Programmed Cell Death as a Response to High Light, UV and Drought Stress in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 227

tron transport - DCMU decreases the ROS production and retards PCD. These results prove
that mitochondria and ROS act as mediators in the UV-C-induced cell death [173] and that
AsA can be considered as an important antioxidant during this process [174], which is con‐
sistent with what has been reported in various types of PCD. It has been also shown that
Arabidopsis proteins AtDAD1 and AtDAD2 (defender against apoptotic death), localized in
the endoplasmic reticulum membrane can suppress DNA fragmentation, indicating an in‐
volvement of the ER in UV-C-triggered PCD pathway (25). The microarray approach has
identified numerous genes responsible for ROS scavenging, signaling, transcription regula‐
tion and involved in DNA replication or conformation changes that have been deregulated
after exposure to UV-C radiation [175]. Metacaspase-8 (AtMC8) has been proven to be
strongly up-regulated by UV-C. Overexpression of AtMC8 in Arabidopsis has resulted in
more severe cell death, while knocking-out AtMC8 has reduced the UV-C-triggered PCD,
which suggests that metacaspase-8 is a part of PCD pathway activated by UV radiation
[176]. The activation of PCD program upon UV helps plants in eliminating damaged cells to
control cell quality and quantity after the trauma.
6. Drought stress - soil water deficit
Drought is one of the most unfavorable environmental factors that affects growth and devel‐
opment of plants and consequently limits plant productivity. Plants have developed specific
acclimation and adaptation mechanisms to survive the soil water deficit. In response to
drought, plants can exhibit either escape (ability to complete the life cycle before severe
stress) or resistance mechanisms. Resistance mechanisms include drought avoidance and
drought tolerance. The latter depends on the cell turgor maintenance by accumulating os‐
molytes and soluble sugars [177]. There are several examples of molecules that help to main‐
tain an osmotic balance under dehydration conditions: sugars, polyols and proline [178].
Proline is accumulated in the cytoplasm and chloroplast stroma while other solutes (sugars,
organic acids, potassium) are cumulated in the vacuole. When the cellular water content de‐
creases, they stabilize cellular structures through hydrophilic interactions and hydrogen
bonding [179]. A similar role is fulfilled by late embryogenesis abundant (LEA) proteins - a
family of unstructured proteins. LEA proteins accumulate in response to dehydration and
ABA treatment. Because of their high hydrophilicity and solubility in water, it has been pro‐
posed that they play a role in protecting cytoplasmic structures during dehydration [180].
The avoidance mechanism is possible by the maintenance of high water potential in plant
tissue despite soil water deficit. It can be achieved by: improved water uptake under stress,
the ability to hold water as well as by the reduction of its loss through smaller leaf area and
lesser stomatal and cuticular conductance. One of the first acclimation responses to drought
is the decrease in leaf growth, which helps to maintain the cell turgor and reduces the tran‐
spiration area. In Arabidopsis , the size of leaf is regulated by both cell division and cell ex‐
pansion. Under drought stress, Arabidopsis leaves have been demonstrated to compensate
for the low rate of expansion by the extension of expansion duration [181]. The cell expan‐
sion is a process of cell wall loosening performed by enzymatic and non-enzymatic compo‐