Chemistry Reference
In-Depth Information
treating each single stimulus in accordance with the plant status or the stress severity. That
is to say, a simple stimulus may diversely deliver complicated information to the plants,
that's exactly why it is very likely for plants to have multiple cellular sensors to perceive
each stress signal or one attribute of that signal. Secondly, the redundancy of signal
perception make it even harder to identify and confirm each sensor relating to each stress
stimulus, since knocking out one receptor may not significantly affect stress signaling
outputs. Thirdly, even if we find a putative sensor, how to prove our hypothesis could also
become a headache. Because different sensors probably vary in molecular identities, signal-
perceiving modes, outputs, and also subcellular localizations, no wonder not much is
known about plant abiotic stress sensors.
2.1.2. Putative sensors for perceiving stress signal
First of all, how can an external signal turn out to be internal? Where are the
receptors/sensors and transporters? These will be the first bunch of questions we are going
to ask. Imaging if we are plant cells, what will be the first weapon we use to maintain inner
homeostasis when suffering from the outside disturbances? The answer will probably be
"plasma membrane". So far, many researchers have demonstrated that the plasma
membrane (PM) is responsible for perceiving and transmitting external stress signals, as
well as responding to them. For example, when plants are under salinity stress, salt reaches
the PM first, which makes the membrane lipids and transport proteins start to regulate
permeability of this membrane triggering primary responses (Cooke and Burden, 1991). In
many plants, changes in PM lipids, such as sterols and fatty acids, have been observed
responding to salt stress and may contribute to the control of membrane fluidity and
permeability, as a primary stress-responsive reaction (Elkahoui et al., 2004). Therefore, it is
suggested that physical properties of membranes (lipid composition, fatty acid composition)
may lead us to find potential sensors perceiving stress signals.
Secondly, let's stress a little bit more on the most common stress signals, cold, drought, and
salinity. All of these three stresses have been detected to induce transient Ca
2+
influx into the
cell cytoplasm (Sanders D et al., 1999; Knight, 2000). Thus we can hypothesis channels
responsible for this Ca
2+
influx possibly acting as a sensor for these stress signals. Based on
what we have discussed above, signaling reception may involve changes in membrane
fluidity and cytoskeleton reorganization, which are also confirmed in early cold signaling
(Sangwan et al., 2001; Wang and Nick, 2001). Coincidentally, cold-induced Ca
2+
influx in
plants occurs only after the occurrence of a rapid temperature drop (Plieth et al., 1999).
Taken together, physical alterations in cellular structures may activate certain Ca
2+
channels
under cold stress, which indirectly suggests that Ca
2+
channels might be a putative sensor.
Except the ion channel as a whole, other types of functional proteins can hardly be ignored
on the list of sensors. So far, studies on plants and other systems have also identified several
kinds of sensors. And in order to find sensors effectively in plant abiotic pathways, we need
to borrow the experience and results of researches on other species. It is known that for
plants, cold, drought, and salt stresses will all induce the accumulation of compatible
osmolytes and antioxidants (Hasegawa et al., 2000). In yeast and in animals, mitogen-
