New Insights in Transcriptional Regulation of the Ethylene Response in Arabidopsis

Ethylene Signaling in Regulating Plant Growth, Development, and Stress Responses

Ethylene is a gaseous plant hormone that plays a crucial role in coordinating various physiological processes in plants. It acts as a key mediator, integrating both endogenous developmental cues and external environmental signals to regulate a wide range of functions, including growth, fruit ripening, leaf abscission, and responses to stress. The signaling pathway is initiated when ethylene binds to its receptor. After decades of research, the key components of ethylene signaling have been identified and characterized. Although the molecular mechanisms of the sensing of ethylene signal and its transduction have been studied extensively, a new area of research is how respiration and epigenetic modifications influence ethylene signaling and ethylene response. Here, we summarize the research progress in recent years and review the function and importance of ethylene signaling in plant growth and stress responses. In addition, we also describe the current understanding of how epigenetic modifications regulate ethylene signaling and the ethylene response. Together, our review sheds light on the new signaling mechanisms of ethylene.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

The CALCINEURIN B-LIKE 4/CBL-INTERACTING PROTEIN 3 module degrades repressor JAZ5 during rose petal senescence

Flower senescence is genetically regulated and developmentally controlled. The phytohormone ethylene induces flower senescence in rose (Rosa hybrida), but the underlying signaling network is not well understood. Given that calcium regulates senescence in animals and plants, we explored the role of calcium in petal senescence. Here, we report that the expression of calcineurin B-like protein 4 (RhCBL4), which encodes a calcium receptor, is induced by senescence and ethylene signaling in rose petals. RhCBL4 interacts with CBL-interacting protein kinase 3 (RhCIPK3), and both positively regulate petal senescence. Furthermore, we determined that RhCIPK3 interacts with the jasmonic acid response repressor jasmonate ZIM-domain 5 (RhJAZ5). RhCIPK3 phosphorylates RhJAZ5 and promotes its degradation in the presence of ethylene. Our results reveal that the RhCBL4-RhCIPK3-RhJAZ5 module mediates ethylene-regulated petal senescence. These findings provide insights into flower senescence, which may facilitate innovations in postharvest technology for extending rose flower longevity.

Histone H3K4 methyltransferase DcATX1 promotes ethylene induced petal senescence in carnation

Petal senescence is controlled by a complex regulatory network. Epigenetic regulation like histone modification influences chromatin state and gene expression. However, the involvement of histone methylation in regulating petal senescence remains poorly understood. Here, we found that the trimethylation of histone H3 at Lysine 4 (H3K4me3) is increased during ethylene-induced petal senescence in carnation (Dianthus caryophyllus L.). H3K4me3 levels were positively associated with the expression of transcription factor DcWRKY75, ethylene biosynthetic genes 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (DcACS1), and ACC oxidase (DcACO1), and senescence associated genes (SAGs) DcSAG12 and DcSAG29. Further, we identified that carnation ARABIDOPSIS HOMOLOG OF TRITHORAX1 (DcATX1) encodes a histone lysine methyltransferase which can methylate H3K4. Knockdown of DcATX1 delayed ethylene-induced petal senescence in carnation, which was associated with the down-regulated expression of DcWRKY75, DcACO1, and DcSAG12, whereas overexpression of DcATX1 exhibited the opposite effects. DcATX1 promoted the transcription of DcWRKY75, DcACO1, and DcSAG12 by elevating the H3K4me3 levels within their promoters. Overall, our results demonstrate that DcATX1 is a H3K4 methyltransferase that promotes the expression of DcWRKY75, DcACO1, DcSAG12 and potentially other downstream target genes by regulating H3K4me3 levels, thereby accelerating ethylene-induced petal senescence in carnation. This study further indicates that epigenetic regulation is important for plant senescence processes.

Chemical induction of leaf senescence and powdery mildew resistance involves ethylene-mediated chlorophyll degradation and ROS metabolism in cucumber

Timely initiation of leaf senescence is an integral part of plant development and, importantly, an adaptive strategy by which plants cope with various stresses, e.g. to limit the spread of pathogens. Powdery mildew is a major cucumber disease that promotes the initiation/progression of leaf senescence and reduces leaf photosynthesis, resulting in severe losses of yield and quality. However, how powdery mildew induces leaf senescence and how cucumber plants respond to enhance their resistance remain unclear. Here, with established agrochemical induction and pathogen inoculation systems, we demonstrate that both probenazole (PBZ) and powdery mildew activate ethylene (ET) biosynthesis and signal transduction, consequently promoting leaf senescence and enhancing plant resistance to powdery mildew through CsEIN3 to directly upregulate the expression of CsCCGs and CsRBOHs. Our analysis convincingly suggests that the regulation of leaf senescence and powdery mildew resistance is interconnected and mediated mainly by ET in cucumber.

Contributions of sugar transporters to crop yield and fruit quality

The flux, distribution, and storage of soluble sugars regulate crop yield in terms of starch, oil, protein, and total carbohydrates, and affect the quality of many horticultural products. Sugar transporters contribute to phloem loading and unloading. The mechanisms of phloem loading have been studied in detail, but the complex and diverse mechanisms of phloem unloading and sugar storage in sink organs are less explored. Unloading and subsequent transport mechanisms for carbohydrates vary in different sink organs. Analyzing the transport and storage mechanisms of carbohydrates in important storage organs, such as cereal seeds, fruits, or stems of sugarcane, will provide information for genetic improvements to increase crop yield and fruit quality. This review discusses current research progress on sugar transporters involved in carbohydrate unloading and storage in sink organs. The roles of sugar transporters in crop yield and the accumulation of sugars are also discussed to highlight their contribution to efficient breeding.

Histone acetylation: a requirement for petunia floral scent

This article comments on: Patrick RM, Huang X-Q, Dudareva N, Li Y. 2021. Dynamic histone acetylation in floral volatile synthesis and emission in petunia flowers. Journal of Experimental Botany 72, 3704–3722.

EIN2-directed histone acetylation requires EIN3-mediated positive feedback regulation in response to ethylene

Ethylene is an important phytohormone with pleotropic roles in plant growth, development, and stress responses. ETHYLENE INSENSITIVE2 (EIN2) mediates the transduction of the ethylene signal from the endoplasmic reticulum membrane to the nucleus, where its C-terminus (EIN2-C) regulates histone acetylation to mediate transcriptional regulation by EIN3. However, no direct interaction between EIN2-C and EIN3 has been detected. To determine how EIN2-C and EIN3 act together, we followed a synthetic approach and engineered a chimeric EIN2-C with EIN3 DNA-binding activity but lacking its transactivation activity (EIN2C-EIN3DB). The overexpression of EIN2C-EIN3DB in either wild-type or in the ethylene-insensitive mutant ein3-1 eil1-1 led to a partial constitutive ethylene response. Chromatin immunoprecipitation sequencing showed that EIN2C-EIN3DB has DNA-binding activity, indicating that EIN3DB is functional in EIN2C-EIN3DB. Furthermore, native EIN3 protein levels determine EIN2C-EIN3DB binding activity and binding targets in a positive feedback loop by interacting with EIN2C-EIN3DB to form a heterodimer. Additionally, although EIN3 does not direct affect histone acetylation levels in the absence of EIN2, it is required for the ethylene-induced elevation of H3K14Ac and H3K23Ac in the presence of EIN2. Together, we reveal efficient and specific DNA-binding by dimerized EIN3 in the presence of ethylene to mediate positive feedback regulation, which is required for EIN2-directed elevation of histone acetylation to integrate into an EIN3-dependent transcriptional activation.

Comparative Study of Several Fe Deficiency Responses in the Arabidopsis thaliana Ethylene Insensitive Mutants ein2-1 and ein2-5

Iron (Fe) is an essential micronutrient for plants since it participates in essential processes such as photosynthesis, respiration and nitrogen assimilation. Fe is an abundant element in most soils, but its availability for plants is low, especially in calcareous soils. Fe deficiency causes Fe chlorosis, which can affect the productivity of the affected crops. Plants favor Fe acquisition by developing morphological and physiological responses in their roots. Ethylene (ET) and nitric oxide (NO) have been involved in the induction of Fe deficiency responses in dicot (Strategy I) plants, such as Arabidopsis. In this work, we have conducted a comparative study on the development of subapical root hairs, of the expression of the main Fe acquisition genes FRO2 and IRT1, and of the master transcription factor FIT, in two Arabidopsis thaliana ET insensitive mutants, ein2-1 and ein2-5, affected in EIN2, a critical component of the ET transduction pathway. The results obtained show that both mutants do not induce subapical root hairs either under Fe deficiency or upon treatments with the ET precursor 1-aminocyclopropane-1-carboxylate (ACC) and the NO donor S-nitrosoglutathione (GSNO). By contrast, both of them upregulate the Fe acquisition genes FRO2 and IRT1 (and FIT) under Fe deficiency. However, the upregulation was different when the mutants were exposed to ET [ACC and cobalt (Co), an ET synthesis inhibitor] and GSNO treatments. All these results clearly support the participation of ET and NO, through EIN2, in the regulation of subapical root hairs and Fe acquisition genes. The results will be discussed, taking into account the role of both ET and NO in the regulation of Fe deficiency responses.

Chromatin regulation in plant hormone and plant stress responses

The gene expression is tightly regulated temporally and spatially to ensure the plant and animal proper development, function, growth, and survival under different environmental conditions. Chromatin regulation plays a central role in the gene expression by providing transcription factors and the transcription machinery with dynamic access to an otherwise tightly packaged genome. In this review, we will summarize recent progress in understanding the roles of chromatin regulation in the gene expression, and their contribution to the plant hormone and stress responses. We highlight the most recent publications within this topic and underline the roles of chromatin regulation in gene expression.

Combating Spring Frost With Ethylene

The sustainable fruit production in temperate and boreal regions is often imperiled by spring frosts. The risk of frost damage and the resulting economic losses have been increasing in the recent years as a result of the global climate change. Among the many approaches in mitigating frost damages, an ethylene-based compound, ethephon has proven to be effective in delaying bloom time in many fruit species and, thereby, avoid frost damage. However, effective concentrations of ethephon are often associated with harmful effects on fruit trees, which largely limit its use. Relatively, limited research attention has been given to understand the mechanisms underlying this ethylene-mediated bloom delay, thus hindering the progress in exploring its potential in frost protection. Recent advances in omics and bioinformatics have facilitated the identification of critical molecular and biochemical pathways that govern the progression of bud dormancy in deciduous woody perennials. In this review, we summarized our current understanding of the function of ethylene and its interaction with other networks in modulating dormancy and blooming in temperate fruit trees. Some possible mechanisms are also proposed that might potentially guide future studies attempting to decipher the dormancy regulation or searching for methods to alleviate frost damages.

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.