OsMPK6 Acts As Dual Regulator In Rice Disease Resistance

Mitogen-activated protein kinase (MAPK) cascades play important roles in diversedevelopmental and physiological processes of plants, including pathogen-induced defenseresponses. On researching MAPKs in flee, they lag behind model plant Arabidopsis.Although at least 17 rice MAPKs have been identified and nine of these MAPK geneshave been shown to be pathogen or elicitor responsive, the exact role of most of theMAPKs in host-pathogen interaction is unknown.AtMPK4 is an important negative gene of SAR in Arabidopsis, which confernon-specific resistance. Provious study showed there was a resistance pathway exists inrice as Arabidopsis. In this study, we isolated a rice homolog of AtMPK4 fromMinghui63, designed as OsMPK6. The deduced amino sequence from the full eDNAshared 84% sequence identity to AtMPK4 which showed the highest sequence similarity,Cluster analysis indicated it belong to the same group with AtMPK4 and NtMPK4. Theseresults hinted OsMPK6 would possess similar function as them. And OsMPK6constitutively expressed in multi tissues of rice.OsMPK6 expression is induced rapidly by ABA, GA3, and IAA, drought stress alsoresulted in the accumulation of OsMPK6 transcripts. However, the heavy, metals, copper,cadmium and mercury, and high concentrations of NaCI and sucrose had no distinctinfluence on the expression of OsMPK6. Low(4℃) and high(42℃) temperature slightlyinduced OsMPK6 expression at 2 and 1 h after treatment, respectively.As rapidly as 5 min after treatment, the plant defense signal molecules SA, BTH,INA, JA, ET and H₂O₂ induced the expression of OsMPK6. Comparing the expressionlevel of OsMPK6 after treatment with a single compound, interactions between SA, BTH,or INA and JA or ET showed no distinct effect on the expression of OsMPK6, CHX alsoinduced OsMPK6 expression at 60 to 120 min after treatment as compared to the control,which suggests that de novo synthesized negative regulator also may be involved in theregulation of OsMPK6 expression.Comparing with the expression level of OsMPK6 in mock-inoculated plants,incompatible host-pathogen interaction further suppressed OsMPK6 expression andcompatible host-pathogen interaction slightly induced OsMPK6 expression. OsMPK6transcripts are induced and increased after inoculated blast. The difference expression ofOsMPK6 between bacterial blight and blast infection imply OsMPK6 could be functional in different way against bacterial and fungus pathogen and OsMPK6 would participate inbasic defense response, its expression was regulated with different R genes.OsMPK6-overexpression and -suppression enhanced resistance to Xoo, theenhanced resistance cosegregated in T₁ family. OsMPK6-overexpression and-suppression plant conferred a broad spectrum of resistance to various Xoo strains, andshowed resistace to Xoo in rice full living stage.OsMPK6 positively regulate resistance response. Overexpression of OsMPK6accompanying the development of lesion mimics on the leaves also enhanced theresistance to bacterial blight, whereas the OsMPK6-overexpressed plants that were free oflesion mimics were susceptible. Northern analysis results showed that differentialexpression of some defense-related genes in OsMPK6-overexpressed plants with orwithout lesion mimics, the resistant OsMPK6-overexpressed plants enhanced OsAOS2transcripts which is a important gene composed JA, explained OsMPK6 regulateresistance response by JA pathway, or local resistance.OsMPK6 negatively regulate resistance response. OsMPK6-suppressedplants/mutant developed lesion mimics in adult stage, showed differential expression ofsome defense-related genes after inoculated Xoo. OsMPK6-suppressed plants enhancePAL expression that is key gene composed SA, explained OsMPK6 regulate resistanceresponse by SA pathway, or system resistance. OsMPK6-suppressed plants seedingwithout lesion mimics displayed resistance to Xoo, implied resistance does not require thedevelopment of lesion mimic. OsMPK6-suppressed plants don't change OsAOS2transcripts.OsMPK6 both positively and negatively regulate resistance response using differentpathway. However, all of these resistant plants had elevated salicylic acid (SA) levels.And a subset of genes in the NH1 (an Arabidopsis NPR1 homologue) pathway showedenhanced expression, resulting in Program cell death and ROS. All the data suggestedOsMPK6 might be a coordinator between JA and SA pathway.Observation of the onion epidermal strips under a confocal microscope showed thatOsMPK6 protein localized mainly in the nuclei and lightly in the cytoplasm...