Agriculture Reference
In-Depth Information
2 Threats in the Field: The ROS-Stress Connection
Plants as sessile organisms are subjected to various forms of environmental stresses
such as salinity, UV radiation, drought, heavy metals, temperature variations (heat-
shock, chilling, frost), nutrient deficiency, air pollution, herbicides and pathogen
attacks, from which they cannot escape. They need to be able to develop defense
mechanisms to cope with such unfavorable factors (Nishizawa et al.
2008
). It is
generally accepted that the imposition of the above mentioned stresses leads to ex-
cess concentrations of reactive oxygen species (ROS; Nishizawa et al.
2008
). Crop
yield and quality are negatively-affected by all these different types of stresses,
potentially leading to oxidative damage (Bolouri-Moghaddam et al.
2010
). In other
words, oxidative damage is likely the main reason for yield and quality losses under
stress.
2.1 ROS Identity, Origin and Function
In plants, ROS are continuously produced in chloroplasts during the process of
photosynthesis, during the respiration process in mitochondria, in peroxisomes and
at the plasma membrane (NADPH oxidases). ROS include hydroxyl radicals (
•
OH),
superoxide radicals (O
2
•-
), singlet oxygen (
1
O
2
) and hydrogen peroxide (H
2
O
2
).
Among these, the
•
OH is the most reactive (
in vivo
half-life 10
−9
s) and danger-
ous species, immediately attacking virtually any molecule in its neighbourhood. By
contrast, H
2
O
2
is much more stable and it is able to cross membranes. To deal with
ROS, plants develop mechanisms that keep the equilibrium between the production
and the scavenging of ROS in the cell: ROS homeostasis (Gill and Tuteja
2010
).
In general, two antioxidant protective mechanisms are discriminated: enzymatic
and non-enzymatic mechanisms. The former includes the action of superoxide dis-
mutase (SOD), ascorbate (AsA) peroxidase (APX), glutathione (GSH) peroxidase
(GPX), thioredoxin peroxidase (TPX) and catalase (CAT) (Nishizawa et al.
2008
).
Non-enzymatic antioxidants include polyphenols, AsA, GSH, flavonoids, carot-
enoids, α-tocopherol, sugar-sterols and sugar-phenols and soluble carbohydrates,
such as fructans and Raffinose Family Oligosaccharides (RFOs) (Stoyanova et al.
2011
).
The ROS equilibrium determines whether it causes damage or rather acts as a
signal to induce defense responses under abiotic and biotic stresses (Gill and Tu-
teja
2010
). Therefore, spatio-temporal variations in ROS are greatly important to
understand whether ROS (e.g., H
2
O
2
) could act as a signal or not (Gill and Tuteja
2010
). ROS levels likely differ among different plant organelles, being intimately
linked to the specific metabolism and antioxidant mechanisms taking place at these
different locations. Under stress, the delicate balance is disturbed leading to tempo-
ral ROS accumulation (ROS overshoot) in one or more organelles, and finally also
in the whole cell, including the cytosol, nucleus and the vacuole (see below; Gill
and Tuteja
2010
). This might lead to oxidative damage of biomolecules, including
nucleic acids, proteins and lipids (Gill and Tuteja
2010
; Nishizawa et al.
2008
).
