A Model For the Lowering of Plant Ethylene Concentrations by Plant Growth-promoting Bacteria

“sativa treated with ethylene syn- plant roots or seeds can take up some of the ACC thesis and perception inhibitors, aminoethox- exuded by the plant and degrade it through the action yvinylglycine (AVG) and silver ions, respectively of ACC deaminase (Glick et al. 1998).”

“Plant growthpromoting bacteria containing ACC deaminase and attached to the surface of plant roots or seeds can take up and degrade some of the ACC exuded from the plant by acting as a sink for ACC and therefore decrease ethylene synthesis in plants ( 18 , 40).”

“Inoculation with rhizobacteria having ACC deaminase activity resulted in the development of a better root system, which subsequently had a positive effect on shoot growth (Glick et al., 1998; Belimov et al., 2002).”

“…N2 fixation, synthesis of ACC (1-aminocyclopropane-1-carboxylate) deaminase, phosphate solubilization and also through the production of antimicrobial metabolites or siderophores to inhibit pathogenic microorganisms (Xie et al. 1996; Bashan et al. 1991; Glick et al. 1998; Leong 1986).”

“... phytohormone production, plant growth promotion is known to be mediated by a variety of mechanisms including phosphate solubilization (Valverde et al. 2006), siderophore production (Katiyar and Goel 2004), antagonism toward deleterious root microorganisms (Misaghi et al. 1982), deamination of the precursor molecul…”

“…microbial auxin production on root exudation of amino acids seems not to have been studied, even though it was hypothesized that microbial-derived IAA stimulated the exudation of ACC (an amino acid and the immediate precursor of the phytohormone ethylene) into the rhizosphere (Glick et al., 1998).”

“Much attention has focused on rhizobacteria containing the enzyme ACC deaminase (ACCd) that degrade the ethylene precursor ACC (Glick et al., 1998).”

“down ACC, the direct precursor of ethylene (Glick et al. 1998; Penrose et al. 2001).”

“Moreover, the PGPR also contain enzymes that solubilize minerals which regulate plant growth (Glick et al., 1998; Ma et al., 2009; Oves et al., 2013a).”

“Such rhizosphere bacteria can alleviate stress caused by ethylene due to flooding, salination, drought, heavy metals, toxic organic compounds and pathogens [27, 62-64].”

“fluorescens Maize Improved growth, yield, and nutrition [78]”

“The use of PGPB endophytes with ACC deaminase activity has the potential to replace the chemicals that are currently used by the cut flower industry.”

“Moreover, the high level of ACC synthesized by the plant under stress can activate the production of ACC deaminase by the bacterial strain and therefore increase plant tolerance against stress (Glick et al. 1998).”

“To our knowledge, this is the first attempt to measure rhizosphere ACC concentrations following bacterial inoculation ex vitro, even though these bacteria are postulated to decrease rhizosphere ACC concentrations by taking up ACC as a nutrient source (Glick et al., 1998).”

“However, although certain aspects of plant-bacterial interactions have also been studied in detail, the concept of microorganism-plant interaction to alleviate stress is not well understood, except for mechanisms such as bacterial ACC deaminase (Glick et al. 1998).”

“Pseudomonas brassicacearum strain 520-1, a bacterium described previously as phytopathogenic for tomato (Sikorski et al., 2001), possesses ACC deaminase activity, an important mechanism of plant growth promotion by beneficial root-associated bacteria (Glick et al., 1998; Van Loon and Glick, 2004).”

“ACC-deaminase activity lowers the levels of stress ethylene improving plant growth in stress conditions (Glick et al., 1998; Contesto et al., 2008; Tsuchisaka et al., 2009; Bulgarelli et al., 2013).”

“In this sense, it is well known that ethylene is the hormone controlling the expression of ripening genes (Alexander and Grierson, 2002), and it is implicated in the plant growth promotion (Glick et al., 1998).”

“Plant growth-promoting bacteria (PGPR) communities naturally existing in the rhizosphere is another extremely important tool in decontamination of metal-contaminated soils (Glick et al., 1998; Idris et al., 2004).”

“Glick et al. (1998) proposed a model that explained the mechanism of the lowering of ACC by these PGPR.”

“ACC deaminase producing bacteria have been reported to prevent the inhibition of root elongation by decreasing the level of growth-limiting ethylene through hydrolytic cleavage (deaminase activity) of the ethylene biosynthesis precursor ACC (Adams and Yang, 1979; Glick et al., 1998).”

“Pioneering studies of the stress ethylene model suggested that bacterial ACC deaminase is induced by increasing amounts of its substrate, ACC (Glick et al., 1998).”

“…target the modulation of plant stress genes (Timmusk and Wagner, 1999) or the reduction of the stress hormone ethylene levels through degradation of its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) by the bacterial enzyme ACC deaminase (Glick et al., 1998; Mayak et al., 2004; Glick, 2005).”

“The possibility of a close mutualistic relationship between the plants and the soil bacterium has been suggested, and the role of ACCD in ensuring low levels of ethylene at critical stages of root growth has been proposed [16].”

“[8] also found that the chlorophyll content in the shoot of the canola plant increased when inoculated with Pseudomonas putida G12-2.”

“The IAA released by bacteria enhances plant growth directly by stimulating elongation of the cell or affecting cell division (Glick et al. 1998).”

“…the 1- aminocyclopropane-1-carboxylic acid deaminase gene, which directly stimulates plant growth by cleaving the immediate precursor of plant ethylene (Glick et al., 1998) into P. fluorescens CHAO, not only increases plant growth but can also increase BC properties of PGPB (Wang et al., 2000).”

“This in turn, would lead to more ACC exudation from inside the plant to maintain the equilibrium thus reducing ACC and the amount of ethylene evolved by the plant ( Glick et al. 1998 ).”

“PGPR containing the enzyme ACC deaminase decrease the ethylene level, enhance the survival of some seedlings, and improve root growth and development in various plants such as tomato, pepper, and bean under stressed conditions (Glick et al. 1998; Mayak et al. 2004; Nadeem et al. 2009).”

“under unfavorable conditions and confers resistance to stress, by catalyzing the conversion of ACC into ammonia and α-ketobutyrate (Glick et al. 1998).”

“…Dodd et al., 2010), including auxins (e.g., indole-3-acetic acid; Patten and Glick, 1996; Omer et al., 2004), cytokinins (e.g., zeatin; Akiyoshi et al., 1984; Ivanova et al., 2000; Mok and Mok, 2001), ethylene (Glick et al., 1998; Meldau et al., 2012), and nitric oxide (Stohr and Ullrich, 2002).”

“This differential pattern response is in favor of the hypothesis proposed earlier according to which the AcdS activity of rhizobacteria decreases ethylene production in root by diverting ACC [15].”

“It was previously suggested that plant growth-promoting bacteria that express ACC deaminase act as a sink for ACC, and therein provide one of the major mechanisms that these bacteria use to promote plant growth (Glick et al. 1998).”

“, 2001) and mitigated ethylene’s inhibitory effects on shoot growth (Glick et al., 1998; Belimov et al., 2001, 2009a; Glick, 2005).”

“The production of ACC deaminase, an enzyme that modulates ethylene levels in plants (Glick et al., 1998), may further contribute to the heavy metal tolerance of plants.”

“…that direct length promotion may have been due to cell elongation as a result in ACC deaminase activity reducing ethylene levels in the plants and maximizing plant growth (Abeles et al., 1992; Glick et al., 1997; Penrose and Glick, 2001) by ACC deaminase producing strains L111, L117, and L321.”

“[80] proposed that the synthesis of indole acetic acid (IAA) by PGPR stimulates ACC synthase enzyme that converts S-adenosylmethionine to ACC.”

“1-Aminocyclopropane-1-Carboxylate Deaminase (ACC) The ACC, produced by endophytic bacteria, is linked with an alleviation of plant stress as it contributes to lowering the ethylene level hence promoting the plant growth (Glick et al., 1998).”

“Therefore, the IAA increase in planta was correlated with bacterial ACC deaminase activities but not correlated with bacterial IAA production in the medium [49].”

“This is done by the activation of plants’ precursor of ethylene, called S-adenosyl methionine, which is further converted to 1-amino-cyclopropane-1-carboxylic acid (ACC) by the ACC-synthase pathway, followed by conversion to ethylene by the enzyme ACC-oxidase [81].”

“Overproduction of ethylene significantly reduces the root and shoot development (Glick et al., 1998).”

“…(C) and N source), decreasing root ACC concentration and accumulation of root ethylene (a potent plant stress hormone), thus increasing root growth (Glick et al. 1998) and attenuating longdistance ACC signalling, thus generating a systemic effect on plant growth and functioning (Belimov et al.…”

“For instance, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase produced by endophytes lowers the ethylene levels in host plants, reducing their response to abiotic and biotic stress, and by changing root morphology, leading to stimulation of plant growth [13,23,24].”

“The enzyme catalyzes the degradation of ACC (the precursor of the phytohormone ethylene) into K-ketobutyrate and ammonium [55,56].”

“The stress usually induces ethylene emission (Morgan and Drew, 1997) and PGPR play a major role in reducing ethylene in plants via the action of ACC deaminase that cleaves ACC to α-ketobutyrate and ammonia (Glick et al., 1998).”

“One of the major mechanisms that plant growth-promoting bacteria use to facilitate the growth of plants involves the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase (Glick 1995; Glick et al. 1998, 1999 ).”

“Relatively recently, it was discovered that many plant growth-promoting bacteria contain the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase and that this enzyme can cleave the ethylene precursor ACC to -ketobutyrate and ammonia and thereby lower the level of ethylene in developing or stressed plants (5, 6, 7…”

“ACC deaminase activity is a common characteristic of PGPEs, which promotes plant growth and eases plant stress by reducing the ethylene level via degrading ACC (the precursor of ethylene) to ammonia and ∝-ketobutyrate [51–54].”

“ACCDproducing bacteria convert the ethylene precursor ACC into α-ketobutyrate and ammonia, thus reducing the endogenous level of stress hormone ethylene which modulates plant tolerance to environmental stresses and improves plant growth (Glick et al., 1998; Poel and Straeten, 2014).”

“Bacteria that have ACC deaminase can cleave ACC to ammonia and α-ketobutyrate, and thus prevent the effects of high levels of ethylene [ 15 , 19].”

“Glick et al. concluded from model prediction analysis that almost all PGPR have ACC deaminase activity [108].”

“This root elongation has been linked to the bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which can degrade ACC and reduce the ethylene level in the vicinity, or in plant roots (Glick et al., 1998; Li et al., 2000).”

“Thirdly, Rs-5 was isolated by the criteria of utilizing ACC as the sole nitrogen source, so it should have ACC deaminase activity and should be able to metabolize ACC, a precursor of ethylene created in plants in response to stress (Glick et al. 1998; Shah et al. 1998).”

“…to plant cells, act as a sink for plant ACC to uptake and cleave the ACC secreted by plant cells and thus reduce plant ACC concentration, ethylene evolution and the extent of ethylene inhibition of plant growth, particularly, under a variety of abiotic and biotic stresses (Glick et al. 1998).”

“siderophore is beneficial to plants in compensating for the iron deficiency [27].”