Programmed Cell Death as a Response to High Light, UV and Drought Stress in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 222

length between 400 and 550 nm (light-harvesting role), assembly and stabilization of light
harvesting complex proteins (structural role), and protection of the photosynthetic appara‐
tus from free radicals (antioxidant role) [66].
Apart from the antioxidant function, flavonoids are responsible for fl owers, fruits and seeds
pigmentation, protection against UV, defense against pathogens and signal transduction
during stress. Mutant plants, def fi cient in chalcone synthase and chalcone isomerase that are
unable to accumulate fl avonoids have been demonstrated to be more sensitive to UV light
[125]. Many genes encoding fl avonoid biosynthesis components are induced under stress
conditions. Considerable increase in fl avonoid level has been demonstrated in response to
abiotic stresses such as wounding, drought and nutrient deprivation [126].
Under steady state conditions, ROS are eliminated by antioxidative mechanisms described
above (78). Different abiotic and biotic stresses such as drought, high salinity, heavy metals,
high light, UV radiation, high/low temperature or pathogen attack may disturb the balance
between the ROS production and scavenging. The equilibrium between ROS production
and scavenging influences their mode of action as protective, signaling or damaging factors.
The increase in cellular ROS level can cause significant damage to cell structures, cell death
and in consequence loss in crop production [127]. The vast role of ROS in the response to
environmental conditions and cell-death signaling are well documented [65,128]. There are
results suggesting that H 2 O 2 antagonizes the 1 O 2 -mediated signaling and that the cross-talk
between signaling pathways, transferred by different ROS, may contribute to the overall re‐
sponse of plant exposed to adverse environmental conditions [129]. Moreover, ROS interact
with several other signaling pathways including NO and hormones like SA, JA and ET.
Such interactions and the ROS/hormonal balance determine whether the cell will stay alive
or enter the PCD pathway [14,39,60]. Finally, the role of ROS as messenger molecules cannot
be underestimated, since it has been demonstrated that they trigger the transduction of
stress signals and systemic acclimation to adverse environmental conditions [130,131].
4. High and excess light stress
Light is an essential factor in the regulation of plant growth, development and stress re‐
sponses but it is also responsible for the production of reactive oxygen species leading to
PCD. The cell death phenotype of many lesion mimic mutants of Arabidopsis thaliana and Zea
mays is dependent on light [132-134]. Plant cells have been equipped with sophisticated
light-perception mechanisms and signaling pathways that are very important for the plant
defense. Three families of photoreceptors collecting different light qualities exist in plant
cells: phytochromes (PHY), cryptochromes (CRY) and phototropins (PHOT). They localize
in the plasma membrane, cytoplasm or nucleus. While photoreceptors play mainly a regula‐
tory role, providing information about diurnal and seasonal light-quality changes, the light-
quantity sensing system is located in chloroplasts. The absorption of photons by
photosynthetic apparatus is possible owing to chlorophylls located in light-harvesting com‐
plexes (LHCs) of photosystem II (PSII) and photosystem I (PSI) in the thylakoid membrane