Molecular analysis of MAP Kinase-mediated ethylene signaling Arabidopsis thaliana

Ethylene is one of the five classical plant hormones and functions in a multitude of physiological as well as developmental processes including cell elongation, seed germination, leaf senescence and fruit ripening. A number of stress conditions increases the synthesis of ethylene and thereby affects expression of ethylene-inducible genes. Although the biosynthesis of ethylene production has been elucidated both at the genetic and biochemical level, the signaling processes at the cellular level are less well understood. What is clear at the moment is that sensing of ethylene occurs through several histidine kinase receptors such as ETR1. ETR1 is upstream of CTR1, a protein kinase with similarity to MAPKK kinases. Further downstream in the pathway, EIN2 and EIN3 act in transcriptional regulation of ethylene-inducible genes. Preliminary biochemical and genetic evidence from my lab indicates that a specific MAPKK and two MAPKs are involved in ethylene signal transduction in Medicago and Arabidopsis. As part of this research proposal we will attempt to i) place the MAPKK and MAPKs on the map of the existing signal transduction pathway, ii) investigate the interaction of the MAPKK and MAPKs with the known ethylene signaling components and iii) determine the roles of the MAPKK and MAPKs in ethylene functioning in a physiological context.

In this project, we have provided evidence that CTR1 encodes a functional mitogen-activated protein kinase kinase kinase (MAPKKK), which can phosphorylate and thereby activate the MAPKK MKK4 and MKK5. As MKK4 and MKK5 are upstream activators of the MAPKs MPK3 and MPK6, CTR1 can activate MPK3 and MPK6. The biochemical function and importance of CTR1 is currently under further genetic investigation. Moreover, we have provided evidence that the MAPKKK MEKK1 is not an upstream regulator of MPK3 and MPK6 but functions as an activator of MPK4. In a reverse genetic approach, we have shown that MEKK1 and MPK4 have largely overlapping phenotypes and affect similar sets of genes being involved in reactive oxygen species signalling.