Several plant species require microbial associations for survival under different biotic and abiotic stresses. In this study, we show that Enterobacter sp. SA187, a desert plant endophytic bacterium, enhances yield of the crop plant alfalfa under field conditions as well as growth of the model plant Arabidopsis thaliana in vitro, revealing a high potential of SA187 as a biological solution for improving crop production. Studying the SA187 interaction with Arabidopsis, we uncovered a number of mechanisms related to the beneficial association of SA187 with plants. SA187 colonizes both the surface and inner tissues of Arabidopsis roots and shoots. SA187 induces salt stress tolerance by production of bacterial 2-keto-4-methylthiobutyric acid (KMBA), known to be converted into ethylene. By transcriptomic, genetic and pharmacological analyses, we show that the ethylene signaling pathway, but not plant ethylene production, is required for KMBA-induced plant salt stress tolerance. These results reveal a novel molecular communication process during the beneficial microbe-induced plant stress tolerance.

The plant hormone ethylene plays a pivotal role in virtually every aspect of plant development, including vegetative growth, fruit ripening, senescence, and abscission. Moreover, it acts as a primary defense signal during plant stress. Being a volatile, its immediate biosynthetic precursor, 1-aminocyclopropane-1-carboxylic acid, ACC, is generally employed as a tool to provoke ethylene responses. However, several reports propose a role for ACC in parallel or independently of ethylene signaling. In this study, pharmacological experiments with ethylene biosynthesis and signaling inhibitors, 2-aminoisobutyric acid and 1-methylcyclopropene, as well as mutant analyses demonstrate ACC-specific but ethylene-independent growth responses in both dark- and light-grown Arabidopsis seedlings. Detection of ethylene emanation in ethylene-deficient seedlings by means of laser-based photoacoustic spectroscopy further supports a signaling role for ACC. In view of these results, future studies employing ACC as a proxy for ethylene should consider ethylene-independent effects as well. The use of multiple knockout lines of ethylene biosynthesis genes will aid in the elucidation of the physiological roles of ACC as a signaling molecule in addition to its function as an ethylene precursor.

Background and aims Biological fixation of atmospheric nitrogen (N 2) is the main pathway for introducing N into unmanaged ecosystems. While recent estimates suggest that free-living N fixation (FLNF) accounts for the majority of N fixed in mature tropical forests, the controls governing this process are not completely understood. The aim of this study was to quantify FLNF rates and determine its drivers in two tropical pristine forests of French Guiana. Methods We used the acetylene reduction assay to measure FLNF rates at two sites, in two seasons and along three topographical positions, and used regression analyses to identify which edaphic explanatory variables,

Upon stress, a trade-off between plant growth and defense responses defines the capacity for survival. Stress can result in accumulation of misfolded proteins in the endoplasmic reticulum (ER) and other organelles. To cope with these proteotoxic effects, plants rely on the unfolded protein response (UPR). The involvement of reactive oxygen species (ROS), ethylene (ETH), and sugars, as well as their crosstalk, in general stress responses is well established, yet their role in UPR deserves further scrutiny. Here, a synopsis of current evidence for ROS-ETH-sugar crosstalk in UPR is discussed. We propose that this triad acts as a major signaling hub at the crossroads of survival and death, integrating information from ER, chloroplasts, and mitochondria, thereby facilitating a coordinated stress response. Coordinated Inter-Organelle Stress Responses Facilitate Plant SurvivalThe sessile nature of plants implies that they are inherently subject to changing environments. As such, they need to cope with a variety of (a)biotic stresses. These harmful conditions lead to a set of shared but also distinct responses that can include oxidative stress (see Glossary), osmotic or ionic imbalances, and changes in cellular components, all of which modify the physiological status. Growth and development are hindered under such conditions, either directly, for instance by oxidative damage of essential biomolecules, or indirectly, through reprogramming of energy metabolism. In particular, the functioning of chloroplasts and mitochondria, the 'powerhouses' of the cell, is disturbed upon stress. The associated changes in carbohydrate status and ultimately energy levels, affect growth, but probably also serve as important stress signals (Figure 1, Key Figure) [1]. As such, mitochondria and chloroplasts act as central hubs that integrate external and internal signals to coordinate growth [2-4].Importantly, stress perception and its downstream responses should be considered as contextdependent, and are influenced by the stress type, severity, and duration. Nevertheless, an integral aspect of stress is the accumulation of unfolded or misfolded proteins (i.e., proteotoxic stress) [5]. The ER is essential for protein folding and secretion and has different mechanisms for protein quality control (QC). However, once the amount of unfolded or misfolded proteins surpasses the level that can be controlled by the ERQC, cells have to cope with the cytotoxicity of hampered proteostasis, called ER stress. This also occurs in chloroplasts and mitochondria [6,7]. Restoration of organellar proteostasis requires responses from both the organelle and the nucleus, and depends on intricate crosstalk between subcellular compartments. Hence, a tight communication established via anterograde and retrograde signaling is necessary for coordinated gene expression to restore proteostasis (Box 1). Eukaryotes rely on the evolutionary conserved retrograde signaling pathway called the UPR, that initiates a series of transcriptional Highlights Proteotoxic stress, or the ...

mutants together with GR24 (strigolactone agonist) treatments. Importantly, we conducted a detailed mapping of adventitious root initiation along the hypocotyl and measured ethylene production in strigolactone mutants. ACC treatments resulted in a slight increase in adventitious root formation at low doses and a decrease at higher doses, in both wild-type and strigolactone mutants. Furthermore, the distribution of adventitious roots dramatically changed to the top third of the hypocotyl in a dose-dependent manner with ACC treatments in both wild-type and strigolactone mutants. The ethylene mutants all responded to treatments with GR24. Wild type and max4 (strigolactone-deficient mutant) produced the same amount of ethylene, while emanation from max2 (strigolactone-insensitive mutant) was lower. We conclude that strigolactones and ethylene act largely independently in regulating adventitious root formation with ethylene controlling the distribution of root initiation sites. This role for ethylene may have implications for Abstract Adventitious root formation is essential for cutting propagation of diverse species; however, until recently little was known about its regulation. Strigolactones and ethylene have both been shown to inhibit adventitious roots and it has been suggested that ethylene interacts with strigolactones in root hair elongation. We have investigated the interaction between strigolactones and ethylene in regulating adventitious root formation in intact seedlings of Arabidopsis thaliana. We used strigolactone mutants together with 1-aminocyclopropane-1-carboxylic acid (ACC) (ethylene precursor) treatments and ethylene Yuming Hu and Thomas Depaepe have contributed equally to this work.The original version of this article was revised: The name of the sixth author was misspelled. However, it has been corrected in this version. flood response because both ethylene and adventitious root development are crucial for flood tolerance. Electronic supplementary material

The simplest unsaturated hydrocarbon, ethylene or ethene, is one of the most widely produced organic chemicals worldwide. It serves as a building block for various materials and chemicals, including plastics, ethanol, detergents, and many more. Strikingly, it also acts as a signaling molecule in virtually all physiological processes and during all developmental stages in plant life. Plant biologists consider ethylene to have a tripartite role in plant development; this gaseous molecule can serve as a plant growth regulator, an aging hormone, and as a stress controller, aiding in defense against both biotic and abiotic stressors. Therefore, the regulation of the ethylene status is indispensable in both agricultural and horticultural practices. Since its discovery as a phytohormone, many chemicals have been developed that are able to affect ethylene responses in plants. Here, an extensive overview of the current toolbox of ethylene regulators, their discovery, function, and applications in both the agri‐ and horticultural field is presented. Furthermore, possibilities and considerations related to novel small molecules, such as those emerging from the chemical genetics field, are discussed.

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ACCERBATIN is an ethylene-mimicking small molecule that affects auxin homeostasis and ROS accumulation in etiolated seedlings. In light-grown plants, it exhibits auxin-like herbicidal properties.