Abstract

Abstract
Role of MAPK phosphatase MKP2 and DsPTP1 in plant growth and development
Jianlei Sun, M.Sc
Mitogen-activated protein kinase (MAPK) cascades are a group of protein kinases composed of a
MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK), and a MAPK that play an
important role in signal transduction in response to various stimuli in eukaryotes. As terminal
components of sequential phosphorylation events, the activity of MAPKs is upregulated through
phosphorylation by upstream MAPKKs and downregulated through dephosphorylation catalyzed
by protein phosphatases. In plants, MAPK cascades are relevant to a large number of biological
processes, including plant growth, development, and stress responses.
The Arabidopsis thaliana genome contains five MAPK phosphatases (MKPs), MKP1, MKP2,
DsPTP1, IBR5 and PHS1.Relatively little is known about their properties and biological roles,
especially in plant growth and development compared to their substrate, MAPKs. Thus, our
research group investigated the roles of these five MKPs in plant growth and development by
analyzing single and higher-order mkp mutants. In this thesis, based on our original data obtained
from these protein interaction researches, mutant studies as well as available data from the
literature, we decided to focus on two Arabidopsis MKPs, MKP2 and DsPTP1, and their potential
substrate MAPKs.
Neither mkp2 nor dsptp1 plants displayed obvious developmental phenotypes under standard
growth conditions, but interestingly we found that mkp2 dsptp1 double mutants resulted in a
dramatic albino phenotype, eventually leading to mkp2 dsptp1 seedling lethality. These results
suggest that MKP2 and DsPTP1 play an important role in early plant growth and development.
To determine whether the albino phenotype of mkp2 dsptp1 double mutants is caused by the loss
of MKP2 and DsPTP1, transgenic Arabidopsis plants were constructed that have the expression
of either MKP2 or DsPTP1 in the mkp2 dsptp1 mutant background under its own native promoter.
These complementation lines rescued the albino phenotype of mkp2 dsptp1 mutants, indicating
that the loss of MKP2 and DsPTP1 function is indeed responsible for the albino phenotype of mkp2
dsptp1 mutants. It was observed that chloroplast number in guard cells on the epidermis and the
IV
level of photosynthetic pigments were reduced significantly in mkp2 dsptp1 when compared with
either the wild-type or the complementation lines. These results suggest that MKP2 and DsPTP1
are functionally redundant novel regulators and are required for chloroplast development in
Arabidopsis. To further understand how MKP2 and DsPTP1 integrate MAPK cascades in
chloroplast biogenesis a search for their MAPK substrates was performed. Yeast two-hybrid assays
confirmed our previous finding that MKP2 specifically interacts with two MAPKs, MPK8 and
MPK15. This suggests that deregulation of MPK8 and/or MPK15 potentially leads to the dramatic
albino phenotype with reduced chloroplasts in mkp2 dsptp1. Consistently, it was found that mkp2
dsptp1 mpk8 mpk15 high-order mutants supressed the lethal seedling phenotype of mkp2 dsptp1.
Based on this analyses, I propose that two MAPKs, MPK8 and MPK15 are potential substrates of
MKP2 and DsPTP1 in chloroplast biogenesis. In summary, this work identified two MAPK
phosphatases required for early plant development, chloroplast biogenesis, and also identified
possible MAPK substrates for further study.