the so-called
physiological drought, i.e., the inability to transport the water available at the soil to the
living cells, mainly the ones of the leaf mesophyll; iii) the decrease of the cell water con‐
tent under drought stress is due to water shortage in soil or/and in the atmosphere. Any‐
way, dehydration triggers the biosynthesis of the phytohormone abscisic acid (ABA) and
it has been known for a long time that a significant set of genes, induced by drought,
salt, and cold stresses, are also activated by ABA [23].