<i>EBSn</i> , a Robust Synthetic Reporter for Monitoring Ethylene Responses in Plants

Ethylene is a gaseous plant hormone that controls a wide array of physiologically relevant processes, including plant responses to biotic and abiotic stress, and induces ripening in climacteric fruits. To monitor ethylene in plants, analytical methods, phenotypic assays, gene expression analysis and transcriptional or translational reporters are typically employed. In the model plant Arabidopsis, two ethylene-sensitive synthetic transcriptional reporters have been described, 5xEBS:GUS and 10x2EBS-S10:GUS. These reporters harbour a different type, arrangement and number of homotypic cis-elements in their promoters and thus may recruit the ethylene master regulator EIN3 in the context of alternative transcriptional complexes. Accordingly, the patterns of GUS activity in these transgenic lines differ and neither of them encompasses all plant tissues even in the presence of saturating levels of exogenous ethylene. Herein, we set out to develop and test a more sensitive version of the ethylene-inducible promoter that we refer to as EBSnew (abbreviated as EBSn). EBSn leverages a tandem of 10 non-identical, natural copies of a novel, dual, everted, 11 bp-long EIN3-binding site, 2EBS(-1). We show that in Arabidopsis, EBSn outperforms its predecessors in terms of its ethylene sensitivity, having the capacity to monitor endogenous levels of ethylene and displaying more ubiquitous expression in response to the exogenous hormone. We demonstrate that the EBSn promoter is also functional in tomato, opening new avenues to manipulating ethylene-regulated processes, such as ripening and senescence, in crops.