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of germination and gene expression (Sanchez and Chua, 2001). Recently, NPC4 (a NPC
isoform) was found to modulate responses to ABA and bring enhanced salt and drought
tolerance (Peters et al., 2010). In addition, several ABA signaling proteins have been
identified as potential PA targets (Mishra et al., 2006), which has further suggested PA could
mediate ABA responses. Meanwhile, cooperation between the NADPH oxidase isoforms
RbohD, RbohF and PA brings more information for us to understand PA's function in ABA-
induced ROS generation and stomatal closure (Zhang et al., 2009). Furthermore, PA also
targets other protein kinases like SnRK2 protein kinase (Testerink et al., 2004), MAPK
isoform MPK6 (Yu et al., 2010), sphingosine kinase (SPHK) (Liang Guo, XueminWang, 2012)
to influence diverse signaling transduction pathways.
Other two important second messenger molecules - inositol phosphates Ins(1,4,5)P3 and
DAG (diacylglycerol) are worthy to be mentioned here. Firstly, InsP3 was shown to release
Ca
2+
from an intracellular store in the early 90s, but recently evidences pops up suggesting
that InsP6 was shown to release Ca
2+
at a 10-fold lower concentration than InsP3
(Lemtiri-
Chlieh et al., 2003; Teun Munnik, 2009), who can be generated by phosphorylating InsP3.
By
the way data can be found demonstrating that whole-plant IP3 level goes up significantly
within 1 min after stimuli occur, and keep the tendency for more than 30 min under stress.
Under osmotic stress, in Arabidopsis, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P
2
)
is hydrolyzed to IP3. And IP3 accumulation occurs coincidently in a time frame similar to
stress-induced calcium mobilization (Daryll B. DeWald et al., 2001). In conclusion, what we
can say is under different stress, to identify the most critical second messenger molecules
depends on the research on the network consisting of multiple polyphosphoinositides.
Secondly, diacyglycerol (DAG) is an important class of cellular lipid messengers, but for its
function in plants, data is not sufficiently provided. In
Arabidopsis thaliana
, knocking out
NPC4 results in DAG level decrease and compromises plant response to ABA and
hyperosmotic stresses. On the other hand, overexpressing NPC4 leads to higher sensitivity to
ABA and stronger tolerance to hyperosmotic stress than wild-type. And later experiments
indicate that NPC4-produced DAG is converted to PA and NPC4 might be a positive regulator
in ABA response and promote plant tolerance to drought and salt stresses (Carlotta Peters et
al, 2010). Furthermore, all higher plant genomes sequenced so far lack both InsP3 receptor and
the DAG target, PKC (Munnik & Testerink 2009). In conclusion, we have reasons to believe in
that PA rather than DAG are more likely to play a central role in stress signaling transduction.
At last, we'd love to say more about other types of phospholipases like secreted
phospholipase A
2
and patatin-related phospholipase A (pPLA) who were shown to have
functions in auxin signal transduction by cooperating with auxin receptors ABP1 or TIR1.
(Günther F. E. Scherer et al., 2012 ). And this fact helps us to further confirm the significance
of phytohormones in signaling transduction which will be discussed below.
2.2.3. Phytohormones
When it comes to phytohormones, strictly speaking, we can not conclude them as second
messengers, but the first and foremost idea needs to be posted here is "The most powerful
