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.