Chemistry Reference
In-Depth Information
With the proceeding of the pathway, we can see that abiotic stresses also give birth to
second signaling molecules (discussed below). Therefore, in the next part, we are going to
pay attention to the second messengers and their performance in signal transduction
pathways.
2.2. Second messengers
Several second messengers are active participators in stress signal transduction. Mainly they
are groups of small intracellular signaling molecules or ions, normally locating in the
cytoplasm of a cell and responding to a signal received by a cell-surface sensor, which
activates various kinases to regulate other enzymes' activities. What we mention frequently
as second messengers are reactive oxygen species, lipid phosphates-derived signals, and
cyclic nucleotides-related signals. Besides, some plant hormones also work as secondary
signal molecules under stress conditions.
2.2.1. Reactive oxygen species (ROS)
ROS are species of oxygen which are in a more reactive state than molecular oxygen,
resulting from excitation or incomplete reduction of molecular oxygen. Generally, ROS
contains both free radical (O
2
•−, RO•, HO
2
•, OH•), and non-radical forms (H
2
O
2
,
1
O
2
). For
plants, they tend to be a two-edged weapon. On one hand, they are highly reactive and
toxic, always taken as unwelcome harmful by-products of normal cellular metabolism, and
causes damage to proteins, lipids, carbohydrates, DNA which ultimately results in cell
death in plants. On the other hand, it has also been proved that ROS can affect genes'
expression and signal transduction pathways, which mean that cells may use it as biological
stimuli and signals to activate and regulate various genetic stress-response processes (Foyer
and Noctor 2009).
Since it means a lot to plants' life, where and how it can be produced? In photosynthetic
tissues, the chloroplast is the prime source of ROS. But for the non-photosynthetic tissues,
mitochondria are the leader in production. In chloroplasts, photosystem I and II (PSI and
PSII) are the major sites for the production of
1
O
2
and O
2
•−. In mitochondria, complex I,
ubiquinone and complex III of electron transport chain (ETC) are the major sites for the
generation of O
2
•−. In addition to the mitochondria and NADPH oxidases, additional
cellular sources of ROS production include a host of other intracellular enzymes such as
xanthine oxidase, cyclo-oxygenases, cytochrome p450 enzymes, and lip-oxygenases for
which oxidants act as part of their normal enzymatic function.
Consequently, when plant cells are under stresses, the rate of ROS production usually goes
up, inspiring the activities of antioxidants and scavenging enzymes to keep plants live a
healthy life. Fortunately, plant cells possess very efficient enzymatic (superoxide dismutase,
SOD; catalase, CAT; ascorbate peroxidase, APX; glutathione reductase, GR;
monodehydroascorbate reductase, MDHAR; dehydroascorbate reductase, DHAR;
glutathione peroxidase, GPX; guaicol peroxidase, GOPX and glutathione-S- transferase,
GST) and non-enzymatic (ascorbic acid, ASH; praline; glutathione, GSH; phenolic
