A Cell Wall Fragment Affects Ethylene Response in Arabidopsis
The plant hormone ethylene is a simple alkene which regulates germination, growth, stress responses, pathogen response, tissue development, fruit ripening, and senescence in plants. In an effort to identify new factors that relate to how plants respond to ethylene, an Arabidopsis thaliana mutagenesis screen revealed a mutation that is hypersensitive to ethylene. The mutation that gave rise to this aberrant ethylene response phenotype was found to caused by a new pmr6 allele. PMR6 (POWDERY MILDEW RESISTANT 6) encodes a putative pectate lyase that was first identified for pmr6 plants being unable to support powdery mildew growth, presumably by effecting the cell wall composition. The identification of PMR6 as a factor involved in ethylene signaling suggested a novel role and the subsequent characterization of this new pmr6-6 allele utilized known ethylene mutants to characterize the aberrant ethylene response. While pmr6-6 plants do produce additional ethylene, ethylene responses are promoted independent of ethylene binding to the receptors in pmr6-6 plants. Ethylene response is found to be promoted by soluble metabolites from ethylene treated seedlings present in growth media. The pmr6-6 plant is sensitive to these soluble metabolites and results in shortening of the hypocotyl of etiolated seedlings while wild type plants with functional PMR6 plants do not express an ethylene response at the same concentration. The ethylene response was found to be dependent on EIN2 and the ethylene transcription factors EIN3 and EIL1 to display an ethylene phenotype. These soluble metabolites were also able to promote the stabilization of EIN3:GFP independent of ethylene perception. The PMR6 substrate may play an important role in plant defense response and may be one way that plants detect and respond to changes in the cell wall integrity or to changes in growth that produce cell wall fragments in the extracellular space.