Chemistry Reference
In-Depth Information
kinases). In the future, we still have lots of work to elucidate how ROS regulate MAPK
signaling in abiotic stress field.
All in all, even if we are holding a lot of evidence about the functions of ROS in abiotic
stress, we still have to face up to those obscure steps relating to different mechanisms, not to
mention hundreds of stress responsive genes involved in.
2.2.2. Lipid-derived signal messengers
It is well-known that cellular membranes contains a wide range of different lipids, including
sphingo-, neutral-, glyco-, and phospholipids, all with unique biophysical properties.
Beyond the structural role, some of them are equipped with direct signal-transducing
properties. What we discussed in the sensors part is that membrane lipids can directly
response to abiotic stress stimuli by modulating membrane fluidity or its other
physiochemical properties, but in this part we will take another angle to demonstrate its
significant function in the process of generating intracellular signaling molecules. Moreover
lipids and their biogenesis and degradation enzymes play many direct or indirect roles to
regulate or affect signaling and stress tolerance. In signal transduction, signaling lipids are
distinguished for their low abundance and rapid turnover. They are rapidly formed
responding to diverse stimuli through lipid kinases or phospholipases' activation. Thanks to
the lipid-binding domains, these lipid signals can activate enzymes or recruit proteins to
membranes leading to the activation of downstream signaling pathways resulting in specific
cellular events and physiological responses. Studies on them find out, for plants, lipid
signaling form a complex regulatory network responding to abiotic stress.
Basically, in eukaryotes, typical signaling lipids includes phosphatidylinositol lipids
(polyphosphoinositides; PPIs), certain lyso-phospholipids, diacylglycerol (DAG), and
phosphatidic acid (PA) (Munnik and Testerink, 2009; Xue et al.,2009; Munnik and Vermeer,
2010). Among them, PA is of great importance as a lipid second messenger in plants
involved in various biotic and abiotic stress conditions. Based on thousands of researches, it
is easy to detect almost every environmental cue can trigger a rapid PA response (Testerink
and Munnik, 2005; Arisz et al., 2009; Li et al., 2009; Mishkind et al., 2009). How can PA be
produced? Two ways in brief. Directly PA is generated through activation of phospholipase
D (PLD), and indirectly a phospholipase C/diacylglycerol kinase (PLC/DGK) pathway
regulated by two types of PLC enzyme named as the PI-PLCs (phosphoinositide-PLCs) and
NPCs (non-specific PLCs). After the rapidly bounce up under stress, PA level will go back to
normal when stimuli disappear. (Christa Testerink et al. 2011).
In most of the osmotic stress cases, both PLC/DGK and PLD pathways are activated leading
to fast and transient PA accumulation, but exceptions also exist (Zonia and Munnik, 2004;
Darwish et al., 2009; Hong et al., 2010). Besides responding to osmotic stress, we also see the
PLDĪ±1 enzyme participating in cold, frost, and wound stress signaling (Bargmann et al.,
2009; Hong et al., 2010) and probably by promoting responses to ABA, especially in stomata
(Mishra et al., 2006). On the other side, PLC/DGK pathways also get activated by salinity
(Arisz, 2010). Earlier we knew that AtPLC1, one of the PI-PLCs, was shown to be induced by
salinity and drought (Hirayama et al., 1995), which is necessary for ABA-induced inhibition
