Feedback loop promotes sucrose accumulation in cotyledons to facilitate sugar-ethylene signaling-mediated, etiolated-seedling greening

De-etiolation is indispensable for seedling survival and development. However, how sugars regulate de-etiolation and how sugars induce ethylene (ET) for seedlings to grow out of soil remain elusive. Here, we reveal how a sucrose (Suc) feedback loop promotes de-etiolation by inducing ET biosynthesis. Under darkness, Suc in germinating seeds preferentially induces 1-amino-cyclopropane-1-carboxylate synthase (ACS7; encoding a key ET biosynthesis enzyme) and associated ET biosynthesis, thereby activating ET core component ETHYLENE-INSENSITIVE3 (EIN3). Activated EIN3 directly inhibits the function of Suc transporter 2 (SUC2; a major Suc transporter) to block Suc export from cotyledons and thereby elevate Suc accumulation of cotyledons to induce ET. Under light, ET-activated EIN3 directly inhibits the function of phytochrome A (phyA; a de-etiolation inhibitor) to promote de-etiolation. We therefore propose that under darkness, the Suc feedback loop (Suc-ACS7-EIN3-|SUC2-Suc) promotes Suc accumulation in cotyledons to guarantee ET biosynthesis, facilitate de-etiolation, and enable seedlings to grow out of soil.

Shaping Ethylene Response: The Role of EIN3/EIL1 Transcription Factors

EIN3/EIL1 transcription factors are the key regulators of ethylene signaling that sustain a variety of plant responses to ethylene. Since ethylene regulates multiple aspects of plant development and stress responses, its signaling outcome needs proper modulation depending on the spatiotemporal and environmental conditions. In this review, we summarize recent advances on the molecular mechanisms that underlie EIN3/EIL1-directed ethylene signaling in Arabidopsis. We focus on the role of EIN3/EIL1 in tuning transcriptional regulation of ethylene response in time and space. Besides, we consider the role of EIN3/EIL1-independent regulation of ethylene signaling.

The Interplay of Chromatin Landscape and DNA-Binding Context Suggests Distinct Modes of EIN3 Regulation in Arabidopsis thaliana

The plant hormone ethylene regulates numerous developmental processes and stress responses. Ethylene signaling proceeds via a linear pathway, which activates transcription factor (TF) EIN3, a primary transcriptional regulator of ethylene response. EIN3 influences gene expression upon binding to a specific sequence in gene promoters. This interaction, however, might be considerably affected by additional co-factors. In this work, we perform whole genome bioinformatics study to identify the impact of epigenetic factors in EIN3 functioning. The analysis of publicly available ChIP-Seq data on EIN3 binding in Arabidopsis thaliana showed bimodality of distribution of EIN3 binding regions (EBRs) in gene promoters. Besides a sharp peak in close proximity to transcription start site, which is a common binding region for a wide variety of TFs, we found an additional extended peak in the distal promoter region. We characterized all EBRs with respect to the epigenetic status appealing to previously published genome-wide map of nine chromatin states in A. thaliana. We found that the implicit distal peak was associated with a specific chromatin state (referred to as chromatin state 4 in the primary source), which was just poorly represented in the pronounced proximal peak. Intriguingly, EBRs corresponding to this chromatin state 4 were significantly associated with ethylene response, unlike the others representing the overwhelming majority of EBRs related to the explicit proximal peak. Moreover, we found that specific EIN3 binding sequences predicted with previously described model were enriched in the EBRs mapped to the chromatin state 4, but not to the rest ones. These results allow us to conclude that the interplay of genetic and epigenetic factors might cause the distinct modes of EIN3 regulation.