The signal transduction pathway of the plant stress and defense hormone, ethylene, has been extensively elucidated using the plant genetic model Arabidopsis over the last two decades. Among others, a MAPKKK CTR1 was identified as a negative regulator that has led to the speculation of MAPK involvement in ethylene signaling. However, it remained unclear how the MAPK modules acting downstream of the receptors to mediate ethylene signaling. We have recently presented new evidence that the MKK9-MPK3/6 modules identified by combined functional genomic and genetic screens mediate ethylene signaling, which is negatively regulated by the genetically identified CTR1-dependent cascades. Our genetic studies show consistently that the MKK9-MPK3/MPK6 modules act downstream of the ethylene receptors. Biochemical and transgenic analyses further demonstrated that the positive-acting and negative-acting MAPK activities are integrated and act simultaneously to control the key transcription factor EIN3 through dual phosphorylations to regulate the EIN3 protein stability and downstream transcription cascades. This study has revealed a novel molecular mechanism that defines the specificity of complex MAPK signaling. Comprehensive elucidation of MAPK cascades and the underlying molecular mechanisms would provide more precise explanations for how plant cells utilize MAPK cascades to control specific downstream outputs in response to distinct stimuli.
Ethylene was the first plant hormone for which a receptor‐dependent signalling pathway was established. The signal transduction pathway is framed around genetically identified factors encompassing membrane receptors through to intra‐cellular regulators and then nuclear transcription factors. Recently, the cellular and biochemical connections among these genetic factors have been characterized to reveal a complex and intertwined signalling scheme. For ethylene signalling, the short‐lived ETHYLENE INSENSITIVE2 (EIN2) and ETHYLENE INSENSITIVE3 (EIN3) proteins play central roles, and so regulation of the turnover of these proteins by specific E3 ligases serves as a key regulatory step in the pathway. Two antagonistic mitogen‐activated protein kinase (MAPK) cascades that modify EIN3 appear to mediate intra‐cellular signalling from membrane receptors to nuclear transcription factors. The identification of further genomic and genetic components and elucidation of their cellular and biochemical roles in EIN2 and EIN3 regulation are thus required to complete the whole ethylene signal transduction pathway. Such a comprehensive understanding of ethylene signal transduction will provide useful information for the selection of genes that can enhance plant adaptations to unfavourable environmental conditions and so secure food production and plant‐based renewable bio‐energy for human society.
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