Virginia Luna

Isolation and characterization of endophytic plant growth-promoting (PGPB) or stress homeostasis-regulating (PSHB) bacteria associated to the halophyte Prosopis strombulifera.

This study was designed to isolate and characterize endophytic bacteria from halophyte Prosopis strombulifera grown under extreme salinity and to evaluate in vitro the bacterial mechanisms related to plant growth promotion or stress homeostasis regulation. Isolates obtained from P. strombulifera were compared genotypically by BOX-polymerase chain reaction, grouped according to similarity, and identified by amplification and partial sequences of 16S DNAr. Isolates were grown until exponential growth phase to evaluate the atmospheric nitrogen fixation, phosphate solubilization, siderophores, and phytohormones, such as indole-3-acetic acid, zeatin, gibberellic acid and abscisic acid production, as well as antifungal, protease, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. A total of 29 endophytic strains were grouped into seven according to similarity. All bacteria were able to grow and to produce some phytohormone in chemically defined medium with or without addition of a nitrogen source. Only one was able to produce siderophores, and none of them solubilized phosphate. ACC deaminase activity was positive for six strains. Antifungal and protease activity were confirmed for two of them. In this work, we discuss the possible implications of these bacterial mechanisms on the plant growth promotion or homeostasis regulation in natural conditions.

Phytohormone production by three strains of Bradyrhizobium japonicum and possible physiological and technological implications.

The aim of this work was to evaluate phytohormone biosynthesis, siderophores production, and phosphate solubilization in three strains (E109, USDA110, and SEMIA5080) of Bradyrhizobium japonicum, most commonly used for inoculation of soybean and nonlegumes in USA, Canada, and South America. Siderophore production and phosphate solubilization were evaluated in selective culture conditions, which had negative results. Indole-3-acetic acid (IAA), gibberellic acid (GA3), and abscisic acid (ABA) production were analyzed by gas chromatography–mass spectrometry (GC-MS). Ethylene and zeatin biosynthesis were determined by GS–flame ionization detection and high-performance liquid chromatography (HPLC-UV), respectively. IAA, zeatin, and GA3 were found in all three strains; however, their levels were significantly higher (pu2009<u20090.01) in SEMIA5080 (3.8xa0μg ml−1), USDA110 (2.5xa0μg ml−1), and E109 (0.87xa0μg ml−1), respectively. ABA biosynthesis was detected only in USDA110 (0.019xa0μg ml−1). Ethylene was found in all three strains, with highest production rate (18.1xa0ng ml−1 h−1) in E109 cultured in yeast extract mannitol medium plus l-methionine. This is the first report of IAA, GA3, zeatin, ethylene, and ABA production by B. japonicum in pure cultures, using quantitative physicochemical methodology. The three strains have differential capability to produce the five major phytohormones and this fact may have an important technological implication for inoculant formulation.

Expression of Seed Dormancy in Grain Sorghum Lines with Contrasting Pre-Harvest Sprouting Behavior Involves Differential Regulation of Gibberellin Metabolism Genes

Grain sorghum [Sorghum bicolor (L) moench] exhibits intraspecific variability for the rate of dormancy release and pre-harvest sprouting behavior. Two inbred lines with contrasting sprouting response were compared: IS9530 (resistant) and RedlandB2 (susceptible). Precocious dormancy release in RedlandB2 is related to an early loss of embryo sensitivity to ABA and higher levels of gibberellins in imbibed grains as compared with IS9530. With the aim of identifying potential regulatory sites for gibberellin metabolism involved in the expression of dormancy in immature grains of both lines, we carried out a time course analysis of transcript levels of putative gibberellin metabolism genes and hormone content (GA(1), GA(4), GA(8) and GA(34)). A lower embryonic GA(4) level in dormant IS9530 was related to a sharp and transient induction of two SbGA2-oxidase (inactivation) genes. In contrast, these genes were not induced in less dormant RedlandB2, while expression of two SbGA20-oxidase (synthesis) genes increased together with active GA(4) levels before radicle protrusion. Embryonic levels of GA(4) and its catabolite GA(34) correlated negatively. Thus, in addition to the process of gibberellin synthesis, inactivation is also important in regulating GA(4) levels in immature grains. A negative regulation by gibberellins was observed for SbGA20ox2, SbGA2ox1 and SbGA2ox3 and also for SbGID1 encoding a gibberellin receptor. We propose that the coordinated regulation at the transcriptional level of several gibberellin metabolism genes identified in this work affects the balance between gibberellin synthesis and inactivation processes, controlling active GA(4) levels during the expression of dormancy in maturing sorghum grains.

Fluctuating temperatures terminate dormancy in Cynara cardunculus seeds by turning off ABA synthesis and reducing ABA signalling, but not stimulating GA synthesis or signalling

Fluctuating temperatures terminate seed dormancy in many species, including Cynara cardunculus (L.) var. sylvestris. Termination of physiological dormancy requires low ratios of abscisic acid (ABA)/gibberellins (GA). In a previous paper we have shown that physiological responses to fluctuating temperatures comprise a reduction of abscisic acid (ABA) content and sensitivity. However, a possible stimulation of GA synthesis was also suggested as part of the mechanism. That possible stimulation, as well as the identification of potential regulatory sites for ABA and GA metabolism and signalling involved in the termination of dormancy by fluctuating temperatures, are yet to be determined. In this study, we measured GA content and sensitivity in seeds incubated under constant and fluctuating temperatures. We also assessed the expression of several genes involved in ABA and GA metabolism and signalling. Our results show that fluctuating temperatures reduce ABA/GA ratios through a reduction in ABA accumulation during incubation but without altering GA synthesis as compared to that observed under constant temperatures. On the other hand, fluctuating temperatures did not increase sensitivity to GA. Fluctuating temperatures reduced the expression of CycaNCED and CycaABI5 (ABA synthesis and signalling genes) with a temporal pattern that coincides with the interruption of ABA accumulation that precedes germination of seeds incubated under fluctuating temperatures. However, fluctuating temperatures did not modify the expression of CycaCYP707A2 (ABA inactivation) as compared to that observed under constant temperatures. Consistent with our determinations of GA content and sensitivity, fluctuating temperatures did not modify the expression of GA synthesis (CycaGA3ox )a nd signalling genes (CycaRGL2 and CycaGAI) in relation to that observed at constant temperatures. These results show that fluctuating temperatures terminate dormancy in Cynara cardunculus seeds through an interruption in ABA accumulation and a reduction in ABA signalling exerted at the level of CycaNCED and CycaABI5 expression.

Basic and Technological Aspects of Phytohormone Production by Microorganisms: Azospirillum sp. as a Model of Plant Growth Promoting Rhizobacteria

The functional analysis of phytohormones production, interaction, and regulation in higher plant has re-emerged in the last 10 years due to incredible advances in integrative study models; however, plants are not alone in natural conditions and usually are colonized or influenced directly by different types of microorganisms such as rhizobacteria many of which have the ability to produce phytohormones. This chapter concentrates information related to the biosynthesis, metabolism, regulation, physiological role, and agronomical impact of phytohormones produced by microorganisms with an emphasis on the plant growth promoting rhizobacteria belonging to the Azospirillum genus, considered to be one of the most representative and worldwide studied bacterial genera during the last 30 years. This review contains exhaustive information about the phytohormones auxins, gibberellins, cytokinins, ethylene, abscisic acid and jasmonates, as well as the plant growth regulators polyamines and nitric oxide; biosynthesized or metabolized by Azospirillum sp. and other growth promoting rhizobacteria in chemically defined medium or plant interaction conditions.

Tomato mutants sensitive to abiotic stress display different abscisic acid content and metabolism during germination

Laboratorio de Fisiologi´a Vegetal, Departamento de Ciencias Naturales, Facultad de Ciencias Exactas,Fi´sico-Qui´micas y Naturales, Universidad Nacional de Ri´o Cuarto, 5800 Ri´o Cuarto, ArgentinaAbstractWe report the determination of abscisic acid (ABA)and its metabolites, phaseic acid (PA), dihydropha-seic acid (DPA) and ABA glucose ester (ABA-GE),in non-dormant dry and imbibed seeds of tomato(Solanum lycopersicum Mill.) cv. Moneymaker (wildtype), and its tss1, tss2 and tos1 mutants. High ABA indryseedsmayoriginate from ABAaccumulation inthesheath tissue, which was in contact with an ABA-containing medium, the endocarpus. The highestgermination percentages at 72h, observed in tss1and tss2, coincided with minimal ABA content. Wild-type and mutant seeds showed different ABA andcatabolic patterns, and these were correlated withtheir sensitivity to abiotic stress. Whereas dry seedsshowed a high basal ABA, imbibed seeds showedhigher ABA metabolite content, particularly DPA. Thedramatic decrease of ABA following seed imbibitionsuggests an activation of ABA catabolism during theearlystagesofthegerminationprocess.Theobservedvariation of ABA metabolites among dry and imbibedseeds of Solanum lycopersicum cv. Moneymaker andits tss1, tss2 and tos1 mutants shows that ABAmetabolism is differentially regulated in thesegenotypes.Keywords: abiotic stress, abscisic acid, abscisic acidmetabolites, germination, Solanum lycopersicum,tomato mutantsIntroductionSeveral metabolites of abscisic acid (ABA) havebeen isolated from various plant materials. ABAcatabolism proceeds through two major pathways: the‘oxidative pathway’ involving oxidation at differentpositions, and the ‘conjugation pathway’. The path-way used depends on the plant species, develop-mental stage or tissue type. The oxidative pathway,considered the more common in plant catabolism(Nambara and Marion-Poll, 2005), is initiated byhydroxylation at C-8