| Plant mitogen-activated protein kinases (MAPKs) are involved in a plethora of signaling transduction pathways associated with both abiotic. Up to date, most of the plant MAPKs that have been characterized are from dicotyledonous model species. In Arabidopsis 20 MAPKs fitting into four groups have been identified. However, in rice only four MAPKs have been identified and of these four only two have been characterized relating to the host defense response. In this study, we first identified 17 members of the OsMPK s gene family by conducting an in silico search of rice genome databases and chose one of these OsMPKs for detailed analysis. Phylogenetic analysis reveals that rice MAPK are more divergent than Arabidopsis MAPKs and can be placed in five groups (A, B, C, D and E) and are characterized by either the TEY or TIDY motif in their activation site. Furthermore, quantitative reverse transcription-polymerase chain reaction (QRT-PCR) was used to examine the expression patterns of these 17 OsMPKs in response to virulent and avirulent pathogen infection, host cell death as well as by defense signaling molecules such as JA, SA, ABA and ethylene. Our results show that about half of the OsMPK genes are inducible by pathogen infection and defense signaling molecules.;The OsMPK7 gene was chosen for further analysis because it was found to be both rice blast and JA-inducible in the first part of this project. The OsMPK7 cDNA is 2384 by long and encodes a 599-amino acid protein containing a potential membrane-spanning region. Quantitative RT-PCR analysis of various rice tissues throughout development showed that OsMAPK6 is relatively highly expressed in two-week and two-month old stems as well as in the culm. RNA blot analysis showed that OsMPK7 was rapidly induced by JA within 30-minutes, peaked at four hours, and decreased six hours after treatment. OsMPK6 was also found to be slightly induced 30-minutes after wounding. The OsMPK7 protein was induced and activated four and six hours after treatment with JA, respectively. However, OsMPK7 was not found to be induced by any other defense-signaling molecule. Pre-treatment of cycloheximide did not inhibit the JA-induced expression of OsMPK7 , suggesting that its activation is independent of de novo protein synthesis.;To determine the biological function of OsMPK7 we generated transgenic rice plants with over-expression (using an estradiol inducible promoter) or suppression (using double stranded RNA interference [dsRNAi]) of OsMPK7. Suppression of the OsMPK7 gene product in transgenic plants led to an increase in resistance to the rice blast fungus. No change in PR gene expression was observed in OsMPK7 dsRNAi lines. Overexpression of the OsMPK7 transgene led to a hypersensitivity to JA. The rapid induction of OsMPK7 by the rice blast fungus may be dependent on chemicals secreted by the rice blast spore during germination and may be independent of Avr-gene mediated resistance. It is likely that OsMPK7 is involved with JA signaling and disease resistance response in rice. |
