Numerous species of soil bacteria which flourish in the rhizosphere of plants, but which may grow in, on, or around plant tissues, stimulate plant growth by a plethora of mechanisms. These bacteria are collectively known as PGPR (plant growth promoting rhizobacteria). The search for PGPR and investigation of their modes of action are increasing at a rapid pace as efforts are made to exploit them commercially as biofertilizers. After an initial clarification of the term biofertilizers and the nature of associations between PGPR and plants (i.e., endophytic versus rhizospheric), this review focuses on the known, the putative, and the speculative modes-of-action of PGPR. These modes of action include fixing N2, increasing the availability of nutrients in the rhizosphere, positively influencing root growth and morphology, and promoting other beneficial plant–microbe symbioses. The combination of these modes of actions in PGPR is also addressed, as well as the challenges facing the more widespread utilization of PGPR as biofertilizers.

Plant growth promoting rhizobacteria as biofertilizers

Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

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Biological control of soil-borne pathogens by fluorescent pseudomonads

Nonpathogenic rhizobacteria can induce a systemic resistance in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Rhizobacteria-mediated induced systemic resistance (ISR) has been demonstrated against fungi, bacteria, and viruses in Arabidopsis, bean, carnation, cucumber, radish, tobacco, and tomato under conditions in which the inducing bacteria and the challenging pathogen remained spatially separated. Bacterial strains differ in their ability to induce resistance in different plant species, and plants show variation in the expression of ISR upon induction by specific bacterial strains. Bacterial determinants of ISR include lipopolysaccharides, siderophores, and salicylic acid (SA). Whereas some of the rhizobacteria induce resistance through the SA-dependent SAR pathway, others do not and require jasmonic acid and ethylene perception by the plant for ISR to develop. No consistent host plant alterations are associated with the induced state, but upon challenge inoculation, resistance responses are accelerated and enhanced. ISR is effective under field conditions and offers a natural mechanism for biological control of plant disease.

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Systemic resistance induced by rhizosphere bacteria

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. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

Soil microbial populations are immersed in a framework of interactions known to affect plant fitness and soil quality. They are involved in fundamental activities that ensure the stability and productivity of both agricultural systems and natural ecosystems. Strategic and applied research has demonstrated that certain co-operative microbial activities can be exploited, as a low-input biotechnology, to help sustainable, environmentally-friendly, agro-technological practices. Much research is addressed at improving understanding of the diversity, dynamics, and significance of rhizosphere microbial populations and their cooperative activities. An analysis of the co-operative microbial activities known to affect plant development is the general aim of this review. In particular, this article summarizes and discusses significant aspects of this general topic, including (i) the analysis of the key activities carried out by the diverse trophic and functional groups of micro-organisms involved in cooperative rhizosphere interactions; (ii) a critical discussion of the direct microbe–microbe interactions which results in processes benefiting sustainable agroecosystem development; and (iii) beneficial microbial interactions involving arbuscular mycorrhiza, the omnipresent fungus–plant beneficial symbiosis. The trends of this thematic area will be outlined, from molecular biology and ecophysiological issues to the biotechnological developments for integrated management, to indicate where research is needed in the future.

http://www.dzumenvis.nic.in/Microbes%20and%20Plants%20Growth/pdf/Microbial%20co-operation%20in%20the%20rhizosphere.pdf

Microbial co-operation in the rhizosphere

Treatment with Pseudomonas putida WCS358r, a rifampicin‐resistant derivative of strain WCS358, significantly reduced fusarium wilt of carnation grown in rockwool if disease incidence was moderate, but not if disease incidence was high. Differences in disease incidence could intentionally be established by varying the inoculum density of the pathogen Fusarium oxysporum f. sp. dianthi (Fod). The effectiveness of disease suppression by WCS358r increased with decrease of inoculum density and consequently decrease of disease incidence. WCS358r and a Tn5 marked derivative of WCS358 (B243) reduced fusarium wilt of carnation most effectively if a low iron availability for the pathogen was established by adding unferrated or only partially ferrated ethylenediamine [di(o‐hydroxyphenylacetic) acid]. A Tn5 mutant of WCS358 defective in siderophore biosynthesis (JM218) did not reduce disease incidence. Siderophore production and inhibition of Fod by WCS358r in vitro decreased with increasing iron availability, support...

Suppression of fusarium wilt of carnation by Pseudomonas putida WCS358 at different levels of disease incidence and iron availability

Summary-The influence of pseudobactin 358 (PSB358) on the iron uptake and chlorophyll synthesis of hydroponically-grown barley seedlings (Hordeurn uulgare L. cv. Femina) was studied. PSB358 is the siderophore produced by the plant-growth promoting Pseudomonasputida strain WCS358. Iron uptake by iron-deficient barley plants was significantly higher with 59FePSB358 than with 59FeEDDHA as an iron source. However, iron uptake from the ferrated barley phytosiderophores (s9FePS) was the most efficient. Chlorophyll synthesis in barley was stimulated by 4 PM FePSB358 compared with that in barley supplied with 4 pM FeCl,. The unferrated PSB358 (4 pM) positively influenced chlorophyll synthesis in barley if the plants were grown with Fe(OH)s as an iron source, whereas 4 PM EDDHA had a negative effect. Under axenic conditions, chlorophyll synthesis was high and did not differ between plants that were grown with either FePSB358 or FeCl, as iron source. Inoculation of the axenic plants with barley-rhizosphere microorganisms reduced their chlorophyll synthesis and the phytosiderophore accumulation in their nutrient solution. Under these conditions, FePSB358 stimulated chlorophyll synthesis in barley compared with that in barley supplied with FeCl,. It is concluded that both FePSB358 and PSB358 can affect the iron nutrition of barley positively, if, due to microbial degradation of the phytosiderophores, phytosiderophoremediated iron nutrition was not sufficient for the plant’s iron requirement,

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Influence of pseudobactin 358 on the iron nutrition of barley

Two fractions of agglutination activity towards fluorescent pseudomonads were detected in root washes of potato, tomato, wheat, and bean. High-molecular-mass (>106 Da) components in crude root washes agglutinated only particular saprophytic, fluorescent Pseudomonas isolates. Ion-exchange treatment of the crude root washes resulted in preparations of lower-molecular-mass (105 to 106 Da) fractions which agglutinated almost all Pseudomonas isolates examined. Also, components able to suppress agglutination reactions of pseudomonads with the lower-molecular-mass root components were detected in crude root washes of all crops studied. Pseudomonas isolates were differentially agglutinated by both types of root components. The involvement of these two types of root components in short-term adherence and in colonization was studied in potato, tomato, and grass, using Pseudomonas isolates from these crops. Short-term adherence of isolates to roots was independent of their agglutination with either type of root components. With agglutination-negative mutants, the high-molecular-mass components seemed to be involved in adherence of Pseudomonas putida Corvallis to roots of all crops studied. Short-term adherence to roots of four Pseudomonas isolates could be influenced by addition of both crude and ion-exchange-treated root washes, depending on their agglutination phenotype with these root wash preparations. Potato root colonization by 10 different isolates from this crop, over a period of 7 days, was not correlated with their agglutination phenotype. Agg- mutants of P. putida Corvallis were not impaired in root colonization. It is concluded that the root agglutinins studied can be involved in short-term adherence of pseudomonads to roots but do not play a decisive role in their root colonization.

Agglutination, adherence, and root colonization by fluorescent pseudomonads.

For application of genetically engineered fluorescent Pseudomonas spp., specific markers are required for monitoring of wild-type Pseudomonas strains and their genetically modified derivatives in natural environments. In this study, the specific siderophore receptor PupA of plant growth-promoting Pseudomonas putida WCS358 was used as a marker to monitor wild-type strain WCS358. After introduction into natural soil and rhizosphere environments, strain WCS358 could be recovered efficiently on a medium amended with 300 microM pseudobactin 358. Although low population densisties of indigenous pseudomonads (less than or equal to 10(3)/g of soil or root) were recovered on the pseudobactin 358-amended medium, subsequent agglutination assays with a WCS358-specific polyclonal antiserum enabled accurate monitoring of populations of wild-type strain WCS358 over a range of approximately 10(3) to 10(7) CFU/g of soil or root. Genetic analysis of the background population by PCR and Southern hybridization revealed that natural occurrence of the pupA gene was limited to a very small number of indigenous Pseudomonas spp. which are very closely related to P. putida WCS358. The PupA marker system enabled the study of differences in rhizosphere colonization among wild-type strain WCS358, rifampin-resistant derivative WCS358rr, and Tn5 mutant WCS358::xylE. Chromosomally mediated rifampin resistance did not affect the colonizing ability of P. putida WCS358. However, Tn5 mutant WCS358::xylE colonized the radish rhizosphere significantly less than did its parental strain.

Siderophore receptor PupA as a marker to monitor wild-type Pseudomonas putida WCS358 in natural environments

The influence of the siderophore produced by plant growth‐promoting Pseudomonas putida strain WCS358, pseudobactin 358 (PSB358), on chlorophyll synthesis and iron(III) reduction in carnation cultivars Lena and Pallas grown in hydroponics was studied. Ferric pseudobactin 358 (4 μM) stimulated chlorophyll synthesis in cv. Lena, but not in cv. Pallas. FeEDDHA stimulated chlorophyll synthesis in both cultivars. Differences between the two carnation cultivars in the utilization of FePSB358 as an iron source are correlated with differences in iron efficiency reactions of the two cultivars: Fe‐deficient plants of cv. Lena produced more and longer root hairs than Fe‐deficient plants of cv. Pallas, and the ferric reducing activity of cv. Lena was higher than that of cv. Pallas.

P. A. H. M. Bakker

Papers

Suppression of fusarium wilt of carnation by Pseudomonas putida WCS358 at different levels of disease incidence and iron availability

Influence of pseudobactin 358 on the iron nutrition of barley

Agglutination, adherence, and root colonization by fluorescent pseudomonads.

Siderophore receptor PupA as a marker to monitor wild-type Pseudomonas putida WCS358 in natural environments

Ferric pseudobactin 358 as an iron source for carnation