Academic literature on the topic 'Plant growth promoting rhizobacteria (PGPR)'

Plant growth promoting rhizobacteria (PGPR) colonise plant roots and exert beneficial effects on plant growth and development. The mechanisms of action of these PGPR are not conclusively known, however, there is evidence for the role of indole-3-acetic acid (IAA) and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase production by rhizobacteria in plant growth promotion. In this study, novel-PGPR were isolated from the rhizosphere of native species as well as agricultural crop species, as opposed to other work in this field in which potential PGPR are isolated from the rhizosphere of the target plant species. One hundred and sixty six bacteria were isolated from four rhizosphere soils in Western Australia and 72 isolates were assayed for the production of IAA. In the presence of the auxin precursor L-tryptophan (L-TRP) IAA production ranged from 0-37 1-Lg/ml. Five rhizosphere soils were screened for bacteria capable of utilizing ACC as a sole nitrogen source and 13 isolates were obtained. To ensure that the isolates were not potentially deleterious to host plants, 14 IAA producing (IAA-PGPR) and all rhizobacteria capable of using ACC as a sole nitrogen source (ACC-PGPR) were tested for their effects on germinating clover and wheat seedlings. Two IAA-PGPR isolates, NCH7 and PMK4, were inhibitory to wheat seedling germination and one ACC-PGPR isolate was inhibitory to clover seedlings. Based on these findings, 6 IAA-PGPR and 4 ACC-PGPR were screened for their effects on germinating wheat seedlings in gnotobiotic growth pouch assays. Prior to these tests, spontaneous rifampicin resistant mutants were generated for 6 isolates. The mutants, or the wild type isolates where rifampicin mutants were not generated, were (re)tested for their ability to produce IAA and utilize ACC. All 10 isolates produced IAA in the presence of L-TRP ranging from 0.11-2.97 1-Lg IAA/ 1-Lg cellular protein and 7 of the isolates grew on ACC amended medium. Bacterial growth was greatly increased in some isolates in the L-TRP amended media used in the auxin assay, suggesting some of the isolates have a requirement for tryptophan for optimal growth. The largest increases in root lengths in the gnotobiotic growth pouch assays were observed for seed treated with thhe ACC-PGPR, AWMK3 (81% increase). The IAA-PGPR treatments that increased root lengths were PMK4R (76%), WMK10R (66%) and NCH45 (33%). Increases in shoot lengths were recorded for seed treated with isolates WMK10R (42%), AWMK3 (11%), APMK2R (9%) and PMK9 (9%). A reduction in germination was observed in seed treated with some isolates, particularly PMK4R and WMK10R, which reduced germination by 34% and 20%, respectively. Five of the PGPR isolated in this study were tested in the field on 2 wheat cultivars at 3 locations in Konjonup and Wongan Hills and as a co-inoculant with a commercial rhizobial strain on peas at Kojonup. All the PGPR were delivered in the field using the clay based A1osca™ carrier technology. The increases in yields in response to the inoculation with the PGPR on peas and wheat were small and not significantly different from the controls. However, the yield of wheat was improved by four of the PGPR (NCH45, NCH54, PMK9, WMK10) at the Wongan Hills heavy soil site by 2 to 23% and by NCH54 and PMK9 at the Wongan Hills light soil site by 4% and 3%, respectively compared with the uninoculated controls. On the peas at Kojonup, nodulation was improved with the isolate PMK4 and these plots were visually more vigorous than the other treatments, however this growth was not significant. At harvest, four of the PGPR (NCH45, PMK4, PMK9 WMK1 0) improved pea yields compared to the Alosca™ control by 6-13%. These results suggest that further testing is warranted. Improvements to experimental design and sampling have been recommended to allow for the detection of statistically significant small percentage increases if they occur. The findings in this study demonstrate that novel PGPR can be isolated from non-target as well as target plant species and that the screening of rhizobacteria based on their in vitro auxin production and growth promoting effects in growth pouch assays is valid for the selection of effective PGPR.

Wastewater or fish effluent (FE) from freshwater aquaculture can be a good and cheap liquid fertiliser for plants. However, while it represents a good source of nutrients to support plant growth in a system called aquaponics, it appears that its use needs to be optimised to take full advantage of the potential benefits. Apart from mineral amendments, the use of beneficial microorganisms that can have a direct impact on plant growth and nutrient utilisation could be a promising option. Plant growth promoting rhizobacteria (PGPR) are a group of rhizospheric bacteria, when introduced in association with the host plant at optimum density, can enhance plant growth and health. One well-known and versatile PGPR is Azospirillum brasilense that has numerous beneficial effects on plants. The production of phytohormones by the bacterium has been proposed as one of the major mechanisms responsible for the plant growth promoting effects observed in plants inoculated with Azospirillum. Hence, this PGPR could be a valuable input in vegetable production under an aquaponics system. In addition, despite the widespread studies conducted with this PGPR in various crops, there is no published report on vegetables fertilised with fish effluent or under an aquaponics system. This study focuses on evaluating the role of PGPR, particularly A. brasilense, on the growth and development of selected vegetable crops fertilised with fish effluent and using an aquaponics system. Strains of A. brasilense Sp7, Sp7-S and Sp245, Herbaspirillum seropedicea and Burkholderia phytofirmans PsJNT were used to inoculate seeds and/or seedlings by soaking and/or drenching. Inoculated and uninoculated seeds and seedlings were germinated and raised in controlled growing cabinets and a greenhouse, respectively. PGPR-inoculated vegetable seeds generally germinated faster and had better early seedling growth than uninoculated controls. Cucumber seeds inoculated with strains Sp7, Sp245 and H. seropedicea exhibited increase in germination percentage and shoot length by 9 and 20%, respectively, while all PGPR improved the germination vigour index, and enhanced length and weight of seedling roots by 25 and 23%, respectively. In tomato, Sp7-S enhanced the germination value, while most PGPR, except Sp7, significantly improved the germination vigour, root length (28%) and weight (37%) with superior vigor. In lettuce, Sp7-S, Sp245 and H. seropedicea inoculation resulted in longer roots (26%). Germination vigour was also improved by inoculation, except for B. phytofirmans. This improved germination and early seedling growth characteristics may influence future crop establishment and production. Of the two laboratory-based inoculation methods used, soaking appeared to be a better technique for enhanced early seedling growth by strains of A. brasilense. This effect could be related to their unique metabolic characteristics of the strains. The growth promoting effects of A. brasilense strains on the early seedling growth of vegetables varied between the bacterial strains and crop species, In particular, strains Sp7-S and Sp245 strongly enhanced root (85%) and shoot (75%) growth, germination value and vigour in tomato when inoculated by soaking. Sp245 increased endogenous plant IAA (indole-3-acetic acid) content of cucumber and lettuce by up to 100%, irrespective of inoculation method. This work demonstrates that the strains can be used for inoculation within the studied range of cell concentrations with or without plant growth promoting (PGP) effects. However, strain Sp7 appeared to be more influential at lower inoculum concentrations (about log10 6), while Sp7-S and Sp245 at log10 7 cfu mL-1 or higher. For instance, cucumber seeds inoculated with Sp7 log10 8 and 6, Sp7-S and Sp245 log10 8 and 7 cfu mL-1 increased seedling growth, vigour index and endogenous plant IAA by up to 55%. In lettuce, the inoculation with log10 6 of Sp7, log10 7 and 6 of Sp7-S, and log10 8 and 7 of Sp245 yielded superior seedling growth with improved seedling vigour, while log10 7 and 8 of Sp7 and Sp7-S, respectively, increased plant IAA concentration by more than 20%. In tomato, Sp7 at log10 6, Sp7-S and Sp245 at log10 7 enhanced the root biomass, while inoculation with all concentrations of Sp7 and Sp7-S, and log10 8 of Sp245 significantly increased plant IAA content by up to 300%. The inoculation with the bacterial cell suspension exerted more beneficial effects on the early seedling growth, vigor and endogenous plant IAA. In cucumber, seeds inoculated with bacterial cell and those treated with IAA solutions produced longer roots and shoots by 163 and 60%, respectively. Seedlings also exhibited superior vigor. These treatments, together with culture supernatant, and combined cell and supernatant, also increased endogenous plant IAA content, in which the combined cell and supernatant produced up to four-fold greater plant IAA concentrations. In lettuce, seeds inoculated with cell suspension produced longer roots (86%) with superior seedling vigour and elevated plant IAA. In tomato, inoculation with cell suspension and treatment with IAA solutions enhanced length of roots length by up to 52 and 188%, respectively, while all treatments significantly increased the plant IAA content by 70%. These results also demonstrate that bacterial cell suspension and combined cell and supernatant showed consistent effects on the expression of plant IAA. This work suggests that the endogenous IAA levels in the seeds during germination have been altered by the activity of live bacteria and phytohormones present in the supernatant. The altered root morphology of the seedlings due to A. brasilense inoculation might have enhanced the capacity of roots to absorb water and essential minerals leading to enhanced plant growth and metabolic activity. For instance, inoculated cucumber seedlings produced longer roots (23%), greater root biomass (19%), higher total phosphorus (15%), endogenous plant IAA (101%) and peroxidase activity (134%). In lettuce, inoculation increased root length (22%), peroxidase activity (53%) and plant IAA (38%). In addition, strain Sp7 enhanced the chlorophyll and protein contents by 25 and 42%, respectively. In tomato, inoculation resulted in longer roots (67%), larger leaves (22%), higher dry matter accumulation (33%), protein (15%) and endogenous plant IAA (94%) contents. Taller seedlings (12%) with larger stems (15%) and more developed leaves (9%) with greater fresh biomass (18%) were observed with Sp7 inoculation, while two-fold increase in peroxidase activity due to strain Sp245 was detected. On the other hand, inoculated basil seedlings grown in soil produced longer roots (90%), taller seedlings (19%) with more (25%) and larger (61%) leaves, which resulted in greater seedling biomass (61%) and phosphorus content (3%), and higher peroxidase activity (122%) particularly for those inoculated with Sp245 and Sp7, respectively. These plant growth promoting effects were also observed in basil grown in an aquaponics system. These include larger stems and leaves (25%), fresh weight yield (17%), peroxidase activity (73%), phosphorus (5%) and protein (23%) contents due to inoculation. The amount of endogenous plant IAA (27%) and chlorophyll (13%) contents were also increased by Sp7 and Sp7-S inoculation, respectively. This further suggests that A. brasilense could be a valuable agent to help maximize the usefulness of fish effluent or wastewater from freshwater aquaculture for vegetable seedling production. The 16S rDNA terminal restriction fragment length polymorphism (T-RFLP) analysis revealed that inoculation with A. brasilense has no adverse effect to the existing rhizobacterial communities (measured by the changes in the distribution of detectable operational taxonomic unit (OTU) (represented by TRF)) in the root rhizosphere of vegetables (i.e. lettuce, cucumber and basil) grown under different systems (i.e. sterile artificial substrate, soil and aquaponics). This highlights that this PGPR did not cause disturbance to the resident microbial communities or imbalance of the normal functioning of the system. In aquaponics, the presence of a substantial density of A. brasilense strains in the root rhizosphere of basil and the enhanced plant growth and physiological parameters of inoculated basil may imply that Azospirillum have successfully established a beneficial association with the existing bacterial populations. Moreover, this study demonstrates the potential of Azospirillum to be a practical agent for enhancing plant growth and development of vegetables fertilised with fish effluent and under aquaponics system. Directing future research endeavors to better understand the basic mechanisms occurring in the Azospirillum-plant interaction rather than exploring large scale application of this PGPR would support further development of the bioinoculant technology.

Der Mangel an Nährstoffen im Boden, hauptsächlich an Phosphor (P) und Stickstoff (N), verbunden mit einem hohen Salzgehalt und der generellen Verarmung landwirtschaftlicher Böden , sind ein ernstes Problem für die landwirtschaftliche Produktion weltweit. Daher besteht ein dringender Bedarf an Forschung und Entwicklung geeigneter landwirtschaftlicher Praktiken, um ungünstige Bodenbedingungen zu verringern und wenn möglich die Fruchtbarkeit von Kulturland wiederherzustellen. Die Verwendung von Rhizobakterien, die das Pflanzenwachstum (PGPR) fördern, kann sich bei der Entwicklung von Strategien zur Erleichterung des Pflanzenwachstums unter normalen Wachstumsbedingungen sowie unter abiotischen Stress als nützlich erweisen. Diese Bakterien bieten ihren pflanzlichen Wirten Vorteile, indem sie die Aufnahme von Bodenmineralien fördern und Pflanzen vor schädlichen Umwelteinflüssen schützen. Die vorliegende Arbeit bewertet die Rolle von in Kamerun natürlich vorkommenden PGPR an Mais und untersucht deren Potenzial als Bioimpfstoffe zur Steigerung des Pflanzenwachstums in Kamerun. Wir prüfen die Hypothese, dass einheimische Bakteriengemeinschaften aus Kamerun einen hohen Anteil an Bakterien aufweisen, deren Eigenschaften Kulturpflanzen helfen, mit ungünstigen Bedingungen umzugehen. In der vorliegenden Arbeit wurden dazu Bakteriengemeinschaften der Rhizosphäre von in Kamerun angebautem Mais isoliert und untersucht. Zum ersten Mal erfolgte eine umfassende phylogenetische Zuordnung aller kultivierbaren Bakterien, auf Grundlage ihrer potenziellen Fähigkeiten zur Förderung des Pflanzenwachstums.<br>Nutrient deficiencies in soil, mainly in phosphorus (P) and nitrogen (N), coupled to salinity and the impoverishment of agricultural soils, are a severe problem for agricultural production worldwide. Therefore, there is an urgent need for research and development of more suitable agricultural practices in order to reduce unfavorable conditions, and if possible, to restore the fertility of cultivated lands. The use of rhizobacteria, which promote plant growth (PGPR), can prove useful in developing strategies to facilitate plant growth under normal as well as under abiotic stress conditions. These bacteria offer benefits to plant hosts by promoting the uptake of soil minerals and protecting plants from environmental stresses. The thesis evaluates the role of native PGPR associated with maize as potential bio-inoculants for plants growth in Cameroon. We hypothesized that native bacterial communities from Cameroon include a high potential of bacteria helping the plant cope with unfavorable conditions. Here, we provide for the first time a comprehensive phylogenetic affiliation of cultivable bacterial communities associated with maize rhizosphere grown in Cameroon in relationship to their potential plant growth-promoting abilities.

L'agricoltura moderna sta affrontando sfide come la perdita di fertilità del suolo, la variabilità climatica e gli attacchi di agenti patogeni in continuo aumento. Le pratiche agricole si stanno evolvendo verso sistemi sostenibili e rispettosi dell'ambiente. L'uso di biostimolanti (PBS, plant biostimulant) è una soluzione innovativa per affrontare le sfide di un’agricoltura sostenibile che garantisce un assorbimento ottimale dei nutrienti, una resa delle colture e tolleranza agli stress abiotici. In particolare, tra i diversi tipi di biostimolanti presenti sul mercato, i rizobatteri, classificati come Plant Growth Promoting Rhizobacteria (PGPR), offrono un nuovo approccio per promuovere la crescita delle piante, la mitigazione degli stress e l’aumento della resa colturale. Pertanto i PGPR sono considerati come una sorta di "probiotici" vegetali, poiché contribuiscono in modo efficiente alla nutrizione e all'immunità delle piante. L'obiettivo principale di questa tesi è isolare e identificare batteri presenti nella rizosfera di pomodoro (Solanum lycopersicum L.) che mostrano proprietà PBS, nonché valutare i meccanismi coinvolti nell'azione di promozione della crescita delle piante (Capitolo 2) e la genetica alla base di questi meccanismi (Capitolo 3 e 4). Infatti, una profonda comprensione dei meccanismi d’azione dei PGPR potrebbe colmare la mancanza di coerenza del dato di efficacia tra gli studi di laboratorio e gli studi in campo e stimolare la ricerca per la produzione e la commercializzazione di nuovi prodotti biostimolanti microbici.<br>Modern agriculture faces challenges such as loss of soil fertility, fluctuating climatic factors and increasing pathogen and pest attacks. Agricultural practices have been evolving towards organic, sustainable and environmentally friendly systems. The use of natural plant biostimulants (PBS) is an innovative solution to address the challenges in sustainable agriculture, to ensure optimal nutrient uptake, crop yield, quality and tolerance to abiotic stress. In particular, among different types of biostimulants present on the market, plant growth promoting rhizobacteria (PGPR) offer a novel approach for promoting plant growth, mitigate stress and increase crop yield. Hence, PGPR inoculants are now considered as a kind of plant ‘probiotics’, since they efficiently contribute to plant nutrition and immunity. The main goal of this thesis was to isolate and identify bacteria symbionts of tomato (Solanum lycopersicum L.) rhizosphere, which showed PBS properties and evaluate mechanism involved in the action of PGPR (Chapter 2), underlying genetics and physiological pathways (Chapter 3 and 4). Indeed, a deeply understanding of the mechanisms of plant growth promotion, could fulfill the lack of consistency between lab, greenhouse and field studies, and support commercialization of novel plant biostimulant products.

L'agricoltura moderna sta affrontando sfide come la perdita di fertilità del suolo, la variabilità climatica e gli attacchi di agenti patogeni in continuo aumento. Le pratiche agricole si stanno evolvendo verso sistemi sostenibili e rispettosi dell'ambiente. L'uso di biostimolanti (PBS, plant biostimulant) è una soluzione innovativa per affrontare le sfide di un’agricoltura sostenibile che garantisce un assorbimento ottimale dei nutrienti, una resa delle colture e tolleranza agli stress abiotici. In particolare, tra i diversi tipi di biostimolanti presenti sul mercato, i rizobatteri, classificati come Plant Growth Promoting Rhizobacteria (PGPR), offrono un nuovo approccio per promuovere la crescita delle piante, la mitigazione degli stress e l’aumento della resa colturale. Pertanto i PGPR sono considerati come una sorta di "probiotici" vegetali, poiché contribuiscono in modo efficiente alla nutrizione e all'immunità delle piante. L'obiettivo principale di questa tesi è isolare e identificare batteri presenti nella rizosfera di pomodoro (Solanum lycopersicum L.) che mostrano proprietà PBS, nonché valutare i meccanismi coinvolti nell'azione di promozione della crescita delle piante (Capitolo 2) e la genetica alla base di questi meccanismi (Capitolo 3 e 4). Infatti, una profonda comprensione dei meccanismi d’azione dei PGPR potrebbe colmare la mancanza di coerenza del dato di efficacia tra gli studi di laboratorio e gli studi in campo e stimolare la ricerca per la produzione e la commercializzazione di nuovi prodotti biostimolanti microbici.<br>Modern agriculture faces challenges such as loss of soil fertility, fluctuating climatic factors and increasing pathogen and pest attacks. Agricultural practices have been evolving towards organic, sustainable and environmentally friendly systems. The use of natural plant biostimulants (PBS) is an innovative solution to address the challenges in sustainable agriculture, to ensure optimal nutrient uptake, crop yield, quality and tolerance to abiotic stress. In particular, among different types of biostimulants present on the market, plant growth promoting rhizobacteria (PGPR) offer a novel approach for promoting plant growth, mitigate stress and increase crop yield. Hence, PGPR inoculants are now considered as a kind of plant ‘probiotics’, since they efficiently contribute to plant nutrition and immunity. The main goal of this thesis was to isolate and identify bacteria symbionts of tomato (Solanum lycopersicum L.) rhizosphere, which showed PBS properties and evaluate mechanism involved in the action of PGPR (Chapter 2), underlying genetics and physiological pathways (Chapter 3 and 4). Indeed, a deeply understanding of the mechanisms of plant growth promotion, could fulfill the lack of consistency between lab, greenhouse and field studies, and support commercialization of novel plant biostimulant products.

En las zonas semiáridas mediterráneas del sureste de España, las escasas e irregulares precipitaciones, y un largo y seco periodo de verano han contribuido drásticamente a la aceleración de los procesos de degradación del suelo. Los cambios ambientales como consecuencia de la pérdida de las comunidades naturales de plantas, vienen a menudo acompañados o precedidos por la degeneración de las propiedades físicas y químicas del suelo, además de por una pérdida o reducción de la actividad microbiana. Actualmente se acepta que la diversidad y actividad de la microbiota del suelo es la base de uno de los mecanismos que más contribuyen a la conservación del suelo, al desarrollo y mantenimiento de la cubierta vegetal y por ende, a la estabilidad y funcionamiento del ecosistema. Así pues, el objetivo principal de este trabajo fue evaluar, en áreas degradas, la eficacia de diversas cepas de rizobacterias promotoras del crecimiento vegetal (RPCV) junto a la aplicación de enmiendas orgánicas sobre el desarrollo de la cubierta vegetal y la calidad de las propiedades del suelo, así como verificar la efectividad como RPCV de varias cepas de actinobacterias, previamente aisladas de diferentes suelos de la Región de Murcia. Con este fin, se llevaron a cabo cinco ensayos diferentes: tres de ellos en condiciones de campo, utilizando diferentes enmiendas orgánicas y RPCV, un cuarto ensayo consistente en el aislamiento de cepas de actinobacterias de la rizosfera de un arbusto autóctono presente en dos localidades diferentes de la Región de Murcia, Rhamnus lycioides L. y un quinto y último ensayo focalizado en la verificación como RPCV de las cepas de actinobacterias previamente aisladas así como el estudio de la incidencia relativa del de origen de las cepas y el suelo sujeto a plantación en la efectividad de las mismas. En todos los experimentos desarrollados en condiciones de campo, se evaluaron tanto el crecimiento y la absorción de nutrientes por parte de la planta, así como las respuestas al estrés originado por la escasez de agua. Del mismo modo, se determinaron las propiedades físico-químicas, químicas y biológicas del suelo. Con respecto al ensayo de aislamiento de actinobacterias de suelo rizosférico, se llevaron a cabo diversas técnicas que permitieron aislar y purificar diferentes cepas, así como caracterizarlas e identificarlas. Como resultados principales del trabajo, se puede destacar que en los tres primeros ensayos, las rizobacterias empleadas promovieron, satisfactoriamente, el crecimiento de las plantas así como la absorción de nutrientes y su tolerancia al estrés. En el primer experimento, en el que se ensayó sobre Cistus albidus L. una mezcla de dos rizobacterias inmovilizadas en arcilla (Azospirillum brasilense y Pantoea dispersa) como inoculante microbiano y residuo de oliva como enmienda, se observó un efecto aditivo en el tratamiento combinado, consistente en la inoculación microbiana y la adición del residuo orgánico al mismo tiempo, que permitió acrecentar las propiedades bioquímicas y microbiológicas del suelo. En el segundo ensayo en campo, en el que se probaron las mismas rizobacterias y la misma enmienda sobre Pinus halepensis Mill., se determinó que la eficacia de la inoculación microbiana fue el tratamiento más efectivo sobre el desarrollo de la planta y sobre las propiedades del suelo. El tercer ensayo se desarrolló para verificar la eficacia de diferentes cepas libres de RPCV (Bacillus megaterium, Enterobacter sp., Bacillus thuringiensis y Bacillus sp.) y la adición de residuo de remolacha azucarera compostado como enmienda orgánica sobre Lavandula dentata L. En este caso, la selección de las rizobacterias efectivas y la combinación de su inoculación junto con la aplicación de la enmienda orgánica se consideró el punto crucial del que dependería la eficacia de esta técnica de revegetación. Con respecto al cuarto ensayo, desarrollado en condiciones de laboratorio, la metodologías utilizadas para el aislamiento caracterización e identificación de diferentes especies de actinobacterias se consideraron las adecuadas, obteniéndose cuatro cepas pertenecientes al género Streptomyces que reunían las condiciones necesarias para ser consideradas potenciales RPCV. En el quinto y último ensayo, en condiciones de campo, se determinó que las bacterias previamente aisladas preservaban las habilidades descritas en condiciones de laboratorio, verificándose su rol como RPCV. Sin embargo, deberían considerarse tanto el origen de la cepa como la fertilidad biológica del suelo sujeto a plantación como factores fundamentales para la selección de cepas de actinobacterias destinadas a uso en revegetación en ambientes semiáridos.<br>In Mediterranean semiarid zones of Southeast Spain, limited and irregular rainfalls and a long and dry summer periods have contributed drastically to the acceleration of soil degradation processes. Environmental changes as a consequence of loss of natural plant cover are often accompanied by the physical and chemical soil properties degeneration, and by a loss or reduction of microbial activity. It is a corroborated fact that the proper functioning and stability of terrestrial ecosystems depends, to a large extent, of the diversity and composition of their vegetal cover. However, the ecological mechanisms that adjust and maintain the peculiar diversity of plant species in an ecosystem throughout the time are only known in a fragmentary way. Nowadays, it is permissible to think that the soil microbiota diversity and activity constitute the basis of one of the mechanisms that influences on soil preservation, on the development and maintenance of the vegetal cover and, consequently, on the ecosystem stability and functioning. The main objective in this Thesis was to evaluate, in degraded areas, the effectiveness of diverse plant growth promoting rhizobacteria (PGPR) strains and the addition of an organic waste on plant performance and on the soil quality properties, as well as to verify the efficacy of some actinobacteria strains as PGPR, previously isolated from different soils of Murcia. So, five different assays were developed: three field experiments involved the use of different organic amendments and PGPR strains; a fourth assay based on the isolation of different actinobacterial strains from the rhizosphere of an autochthonous shrub, that occurs naturally in two distinct sites of Murcia, Rhamnus lycioides L. and a fifth and last experiment focused on the verification as PGPR of the previously isolated actinobacteria strains as well as the study of the relative incidence of both the strain origin and the characteristics of soil subjected to plantation. In the entire field assays it was evaluated the plants growth, nutrients uptake and the biochemical and/or physiological responses of the plants. The physical, physico-chemical and biological soil properties were also determined. With regard to the experiment focused to the actinobacteria isolation from rhizosphere soil, diverse techniques were carried out allowing isolating and purifying different strains as well as to characterise and identify them. The main results obtained in this Thesis can be summarised as follows: in the assays developed under field conditions, the assayed PGPR satisfactory promoted the plant growth, the nutrients uptake and the tolerance to water stress. In the first assay, it was tested the addition of a mixture of two immobilised PGPR in clay pellets (Azospirillum brasilense and Pantoea dispersa) as microbial inoculant and olive mill residue as organic amendment on the target plant Cistus albidus L., it was observed an additive effect in the combined treatment consisting of the microbial inoculation and the organic amendment applied jointly, allowing to enhance biochemical and microbiological soil properties. In the second field experiment, developed by using the same PGPR and organic residue than in the previous assay, it was determined that the most effective treatment to improve Pinus halepensis Mill. plant performance and soil conditions was the microbial inoculation. The third experiment was developed to verify the effectiveness of diverse PGPR free strains (Bacillus megaterium, Enterobacter sp., Bacillus thuringiensis and Bacillus sp.) and the application of sugar beet residue as organic amendment Lavandula dentata L. performance as target plant. The selection of the most efficient rhizobacteria strains and their combined effect with organic residue seems to be a critical point that drives the effectiveness of using these biotechnological tools in revegetation tasks. Regarding the fourth experiment, developed under laboratory conditions, the methodologies used to the actinobacteria isolation, characterisation and identification were successful. Four strains belonging to genus Streptomyces were obtained and they met the required abilities to consider them PGPR. The actinobacteria strains were tested in a fifth assay developed under field conditions being observed that the PGPR capacities were preserved. However, the strain origin and the biological fertility of plantation soil must be considered to an adequate actinobacteria strain selection to be used in restoration programs under semiarid conditions.

Nitrogen-fixing bacteria in soybean nodules convert atmospheric nitrogen to plant-available forms in exchange for carbon from the plant, but other non-nitrogen-fixing bacteria also reside in nodules, and their role in the nodule is not well understood. This study was conducted to determine the effect of three non-nitrogen-fixing Pseudomonas spp. strains isolated from nodules on soybean, and we hypothesized these strains benefit soybean. A greenhouse study in which two cultivars of soybean (Asgrow AG46X6 and Pioneer P48A60X) were treated with three fluorescent Pseudomonas spp. strains (referred to in this study as Bullseye, Pancake, and Starfish) and an uninoculated control. Soybeans were harvested at two time points: the R2/R3 growth stage and the R6 growth stage. Following each harvest, measures of growth, yield, and nitrogen fixation were taken, and data were analyzed using two non-parametric, multivariate analyses: multiple response permutation procedure (MRPP) and permutational multivariate analysis of variance (PERMANOVA). Both analyses showed soybeans of both cultivars treated with Pancake differed from controls following the first harvest but not the second. When analyzed individually, most metrics for growth, yield, and nitrogen fixation following the first harvest were not significantly different between Pancake and control treatments, but Pancake treatment means were still generally higher than controls. If metrics are considered collectively in conjunction with the results of the multivariate analyses, the results show Pancake generally increased soybean growth and nitrogen fixation. These findings support the hypothesis that non-nitrogen-fixing bacteria from nodules benefit plants, and such bacteria have the potential to serve as biofertilizers.<br>Master of Science in Life Sciences<br>Soybeans are one of the most commonly grown crops in the world, and nitrogen-fixing bacteria colonize the roots of soybeans and initiate the formation of spherical nodules attached to the roots. Inside the nodules, these bacteria convert atmospheric nitrogen to plant-available forms in exchange for sugar from the plant, and such bacteria reduce the need to add nitrogen fertilizer to agricultural fields. Other non-nitrogen-fixing bacteria also reside in nodules, but their role in the nodule is not well understood. If these bacteria benefit soybeans, they have the potential to serve as biofertilizers (microbial inoculants that promote plant growth). This study was conducted to determine whether non-nitrogen-fixing bacteria isolated from nodules benefit soybean. A greenhouse study in which two cultivars of soybean (Asgrow AG46X6 and Pioneer P48A60X) were grown in soil and were either left uninoculated or were inoculated with one of three strains of bacteria from the genus, Pseudomonas (referred to in this study as Bullseye, Pancake, and Starfish). Following harvest, measures of growth, yield, and nitrogen fixation were taken, and data showed the bacteria generally benefited the soybean plants. Although, these results showed the bacteria benefitted the plants, field trials and further testing in the greenhouse should be conducted before using these bacteria as commercial biofertilizers. Additionally, the effects of other non-nitrogen-fixing nodule bacteria on soybeans should also be tested to identify other beneficial strains, and the cost of production should be compared to the potential gains of using such bacteria before they are developed into biofertilizers.