Plant growth-promoting rhizobacteria (PGPR) are the rhizosphere bacteria that can enhance plant growth by a wide variety of mechanisms like phosphate solubilization, siderophore production, biological nitrogen fixation, rhizosphere engineering, production of 1-Aminocyclopropane-1-carboxylate deaminase (ACC), quorum sensing (QS) signal interference and inhibition of biofilm formation, phytohormone production, exhibiting antifungal activity, production of volatile organic compounds (VOCs), induction of systemic resistance, promoting beneficial plant-microbe symbioses, interference with pathogen toxin production etc. The potentiality of PGPR in agriculture is steadily increased as it offers an attractive way to replace the use of chemical fertilizers, pesticides and other supplements. Growth promoting substances are likely to be produced in large quantities by these rhizosphere microorganisms that influence indirectly on the overall morphology of the plants. Recent progress in our understanding on the diversity of PGPR in the rhizosphere along with their colonization ability and mechanism of action should facilitate their application as a reliable component in the management of sustainable agricultural system. The progress to date in using the rhizosphere bacteria in a variety of applications related to agricultural improvement along with their mechanism of action with special reference to plant growth-promoting traits are summarized and discussed in this review.

Plant growth-promoting rhizobacteria (PGPR): emergence in agriculture

The loss of organic material from the roots provides the energy for the development of active microbial populations in the rhizosphere around the root. Generally, saproptrophs or biotrophs such as mycorrhizal fungi grow in the rhizosphere in response to this carbon loss, but plant pathogens may also develop and infect a susceptible host, resulting in disease. This review examines the microbial interactions that can take place in the rhizosphere and that are involved in biological disease control. The interactions of bacteria used as biocontrol agents of bacterial and fungal plant pathogens, and fungi used as biocontrol agents of protozoan, bacterial and fungal plant pathogens are considered. Whenever possible, modes of action involved in each type of interaction are assessed with particular emphasis on antibiosis, competition, parasitism, and induced resistance. The significance of plant growth promotion and rhizosphere competence in biocontrol is also considered. Multiple microbial interactions involving bacteria and fungi in the rhizosphere are shown to provide enhanced biocontrol in many cases in comparison with biocontrol agents used singly. The extreme complexity of interactions that can occur in the rhizosphere is highlighted and some potential areas for future research in this area are discussed briefly.

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Microbial interactions and biocontrol in the rhizosphere

Over 7,000 peer reviewed articles have been published on electrochemical glucose assays and sensors over recent years. Their number makes a full review of the literature, or even of the most recent advances, impossible. Nevertheless, this chapter should acquaint the reader with the fundamentals of the electrochemistry of glucose and provide a perspective of the evolution of the electrochemical glucose assays and monitors helping diabetic people, who constitute about 5 % of the world’s population. Because of the large number of diabetic people, no assay is performed more frequently than that of glucose. Most of these assays are electrochemical. The reader interested also in nonelectrochemical assays used in, or proposed for, the management of diabetes is referred to a 2007 excellent review of Kondepati and Heise [1].

Electrochemical glucose sensors and their applications in diabetes management.

Actinobacteria are Gram-positive bacteria with high G+C DNA content that constitute one of the largest bacterial phyla, and they are ubiquitously distributed in both aquatic and terrestrial ecosystems. Many Actinobacteria have a mycelial lifestyle and undergo complex morphological differentiation. They also have an extensive secondary metabolism and produce about two-thirds of all naturally derived antibiotics in current clinical use, as well as many anticancer, anthelmintic, and antifungal compounds. Consequently, these bacteria are of major importance for biotechnology, medicine, and agriculture. Actinobacteria play diverse roles in their associations with various higher organisms, since their members have adopted different lifestyles, and the phylum includes pathogens (notably, species of Corynebacterium, Mycobacterium, Nocardia, Propionibacterium, and Tropheryma), soil inhabitants (e.g., Micromonospora and Streptomyces species), plant commensals (e.g., Frankia spp.), and gastrointestinal commensals (Bifidobacterium spp.). Actinobacteria also play an important role as symbionts and as pathogens in plant-associated microbial communities. This review presents an update on the biology of this important bacterial phylum.

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Taxonomy, Physiology, and Natural Products of Actinobacteria

Biocontrol fungi (BCF) are agents that control plant diseases. These include the well-known Trichoderma spp. and the recently described Sebacinales spp. They have the ability to control numerous foliar, root, and fruit pathogens and even invertebrates such as nematodes. However, this is only a subset of their abilities. We now know that they also have the ability to ameliorate a wide range of abiotic stresses, and some of them can also alleviate physiological stresses such as seed aging. They can also enhance nutrient uptake in plants and can substantially increase nitrogen use efficiency in crops. These abilities may be more important to agriculture than disease control. Some strains also have abilities to improve photosynthetic efficiency and probably respiratory activities of plants. All of these capabilities are a consequence of their abilities to reprogram plant gene expression, probably through activation of a limited number of general plant pathways.

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Induced Systemic Resistance and Plant Responses to Fungal Biocontrol Agents

Enzymes from filamentous fungi are already widely exploited, but new applications for known enzymes and new enzymic activities continue to be found. In addition, enzymes from less amenable non-fungal sources require heterologous production and fungi are being used as the production hosts. In each case there is a need to improve production and to ensure quality of product. While conventional, mutagenesis-based, strain improvement methods will continue to be applied to enzyme production from filamentous fungi the application of recombinant DNA techniques is beginning to reveal important information on the molecular basis of fungal enzyme production and this knowledge is now being applied both in the laboratory and commercially. We review the current state of knowledge on the molecular basis of enzyme production by filamentous fungi. We focus on transcriptional and post-transcriptional regulation of protein production, the transit of proteins through the secretory pathway and the structure of the proteins produced including glycosylation.

The Molecular Biology of Secreted Enzyme Production by Fungi

The isolation of endophytic actinomycetes from surface-sterilized tissues of 36 plant species was made using humic acid–vitamin (HV) agar as a selection medium. Of the 330 isolates recovered, 212 were from roots, 97 from leaves and 21 isolates from stems with a prevalence of 3.9, 1.7 and 0.3%, respectively. Identification of endophytic actinomycetes was based on their morphology and the amino acid composition of the whole-cell extract. Most isolates were classified as Streptomyces sp. (n = 277); with the remainder belonging to Microbispora sp. (n = 14), Nocardia sp. (n = 8) and Micromonospora sp. (n = 4). Four isolates were unclassified and 23 were lost during subculture. The most prevalent group of isolates were the Streptomyces sp. occurring in 6.4% of the tissue samples of Zingiber officinale. Scanning electron microscopy investigation of this plant revealed that 7.5% of the root and 5% of the leaf samples contained endophytes. Three of the Streptomyces sp. isolates strongly inhibited Colletotrichum musae, five were very active against Fusarium oxysporum and two strongly inhibited growth of both test fungi.

Isolation of endophytic actinomycetes from selected plants and their antifungal activity

Protein secretion in filamentous fungi — trying to understand a highly productive black box

A collection of Streptomyces spp. isolated from the rhizosphere soils of 14 Thai medicinal plants were found to produce, the plant growth hormone indole-3-acetic acid (IAA) in a yeast malt extract medium supplemented with 2 mg/mL L-tryptophan. Streptomyces CMU-H009 recovered from soil associated with lemongrass (Cymbopogon citratus) was very effective in producing IAA. Based on the 16S rDNA sequence analysis, this strain was most closely related to Streptomyces viridis NBRC 13373T (99.3% similarity). The IAA production was maximums (300 mg/mL) when the strain CMU-H009 was cultivated in a yeast malt extract broth amended with 2 mg/mL L-tryptophan a pH 7.0, and incubated at 30 o C with shaking at 125 rpm for 3 d. The culture filtrate from the strain CMU-H009 stimulated a significant increase in the germination and root elongation of maize (Zea mays) and cow pea (Bruguiera parviflora) seeds. The IAA from this strain was extracted, purified and identified by thin layer chromatography.

John F. Peberdy

Papers

The Molecular Biology of Secreted Enzyme Production by Fungi

Isolation of endophytic actinomycetes from selected plants and their antifungal activity

Protein secretion in filamentous fungi — trying to understand a highly productive black box

Indole-3-acetic acid production by Streptomyces sp. isolated from some Thai medicinal plant rhizosphere soils

Efficient protoplast isolation from fungi using commercial enzymes