Chemistry Reference
In-Depth Information
by altered hormonal and oxidative stress responses. On the other hand, recently in heavy
metal stress studies, we find data supporting that MAPK3 and MAPK6 are activated
responding to cadmium through ROS accumulation produced by oxidative stress in
Arabidopsis (Liu XM et al., 2010), which further demonstrate the close relevance between
ROS and MAPKs.
Meanwhile, by searching for the linkage between MAPKs and its various substrates
involving other kinases and transcription factors, we gain much more information about
MAPK cascades. Over-expression of MAP2K2 affects genes for several transcription factors
(such as RAV1, STZ, ZAT10, ERF6, WRKY, and CBF2), disease resistance proteins, cell wall
related proteins, enzymes involved in some secondary metabolisms and an 1-
aminocyclopropane-1-carboxylic acid synthase (ACS). In the case of ACS, the rate-limiting
enzyme of ethylene biosynthesis, the phosphorylation by MAPKs and by CDPKs affects
protein stability and turnover, which again shows us the complicated cross-talk network
(Bernhard Wurzinger et al., 2011). So it is a big challenge to identifying the targets of MAPK
cascades, but the researches on other protein kinases leave us useful clue to find the answer.
2.4.2. Other protein kinases
Whether at the transcript level or activity level, protein kinases are induced by a variety of
abiotic stress, which indicates their powerful participation in signaling process. Moreover no
matter suppressing or overexpressing these kinases, there both exists data showing that in
transgenic plants, stress responses has changed. So far, we know several protein kinases
involved in stress tolerance are stimulated by ABA, such as most of SNF1-related kinases
(SnRKs) like SnRK2, SnRK3 (CIPK), CDPK and MAPK families. But others like Glycogen
synthase kinase 3 (GSK3) ( Jonak and Hirt, 2002; Koh et al., 2007), S6 kinase (S6K) (Mahfouz
et al., 2006), SERK (Marcelo O. Santos et al., 2009) also attract a lot of attention.
Now let's start from SnRKs. The SnRK2 family members are plant-specific kinases relating
to abiotic stresses responses and abscisic acid (ABA)-dependent plant development. They
have been classed into three groups; members of group 1 are not activated by ABA, and
group 2 also will not be activated or weekly activated, while group 3 is strongly activated by
ABA. In
Arabidopsis
the SnRK2 subfamily consists of 10 members. Except SnRK2.9, all
SnRK2s are activated by osmotic and salt stress (Boudsocq, M. et al., 2004). Take SnRK2.6
(OST1) as an example. SnRK2.6 functions in the ABA signaling pathway upstream ABA-
induced ROS production. It is related to the ABA-activated protein kinase AAPK in Vicia
faba and also associates with SNF1 protein kinase. NADPH oxidases function in ABA signal
transduction, also targeted by the SnRK2.6 kinase (Nakashima et al., 2010). Generally
speaking, regulating the response to ABA through SnRK2s pathways is to directly
phosphorylate various downstream targets such as ion channels (SLAC1, KAT1) and ABFs
and other specific TFs required for expression of stress-responsive genes (Anna Kulik et al.,
2011). By the way, the SnRK2 subfamily is conserved in land plants. No wonder their role in
ABA signaling and osmotic stress responses have also been found in pea, barley, rice and
zea
mays
(Shen, Q. et al., 2001; Kobayashi et al., 2004; Huai, J. et al., 2008). As for SnRK3 (CIPK),
