Chemistry Reference
In-Depth Information
activated protein kinase (MAPK) pathways are responsible for producing osmolytes and
antioxidants, which are activated by receptors/sensors such as protein tyrosine kinases, G-
protein coupled receptors, and two-component histidine kinases. For plants, only histidine
kinases may have been explored and clarified in a deeper sense compared with others.
Retrospecting the history of histidine kinase, one important discovery is the cyanobacterium
histidine kinase Hik33 (Suzuki et al., 2000) and the
Bacillus subtilis
histidine kinase DesK
(Aguilar et al., 2001) being identified as thermosensors. Unfortunately, even if several
putative two-component histidine kinases have been found in
Arabidopsis thaliana
(Urao et
al., 2000), none of them can be confirmed as thermosensors. However in yeast, a two-
component histidine kinase named SLN1 has been identified as a type of membrane protein
sensor for osmotic stress perception (Maeda et al., 1994, 1995). And then later researches
found out AtHK1, an Arabidopsis histidine kinase, can complement mutations of
SLN1.
Therefore AtHK1 may participate in osmotic stress signal transduction in plants (Urao et al.,
1999). In conclusion, understanding the function of putative histidine kinases and their
relationship with MAPK pathways not only help us dig out more sensors but also, even
more important, find out how they work in signal transduction pathways.
Thirdly, in plants, the receptor-like kinases and G-protein are worthy to be mentioned in the
searching for stress signal sensors. Why? The stress hormone abscisic acid (ABA) makes us
study on them who may contain putative stress sensors. It is well-known that the ABA is of
great significance in stress signaling, thus, to understand how ABA is perceived certainly
will contribute to revealing the hidden sensing-processes of stress signals. Generally, the
researches on ABA perception mechanisms always relate to putative receptor-linked
components or those putative receptor molecules regulated by stress or ABA.
Here we are going to mention a different way of osmolyte production that involves the
pathways triggering the activation of late embryogenesis-abundant (
LEA
)-type genes
representing damage repair processes (Zhu, 2001; Xiong and Zhu, 2002). And these LEA-like
genes under cold, drought, and salt stress are modulated by phosphoinositols who are
closely connected with the activity of phospholipase C, which in plants might be regulated
by G-proteins. Moreover evidences suggest G-protein coupled receptors may take part in
perceiving a secondary signal derived from these stresses (Ullah et al., 2001; Wang et al.,
2001), which may brings a hint that G-protein may have a position on primary sensors list.
On the other hand,
Arabidopsis
heterotrimeric G-proteinαsubunit GPA1 may be a part of
ABA response in guard cells but has no relation with ABA-induced stomata closure.
Moreover GPA1 interacts with the G-protein couple receptor-like protein GCR1. Researches
on gpa1 mutants and gcr1 mutants bring us much more information on finding receptors for
ABA. Also some small G proteins are referred to as negative regulator of ABA responses in
Arabidopsis, like ROP10. But the really surprising discovery comes into the world in 2009.
In that year, two research groups from the USA and Germany reported in Science that they
had identified a small protein family binding to ABA interacts with ABA Insensitive 1 and 2
(ABI1 and ABI2), two type 2C protein phosphatases (PP2Cs). And they are negative
regulators of ABA signaling (Ma et al. 2009, Park et al. 2009).
