Speaker: Caren Chang, University of Maryland professor in the Department of Cell Biology & Molecular Genetics

Title: Insights into ethylene biology: Reevaluating the precursor ACC as an independent signal and 
revealing a novel function of EIN2 in Arabidopsis

Abstract: Part I. ACC as an endogenous signal in plants: Fact or fiction? Increasing evidence over nearly two decades has suggested that 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of the plant hormone ethylene, is capable of acting as an ethylene-independent signal. Our recent findings, however, overturn the genetic evidence for this in Arabidopsis thaliana, thereby requiring a reevaluation of ACC as a signal. Our CRISPR/Cas9 editing of all eight ACC synthase (ACS) genes in Arabidopsis led us to discover that the phenotypes of the historically used acs T-DNA octuple mutant are likely artifacts arising from having a complex genetic background of two ecotypes and multiple transgenes. While these findings indicate the misinterpretation of phenotypes in the acs T-DNA mutant and serve as a cautionary tale, our new acs CRISPR mutants provide a reliable genetic resource for future studies on both ACC and ethylene in plant growth and development. 

Part II. Ethylene signaling requires Ca²⁺ transport by EIN2 in Arabidopsis. Tremendous advances have been made in understanding the ethylene-signaling pathway, yet several underlying mechanisms remain unknown. ETHYLENE INSENSITIVE2 (EIN2), a central regulator in the pathway, has an N-terminal domain (N-term) localized at the endoplasmic reticulum (ER) membrane and a cytosolic C-terminal signaling domain regulated by phosphorylation. The N-term has sequence similarity to Nramp metal ion transporters (which transport Fe²⁺ and Mn²⁺) but the biochemical function of the N-term has remained a mystery ever since the cloning of EIN2 in 1999. We now have evidence that the Arabidopsis EIN2 N-term is capable of transporting Ca²⁺ across the ER membrane, possibly as a proton-coupled antiporter. EIN2 would be the only known Nramp-like protein able to transport Ca²⁺. Based on Arabidopsis ein2 missense alleles that prevent Ca²⁺ transport while conferring ethylene insensitivity, we propose that Ca²⁺ transport by EIN2 is required for ethylene signaling.

Host: Josh Strable, GDCB assistant professor