The role of Arabidopsis MAP kinase genes in biotic and abiotic stress signalling
Abstract
Plants are constantly faced with environmental stresses which trigger morphological,
physiological, biochemical, and molecular changes that can negatively affect crop
productivity. Plants detect and respond to these stresses via diverse means including
signalling pathways such as the Mitogen-activated-protein-kinase (MAPK) networks which
have shown to play a role in stress signalling, activating many stress-responsive genes
through the agency of the plant stress hormone, abscisic acid (ABA). The work in this thesis
examines the role of selected MAPK genes in abiotic and biotic stress, with novel findings
being made for MPK1 and MPK2.
It is known that MPK1 and MPK2 genes together with other MAP Kinases play a role in
pathogen stress resistance and ABA regulation. In this study their additive function in
enhancing osmotic and pathogen stress tolerance was uncovered. The observed increased
sensitivity of the mpk1/2 double mutant to exogenous ABA was attributed to the synergistic
role of MPK1 and MPK2 with MPK2 as the main contributor in the negative regulation of
the transcription factor ABI4, and also in the suppression of ABA biosynthesis although
mechanism remains to be defined fully. The results suggest that the high expression of ABI4
in mpk2 and mpk1/2 mature seeds contributed to low germination under salinity and osmotic
stresses. It was shown via gene expression of the stress responsive gene RD29A in stressed
seedlings that the A. thaliana MPK2 gene promotes salt tolerance in seedlings by acting as a
positive regulator for RD29A. MPK1 however, negatively regulated RD29A under salinity
stress, and also suppressed the ability of MPK2 to promote RD29A expression in seedlings
during osmotic stress. It was evident in this study that the same genes played different roles
under different stress conditions, this was obvious when target genes acted as negative and
positive regulators of proline and ABA respectively under osmotic stress, but not under salt
stress. Based on the above findings, coupled with results for protein-protein interactions from
the Split ubiquitin yeast-2-hybrid assay which identified upstream activators of MPK1 and
MPK3 as MKK1-, MKK2- MKK3- proteins, and MPK2 as MKK1 protein, it is hypothesised
that pathogens, salt, and/or osmotic stress may potentially be perceived through MKK1,
MKK2 and/or MKK3, to activate MPK1, MPK2 and/or MPK3 protein, during stress
signalling. These stress responses are not governed by simple linear pathways, but are
networks that might more subtly regulate the plant response to biotic and abiotic stresses.