Fact or fiction: The ethylene precursor ACC as a plant signal

Abstract:
Increasing evidence suggests that 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of the plant hormone ethylene, can induce responses in plants that are different from those induced by ACC’s conversion to ethylene. Compelling findings include ethylene-independent phenotypes of an Arabidopsis thaliana ACC synthase (acs) octuple mutant, which produces a reduced amount of ACC due to having T-DNA insertions in six of the eight ACS genes plus an artificial microRNA transgene that targets expression of the remaining two genes. Supporting the idea that these phenotypes are ACC-specific is the fact that they can be alleviated by treatment with exogenous ACC. To further examine the role of ACC in Arabidopsis, we used CRISPR/Cas9 to generate complete knockout mutants of all eight acs genes. These mutants have undetectable levels of ACC and ethylene but unexpectedly lack the reported ACC-specific phenotypes of the original T-DNA-generated mutant. A clue to the basis for this discrepancy came from a metabolomic study in which we found that the acs octuple T-DNA mutant possesses a highly distinct pattern of sulfur-containing compounds, such as glucosinolates. By investigating metabolomic profiles and the genome sequences of the acs T-DNA mutant and its parental lines, we identified the genetic factor responsible for the glucosinolate profile. Although we have yet to identify the source of all of the ACC-related phenotypes, our findings indicate unexpected genetic complexity in the original acs T-DNA mutant and serve as a cautionary tale for interpreting phenotypes of mutants derived from extensive backcrosses. We are now analyzing our clean acs octuple null lines (the CRISPR mutants) for possible ACC-specific phenotypes.

About the Speaker:
Dr. Caren Chang is a professor in the Department of Cell Biology and Molecular Genetics at University of Maryland (UMD). She obtained her bachelor’s degree in biophysics from UC Berkeley and her PhD in molecular biology from Caltech. As a graduate student under Dr. Elliot Meyerowitz in the 1980s, she helped to develop Arabidopsis thaliana as a model system, e.g., she generated the first RFLP map for the Arabidopsis genome with which any gene could be cloned on the basis of mapping its mutant phenotype relative to RFLP markers. As a subsequent postdoc in the Meyerowitz lab, she used the RFLP map to clone the gene corresponding to an ethylene-insensitive mutant, etr1-1, which was the first plant hormone receptor to be identified. In 1994, she joined the faculty at UMD where her research has focused on the molecular mechanisms of ethylene signaling and the evolutionary history of ethylene as a plant hormone. More recently, her laboratory has been examining the unexpected ethylene-independent functions of ACC, the immediate precursor molecule for ethylene. Her research uses approaches in molecular genetics, cell biology, biochemistry, proteomics, transcriptomics, and metabolomics. Dr. Chang is a UMD Distinguished Scholar-Teacher, as well as the co-director of Terrapin Teachers, an initiative at UMD that recruits STEM undergraduates to consider careers in teaching to address the severe shortage of well qualified STEM teachers in secondary schools.

Contact

  Ying Gu
  yug13@psu.edu