File Name: new perspectives and approaches in plant growth promoting rhizobacteria research .zip
Studies of the interactions between plants and their microbiome have been conducted worldwide in the search for growth-promoting representative strains for use as biological inputs for agriculture, aiming to achieve more sustainable agriculture practices.
- Plant Growth-Promoting Bacteria: Mechanisms and Applications
- New Perspectives and Approaches in Plant Growth-Promoting Rhizobacteria Research
- PGPR Amelioration in Sustainable Agriculture
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Soil Sci. Plant Nutr. Jorquera 2 , D.
Plant Growth-Promoting Bacteria: Mechanisms and Applications
Soil Sci. Plant Nutr. Jorquera 2 , D. Gajardo 4 and M. Mora 2. Rhizobacteria are capable of stimulating plant growth through a variety of mechanisms that include improvement of plant nutrition, production and regulation of phytohormones, and suppression of disease causing organisms.
While considerable research has demonstrated their potential utility, the successful application of plant growth promoting rhizobacteria PGPR in the field has been limited by a lack of knowledge of ecological factors that determine their survival and activity in the plant rhizosphere. To be effective, PGPR must maintain a critical population density of active cells. Inoculation with PGPR strains can temporarily enhance the population size, but inoculants often have poor survival and compete with indigenous bacteria for available growth substrates.
PGPR often have more than one mechanism for enhancing plant growth and experimental evidence suggests that the plant growth stimulation is the net result of multiple mechanisms of action that may be activated simultaneously.
Keywords: Agricultural inoculant, phytohormone, phytopathogen biocontrol, plant nutrition, rhizosphere. Plant growth promoting rhizobacteria PGPR influence plant health and productivity by a variety of mechanisms that involve solubilization of mineral nutrients, stimulation of root growth, and suppression of root diseases. Since the first studies on PGPR in the 's, many hundreds of candidate PGPR strains have been screened and evaluated in laboratory, greenhouse and field studies across the world.
Today PGPR are commonly used in developing countries, and inoculants are used on millions of hectares of land Zehnder et al. Nevertheless, implementation of this biotechnology has been hindered by the lack of consistency and variation in responses that are obtained in field trials from site to site, year to year, or for different crops Lambert and Joos, Successful establishment of the introduced bacteria depends on proper PGPR selection that must be tailored to the soil and crop combination.
Other basic problems that are related to inoculum production, storage, and delivery have mostly precluded the use of non-spore forming bacteria as soil inoculants. Lastly, there has been considerable confusion over the precise effects of PGPR, which confounds scientific studies aimed at quantifying their contribution to plant growth. This is largely due to poor understanding of the interactions between PGPR and their plant hosts and the resident microflora, as well as a paucity of information on how environmental factors influence processes that contribute to plant growth promotion.
Two paradigms that have emerged so far from the study of PGPR is that many of the best strains are multifunctional, and secondly, that PGPR traits are commonly distributed among many different species and genera of microorganisms, many of which are indigenous members of the soil microbial community.
In most cases, individual strains vary considerably in performance and there is no clear relationship between taxonomy and PGPR functions that can be used to monitor the population size and activity of these bacteria based on quantification of specific taxonomic groups in the soil.
The possibility that indigenous PGPR affect the relative performance of introduced PGPR inoculants is quite high, so without knowledge of background PGPR activity, the response to soil inoculation is difficult to predict.
Many PGPR simultaneously solubilize phosphorus, produce auxins that stimulate root growth, and produce antibiotics and siderophores that may function in suppression of root disease. Other traits that may contribute to plant growth promotion include production of substances that induce systemic resistance or enzymes degrading hydrogen cyanide or ethylene and reactive oxygen species that are produced by plants during environmental stress.
Lastly, the phenomenon of quorum regulation can affect the expression of each of these traits as PGPR interact with the resident microbial community reviewed by Lugtenberg and Kamilova, In this manner, critical threshold population sizes are likely required to induce the expression of some traits, particularly those involved in biocontrol.
Altogether any and all of the cumulative effects of PGPR that influence root growth rates, root system architecture, root hair formation and longevity, will indirectly affect the ability to acquire water and nutrients and to tolerate root loss to disease. Deciphering which mechanisms are most important and how to manage the soil microflora to obtain expression of these traits is so the remaining great challenge for consistent PGPR use in agricultural systems. In this review, we examine the types of PGPR bacteria that have been identified to date and their functional characteristics.
We also examine briefly inoculum production and delivery technologies and the advantages and disadvantages of various methods for introducing and maintaining high population densities of PGPR that are needed in order to be effective.
PGPR have been subjected to numerous investigations focused on biotech-nological applications in agriculture, horticulture, forestry and environmental protection Zahir et al. Early studies in the 's began with a focus on nitrogen fixing bacteria. Since then, a large number of PGPR belonging to different bacterial classes and genera with multifunctional traits have been described Rodriguez-Diaz et al, PGPR strains are broadly distributed among many taxa including Actinobacteria, Bacteroidetes, Cyanobacteria, Firmicutes and Proteobacteria Tilak et al, , such that determination of the background population size and activity of PGPR in resident microbial communities is difficult to assess based on analysis of microbial community structure or abundance of a particular taxonomic group.
The main aim of biotechnological development based on PGPR has been to develop soil inoculants that can contribute to sustainable agriculture, thereby diminishing the need for use of chemical fertilizers and pesticides Adesemoye and Kloepper, Based on our present knowledge, the interactions between bacteria and plants can be classified into three categories: neutral, negative or positive Whipps, Most rhizobacteria associated with plants are commensals, in which bacteria establish an innocuous interaction that does not have any visible effect on the growth and physiology of the plant Beattie, The rhizosphere also contains rhizobacteria that negatively influence the growth and physiology of the plants, and includes phytopathogens Beattie, In addition to parasitic and disease causing organisms, such bacteria include those that produce phytotoxic substances, such as hydrogen cyanide or ethylene that inhibit root growth.
Counter to these deleterious bacteria are PGPR, which exert a positive effect on plant growth by direct mechanisms such as solubilization of nutrients, nitrogen fixation, production of growth regulators, etc. In addition to these functional classifications, PGPR can be further grouped with respect to the plant compartment that they occupy as either intracellular iPGPR, symbiotics or extracellular ePGPR, free living , in accordance with the degree of association with the root cells.
The iPGPR may live inside the root cells, generally in specialized structures, such as nodules. Extracellular ePGPR are situated either in the rhizosphere, on the root surface rhizoplane or in the intercellular spaces of the root cortex, colonizing the plant tissue intercellularly Gray and Smith, In accordance with the mechanisms presented by PGPR, classification terms have been established Table 1 to describe their activities and mechanisms by which these functions are achieved.
In general, direct mechanisms are those affecting the balance of plant's growth regulators, enhancing plant's nutritional status and stimulating systemic disease resistance mechanisms Zahir et al, ; Glick et al.
Indirect mechanisms are related to biocontrol, including antibiotic production, chelation of available Fe in the rhizosphere, synthesis of extracellular enzymes that hydrolyze the fungal cellular wall and competition for niches within the rhizosphere Zahir et al, ; Glick et al, This classification has led to the application of generic terms including: biofertilizer, phytostimulator and biopesticide to describe the primary function.
Nonetheless, many bacteria have dual roles, which can lead to confusion. The best example of such confusion is found in the body of work on Azospirillum, which initially was based on this bacterium's ability to fix nitrogen, but which was later shown to affect plant growth by production of phytohormones. Since then, it has been classified primarily as a phytostimulator Okon and Kapulnik, ; Spaepen et al.
Similarly, many phosphorus-solubilizing bacteria have been screened and selected based on their ability to solubilize hydroxyapatite on agar media, but they have later been found to affect root growth by production of plant growth hormones. Despite the confusion generated by multifunctional PGPR, it is worthwhile to examine the traits associated with each of the three generic descriptors that are used to classify PGPR. Microorganisms having mechanisms that facilitate nutrient uptake or increase nutrient availability or stimulate plant growth are commonly referred to as biofertilizers.
Biofertilizers are considered as an alternative or complement to chemical fertilization to increase the production of crops in low input agricultural systems. There are some PGPR that can fix nitrogen, solubilize mineral nutrients and mineralize organic compounds. The most well-studied PGPR considered biofertilizers correspond to nitrogen fixation and utilization of insoluble forms of phosphorus.
Agronomic significance of biological nitrogen fixation. Nitrogen N is one of the principal plant nutrients, and its low availability due to the high losses by emission or leaching is a limiting factor in agricultural ecosystems, hence bacteria with ability to make atmospheric N available for plants play a critical role. There are two types of biological fixation: symbiotic and non-symbiotic.
The first is the most important mechanism by which most atmospheric N is fixed, but it is limited to legume plant species and various trees and shrubs that form actinorrhizal roots with Frankia. This process is carried out in well defined nodule structures. Among the most studied symbiotic bacteria are Rhizobium, Bradyrhizobium, Sinorhizobium and Mesorhizobium Zahran, Although the beneficial effects of the symbiotic association of rhizobia with legume plants is known, these bacteria are not considered PGPR, except when associated with non-legume plants Dobbelaere et al.
On the other hand, non-symbiotic biological N fixation, is carried out by free living diazotrophics, and this can stimulate non-legume plants growth Antoun et al. There are studies showing that N-fixing bacteria, free-living as well as Rhizobium strains, can stimulate the growth of non-legumes such as radish Antoun et al.
Non-symbiotic N-fixing rhizospheric bacteria belonging to genera including Azoarcus Reinhold-Hurek et al. Due to the high energy requirement for N fixation and relatively low metabolic activity of free living organisms that must compete for root exudates outside a nodule environment, the ability of nonsymbiotic bacteria to fix significant quantities of N is limited. The presence of a diazotrophic bacterium in the rhizosphere of a certain plant is no longer considered to imply that such bacteria make a substantial contribution to N fixation and N supply for plant growth.
Although the N fixing capacity of certain bacteria can easily be demonstrated under in vitro conditions, its demonstration in greenhouse and field studies is more complex and highly variable. Some observations suggest that rhizobacteria can provide crops with significant quantities of N Dobbelaere et al. Nevertheless, studies in sorghum, maize and wheat inoculated with Azospirillum have revealed a contribution of only 5 kg N ha - 1 yr - 1 Okon and Lanbandera-Gonzalez, This quantity pales in importance when compared with the application of N fertilizers in a range of kg N ha - 1 yr - 1 , which is commonly practiced in modern agriculture.
This applies likely to other free living N fixers. Peoples et al. For this reason, the ability of PGPR to fix N is no longer an important criterion for classification of a bacterium as a biofertilizer.
Enhancing phosphorus availability for plant growth by rhizobacteria. Phosphorus P is an essential plant nutrient with low availability in many agricultural soils.
Today many agricultural soils have a high total P content due to the application of P fertilizers over long periods of time. On the other hand, much of this P is in mineral forms and is only slowly available to plants reviewed by Rodriguez et al.
Most of the insoluble P forms are present as aluminum and iron phosphates in acid soils Mullen, , and calcium phosphates in alkaline soils Goldstein and Krishnaraj, The ability of rhizosphere bacteria to solubilize insoluble P minerals has been attributed to their capacity to reduce pH by the excretion of organic acids e.
These bacteria have been characterized as members of the Bacillus, Burkholderia, Enterobacter, Klebsiella, Kluyvera, Streptomyces, Pantoea and Pseudomonas genera, Chung et al.
These microorganisms grow in media with tricalcium phosphate or similar insoluble materials as the only phosphate source and not only assimilate the element, but also solubilize quantities in excess of their nutritional demands, thereby making it available for plants Chen et al, In this context, there are bacteria capable of producing phytase enzymes for the mineralization of phytates Lim et al, ; Jorquera et al, b.
To date, there are only few studies reporting rhizobacteria capable of mineralizing the phytate. Among the phytase producing rhizobacteria, species belonging to Bacillus, Burkholderia, Enterobacter, Pseudomonas, Serratia and Staphylococcus genera are the most common culturable bacteria Richardson and Hadobas, Hussin et al, ; Shedova et al, Many of these bacteria are remarkably efficient.
Richardson and Hadobas isolated Pseudomonas spp. In a later study utilizing plants with a limited capacity to obtain the P from phytate, Richardson et al. Similarly, Unno et al. Almost all the isolates were classified as members of the Burkholderia genus and some of them significantly promoted the growth of the lupin.
Jorquera et al. Moreover, all strains showed the capacity to produce P hydrolases. The major limitation today for use of these organisms is the lack of consistent effects in mobilizing P under field conditions. This is likely due to competition with the native microflora and environmental factors that either limit the population size or activity of the PGPR. It is now clear from many studies that evaluation and ranking of P-solubilizing bacteria under laboratory conditions do not necessarily correspond to the efficacy of the PGPR for enhancing plant P uptake under field conditions Richardson, ; Rengel, As with nitrogen fixing bacteria, the production of plant growth hormones that improve root surface area can have indirect effects on the ability to efficiently extract P from soil.
Thus, it is likely that many so-called biofertilizers have dual action effects that are mediated by direct solubilization of inorganic P, mineralization of organic P, and stimulatory effects on plant root growth or mycorrhizae formation. The production of phytohormones by PGPR is now considered to be one of the most important mechanisms by which many rhizobacteria promote plant growth Spaepen et al,
New Perspectives and Approaches in Plant Growth-Promoting Rhizobacteria Research
Bernard R. The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria PGPB will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies.
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PGPR Amelioration in Sustainable Agriculture
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These microorganisms have significant potential as important tools for sustainable agriculture. PGPR enhance the growth of root systems and often control certain plant pathogens. As PGPR amelioration is a fascinating subject, is multidisciplinary in nature, and concerns scientists involved in plant heath and plant protection, this book is an ideal resource that emphasizes the current trends of, and probable future of, PGPR developments.
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