Editorial: Ethylene Biology and Beyond: Novel Insights in the Ethylene Pathway and Its Interactions

摘要

This Research Topic presents selected contributions to Ethylene 2018 , the XI International Symposium on the Plant Hormone Ethylene , held in Chania, Greece, on 2nd?6th June 2018, covering exciting new discoveries in the ethylene field. This issue brings novel insights in ethylene signaling in bacteria, algae, and lower plants as well as evidence supporting a specific role for the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) in plants. Ethylene receptors were initially thought to be specific to plants. Interestingly, ethylene receptor homologs have been found in cyanobacteria. It was previously shown that in Synechocystis sp. an ethylene receptor regulates phototaxis and biofilm formation. In this issue, the same group ( Allen et al. ) demonstrates that another cyanobacterium, the filamentous species Geitlerinema sp. which possesses two ethylene receptor homologs, similarly displays ethylene-dependent alterations in phototaxis, suggesting that such signaling could be prevalent in cyanobacteria. Both species are highly sensitive to ethylene although their ethylene binding characteristics resemble that of plants, thus suggesting signal amplification in cyanobacteria. In higher plants, subfamily-I of the Arabidopsis receptors (ETR1 and ERS1) and their interactions with downstream players have been extensively studied, while information on subfamily-II (ETR2, ERS2, and EIN4) remains sparse. Here, Berleth et al. demonstrate that ETR2 displays comparable affinities for CTR1 and EIN2 to that previously reported for ETR1, suggesting similar protein-protein interaction-mediated signal transfer for both subfamilies. In addition, the authors show enhanced stability of type-II receptor homomers and type-II:type-I heteromers as compared to type-I homomers, emphasizing the importance of type-II receptors. Wang and Qiao present a mini-review on the transcriptional regulation of the ethylene response in Arabidopsis. Ethylene signaling involves numerous regulatory steps leading to a diversity of responses in plant growth and development. This review discusses our current understanding of the transcriptional regulation as a major control point in ethylene signaling, focusing on recent insights into the role of chromatin modification in repressing transcription. Dolgikh et al. summarize recent advances on the molecular mechanisms that underlie EIN3/EIL1-directed ethylene signaling in Arabidopsis, and focus on the role of EIN3/EIL in tuning transcriptional regulation of ethylene response in time and space. Furthermore, they consider the role of EIN3/EIL1-independent regulation of ethylene signaling. Qin et al. review hormonal crosstalk of ethylene in primary root growth of Arabidopsis and rice. Based on the proposed model, ethylene restricts primary root growth by governing auxin biosynthesis, transport, and signaling through EIN3/EIL1, ERF1, and HB52 interactions in Arabidopsis. ABA and CKs constrain primary root growth by controlling the posttranscriptional regulation of ACS resulting in stimulated ethylene synthesis. GA and ethylene antagonistically regulate stability of DELLA proteins, which act as growth repressors. Low levels of BRs hinder ethylene synthesis by BZR1 and BES1 suppressed ACS gene expression, while high levels of BRs increase ACS stability. Through a different mechanism, ethylene restricts primary root growth in rice, by augmenting auxin and ABA biosynthesis. Understanding light-dependent differences in ethylene synthesis and signaling is essential to expand our insight into the roles of ethylene in growth and development across the plant life cycle. Harkey et al. performed a meta-analysis of multiple transcriptomic datasets to uncover responses to ethylene that are both light-dependent and light-independent. A set of 139 transcripts with robust and consistent responses to elevated ethylene across root-specific datasets was identified. This “gold standard” group of ethylene-regulated transcripts includes numerous genes encoding proteins that function in ethylene signaling and synthesis. The study further reveals a number of previously uncharacterized factors that may contribute to ethylene response phenotypes. Plants synthesize ethylene in a two-step reaction starting with the conversion of S-adenosyl-L-methionine (SAM) to 1-aminocyclopropane-1-carboxylic acid (ACC). Vanderstraeten et al. demonstrated that ACC affects early vegetative development independently of ethylene signaling. Using ethylene biosynthesis and signaling inhibitors, as well as mutants, ACC-specific ethylene-independent growth responses in both dark- and light-grown Arabidopsis seedlings were revealed. Hence, researchers employing ACC as an ethylene precursor should be mindful of putative ACC effects confounding ethylene responses in vegetative growth. The exact mechanism underlying the ACC response remains to be identified. While ACC may play an ethylene-independent role in plant growth and development, the three-membe