Auxin signal transduction through mitogen-activated protein kinase (MAPK) in roots of Arabidopsis thaliana

The phytohormone auxin regulates growth and development throughout the life of plants, through its effects on mitogenesis, cell expansion and tissue differentiation. Although rapid physiological and cellular responses to auxin have been studied, the auxin signaling pathway is completely unknown. I have discovered that auxin induces the activation of mitogen activated protein kinase (MAPK) in the roots of young seedlings.; I characterized the auxin-induced MAPK response in roots of Arabidopsis thaliana with regard to several properties known in mammalian MAPK pathways, and attempted to determine which MAPK(s) are involved in auxin responses. Arabidopsis contains at least twelve structurally similar MAPK isoforms, the functions of which are not yet known.; Several studies in other plants have implicated the involvement of MAPK activation in plant responses to stresses encountered upon pathogenic attack or harsh environmental conditions. I examined the responses of root MAPKs to some of these stress factors, and found that the simultaneous signaling of stress antagonizes both MAPK activation in response to auxin and auxin-inducible transcription in roots. These data, along with others obtained from experiments using drugs that inhibit mammalian MAPK cascades, revealed evidence for negative regulation of auxin signaling by stress signaling through a mechanism involving MAPK pathways.; Investigations of MAPK responses in Arabidopsis mutants showed that several mutants deficient in auxin sensitivity of growth responses were likewise impaired in MAPK activation in response to auxin. Some of these mutants are deficient in gene products proposed to function in ubiquitin-like protein conjugation and ubiquitin-mediated protein degradation processes necessary for auxin responsiveness. These findings raise the intriguing possibility that cellular processes mediated by ubiquitin or ubiquitin-like protein conjugation are involved in regulating the activity of a MAPK signaling cascade.