|Title||Stress activated MAPKs in plants|
|Source||Agricultural University. Promotor(en): T. Bisseling; H. Hirt. - S.l. : S.n. - ISBN 9789058081810 - 176|
Laboratory of Molecular Biology
|Publication type||Dissertation, internally prepared|
|Keyword(s)||eiwitkinase - spanningen - planten - transductie - signalen - stressreactie - wonden - protein kinase - stresses - transduction - signals - stress response - wounds - plants|
|Categories||Plant Physiology / Plant Molecular Biology|
Plants are exposed to a wide variety of extracellular stimuli and employ a broad set of signaling pathways to give the appropriate response. M itogen a ctivated p rotein k inases (MAPKs) play an important role in the signal transduction of yeast and animals and increasing evidence suggests a similar role of MAPKs in the signal transduction of plants. MAPKs employ their function as part of protein kinase cascades, composed of a MAPK, a M AP Kk inase (MKK), and a M AP Kk inase k inase (MKKK). MKKKs activate MKKs by phosphorylation of conserved threonine or serine residues, and subsequently MKKs activate the MAPKs by phosphorylation of highly conserved tyrosine and threonine residues.
The introduction gives a brief overview of the MAPK cascades employed by yeast and animals and a more extensive overview of our current knowledge about the function of MAPK cascades in plants. To get an overview of the plant MAPKs and their functions the known plant MAPKs were classified and the analyses of all isolated full-length plant MAPK sequences reveals that they can be divided into at least five distinct subfamilies (Chapter 2). For some of these groups it could be shown that MAPKs with similar sequences also perform similar functions. In addition, analysis of e xpressed s equence t ags (ESTs) and partial cDNAs coding for MAPKs revealed the existence of a new plant MAPK subfamily.
The goal of the research described in this thesis was to provide insight in the role of several plant MAPKs in stress responses of plants. One of the most severe environmental stresses to which plants can be exposed is wounding. It can be the result of physical injury, herbivore or pathogen attack and induces a wide range of responses, in general involving the induction of genes active in healing and defense processes. In chapter 3 the involvement of a MAPK in the wound response of alfalfa is discussed. It could be shown that wounding activates the alfalfa s tress a ctivated M AP K (SAMK) both at the post-translational and the transcriptional level. The inactivation but not the activation of SAMK was dependent on de novo transcription and translation.
Another important stress to which plants are exposed is pathogen attack. The various defense responses of plants to pathogens can normally also be activated by specific pathogen derived factors (elicitors). A 13 amino acid oligopeptide fragment from a 42 kDa extracellular glycoprotein of the pathogenic fungus Phytophthora sojae was used. Treatment of parsley cells with this elicitor results in the induction of a broad set of defense responses. The signaling pathways leading to these responses include the activation of several ion channels and the production of reactive oxygen species. A MAPK was identified that is also activated upon elicitor treatment (Chapter 4). This MAPK was shown to act downstream of the ion channel activation and upstream or independently of the oxidative burst. The MAPK is translocated to the nucleus after activation by the elicitor, where it might activate transcription factors responsible for the induction of expression of plant defense genes.
To extend these results to other plant species, the responses of alfalfa cells upon yeast elicitor treatment were analyzed. Two protein kinases with relative molecular masses of 44-kD and 46-kD were found to be rapidly and transiently activated upon elicitor treatment (Chapter 5). These kinases were identified as SAMK and SIMK ( s tress- i nducible M AP k inase), respectively. Yeast elicitor-induced medium alkalinisation, oxidative burst, and MAPK activation could be blocked by the protein kinase inhibitor K252a, demonstrating that protein kinase pathways are responsible for mediating these elicitor responses. However, SAMK and SIMK pathways are not involved in elicitor-induced medium alkalinisation or oxidative burst, because staurosporine, another protein kinase inhibitor, did not affect elicitor-induced activation of SAMK and SIMK pathways but totally inhibited medium alkalinisation and production of reactive oxygen species. These data show that whereas elicitor-induced medium alkalinisation and oxidative burst depend on protein kinase pathways, these protein kinases lie on separate pathways from elicitor-activated SAMK and SIMK cascades.
In summary the data provided in this thesis give proof for the involvement of various MAPKs in stress responses of plants, but further research will be needed to elucidate the exact role of these MAPKs in their respective pathways.