Use of plant growth-promoting rhizobacteria to control stress responses of plant roots

Ethylene is a key gaseous hormone that controls various physiological processes in plants including growth, senescence, fruit ripening, and responses to abiotic and biotic stresses. In spite of some of these positive effects, the gas usually inhibits plant growth. While chemical fertilizers help plants grow better by providing soil-limited nutrients such as nitrogen and phosphate, over-usage often results in growth inhibition by soil contamination and subsequent stress responses in plants. Therefore, controlling ethylene production in plants becomes one of the attractive challenges to increase crop yields. Some soil bacteria among plant growth-promoting rhizobacteria (PGPRs) can stimulate plant growth even under stressful conditions by reducing ethylene levels in plants, hence the term “stress controllers” for these bacteria. Thus, manipulation of relevant genes or gene products might not only help clear polluted soil of contaminants but contribute to elevating the crop productivity. In this article, the beneficial soil bacteria and the mechanisms of reduced ethylene production in plants by stress controllers are discussed.     

Factors associated with detrimental effects of rhizobacteria on plant growth

Some of the factors interfering with the specific response of young common bean plants to two rhizosphere fluorescent pseudomonads were studied. These two bacterial strains produced symptoms in foliar plant parts and reduced yield in beans and several other plant species when inoculated on roots. Sensitivity in the plants subjected to bacterial application was highest at early growth stages (up to ten days old plants) giving typical symptoms for each strain in first expanding leaves and typically stunted plant growth. Symptoms to some extent also appeared on plants treated at the age of 2–3 weeks and the fresh shoot weight was affected also in such plants. Deleterious effects of the tested bacteria were found to be related to the level of inoculum (cfu/ml) used and presence or absence of certain nutrients in the bacterial suspensions used for inoculation. The two tested strains differed in nutritional requirements for affecting plant growth. One strain needed sucrose only. The other tested strain required peptone or yeast extract in addition to sucrose to induce typical symptoms and significant yield reductions. Supply of peptone as the only nutrient source to the bacterial suspensions eliminated the deleterious effects of both bacterial strains as tested on beans under nonsterile conditions. The two strains are assumed to affect test plants by different modes of action.     

Scientific approaches to development of medicinal formulation based on biotechnological substance

The study demonstrated possible design of a medicinal formulation in the form of suppositories comprising human recombinant interferon-alpha2 and dry aloe extract. The approaches to the development of the suppositories were technology-derived. No interaction between the active and auxiliary components was proved by solid state 1H-NMR spectroscopy. The specific activity of the drug was investigated.     

Mechanism of plant growth promotion by rhizobacteria

Plant growth results from interaction of roots and shoots with the environment. The environment for roots is the soil or planting medium which provide structural support as well as water and nutrients to the plant. Roots also support the growth and functions of a complex of microorganisms that can have a profound effect on the growth anti survival of plants. These microorganisms constitute rhizosphere microflora and can be categorized as deleterious, beneficial, or neutral with respect to root/plant health. Beneficial interactions between roots and microbes do occur in rhizosphere and can be enhanced. Increased plant growth and crop yield can be obtained upon inoculating seeds or roots with certain specific root-colonizing bacteria- 'plant growth promoting rhizobacteria'. In this review, we discuss the mechanisms by which plant growth promoting rhizobacteria may stimulate plant growth.     

Applications of free living plant growth-promoting rhizobacteria

Free-living plant growth-promoting rhizobacteria (PGPR) can be used in a variety of ways when plant growth enhancements are required. The most intensively researched use of PGPR has been in agriculture and horticulture. Several PGPR formulations are currently available as commercial products for agricultural production. Recently developing areas of PGPR usage include forest regeneration and phytoremediation of contaminated soils. As the mechanisms of plant growth promotion by these bacteria are unravelled, the possibility of more efficient plant-bacteria pairings for novel and practical uses will follow. The progress to date in using PGPR in a variety of applications with different plants is summarized and discussed here.     

Metagenomic approaches to exploit the biotechnological potential of the microbial consortia of marine sponges

Natural products isolated from sponges are an important source of new biologically active compounds. However, the development of these compounds into drugs has been held back by the difficulties in achieving a sustainable supply of these often-complex molecules for pre-clinical and clinical development. Increasing evidence implicates microbial symbionts as the source of many of these biologically active compounds, but the vast majority of the sponge microbial community remain uncultured. Metagenomics offers a biotechnological solution to this supply problem. Metagenomes of sponge microbial communities have been shown to contain genes and gene clusters typical for the biosynthesis of biologically active natural products. Heterologous expression approaches have also led to the isolation of secondary metabolism gene clusters from uncultured microbial symbionts of marine invertebrates and from soil metagenomic libraries. Combining a metagenomic approach with heterologous expression holds much promise for the sustainable exploitation of the chemical diversity present in the sponge microbial community.     

Physiological characterization of opine-utilizing rhizobacteria for traits related to plant growth-promoting activity

Thirty-two strains of opine-utilizing rhizobacteria were evaluated for physiological traits which have been related to plant growth-promoting activity. Tests included antibiosis against two bacterial and eight fungal pathogens of potato (Solanum tuberosum L.), production of hydrogen cyanide and fluorescent pigment production. On average, 71 and 12% of the bacteria inhibited the growth of Erwinia carotovora subsp. carotovora and Agrobacterium tumefaciens, respectively. The growth of Botrytis sp. was inhibited by 62% of the bacteria, and half of these produced an inhibition zone of more than 7 mm in diameter. Fusarium solani, Colletotrichum coccodes, Phoma exigua, Verticillium dahliae, F. oxysporum, V. albo-atrum and F. sambucinum were antagonized by 43, 34, 31, 25, 19, 18, and 12% of the bacteria, respectively. Only four strains produce hydrogen cyanide. The inhibition of a plant pathogen was not correlated to the production of fluorescent pigment. No strain produced a hypersensitive reaction whereas only three strains induced soft-rot and two produced polygalacturonase. Some opine-utilizing rhizobacteria were strong inhibitors of all plant pathogens, while most were active against specific plant pathogens.     

Starch biosynthesis,its regulation and biotechnological approaches to improve crop yields

Structurally composed of the glucose homopolymers amylose and amylopectin, starch is the main storage carbohydrate in vascular plants, and is synthesized in the plastids of both photosynthetic and non-photosynthetic cells. Its abundance as a naturally occurring organic compound is surpassed only by cellulose, and represents both a cornerstone for human and animal nutrition and a feedstock for many non-food industrial applications including production of adhesives, biodegradable materials, and first-generation bioethanol. This review provides an update on the different proposed pathways of starch biosynthesis occurring in both autotrophic and heterotrophic organs, and provides emerging information about the networks regulating them and their interactions with the environment. Special emphasis is given to recent findings showing that volatile compounds emitted by microorganisms promote both growth and the accumulation of exceptionally high levels of starch in mono- and dicotyledonous plants. We also review how plant biotechnologists have attempted to use basic knowledge on starch metabolism for the rational design of genetic engineering traits aimed at increasing starch in annual crop species. Finally we present some potential biotechnological strategies for enhancing starch content.     

Conventional and biotechnological approaches for control of parasitic weeds

On a worldwide basis, parasitic weeds represent one of the most destructive and intractable problems to agricultural production in both developed and developing countries. About 20 families (3,000–5,000 species) of higher plants are parasitic on the plant kingdom and may cause production losses of 30–80% in staple food and industrial crops on every continent. Compared with the other weeds, parasitic weeds are difficult to control by conventional means because of their life style: Parasites are intimately involved with the host and have so much metabolic overlap with the host that differential treatments are very difficult to develop. In some cases, the parasites are closely associated to the host root, concealed underground, and undiagnosed until they irreversible damage the crop. Several different approaches (cultural, mechanical, chemical, use of resistant varieties, and biological) to control parasitic weeds are currently in use, but are only partially successful. Recent reviews have covered the physiology and interactions between parasitic plants and their hosts, taxonomy, and the biology and classical control of parasitic weeds. The current review will discuss why alternative methods are needed to control parasitic weeds and will summarize conventional and new biotechnology-based control measures against the major world pests Striga, Orobanche, Cuscuta, and mistletoes (Phoradendron and Viscum genera). Effectiveness, advantages and disadvantages, environment safety, and simplicity of these new biotechnological methods will be reviewed.     

Recombinant entomopathogenic agents: a review of biotechnological approaches to pest insect control

Although the use of chemical pesticides has decreased in recent years, it is still a common method of pest control. However, chemical use leads to challenging problems. The harm caused by these chemicals and the length of time that they will remain in the environment is of great concern to the future and safety of humans. Therefore, developing new pest control agents that are safer and environmentally compatible, as well as assuring their widespread use is important. Entomopathogenic agents are microorganisms that play an important role in the biological control of pest insects and are eco-friendly alternatives to chemical control. They consist of viruses (non-cellular organisms), bacteria (prokaryotic organisms), fungi and protists (eukaryotic organisms), and nematodes (multicellular organisms). Genetic modification (recombinant technology) provides potential new methods for developing entomopathogens to manage pests. In this review, we focus on the important roles of recombinant entomopathogens in terms of pest insect control, placing them into perspective with other views to discuss, examine and evaluate the use of entomopathogenic agents in biological control.     

Isolation and characterization of phosphate solubilizing rhizobacteria to improve plant health of tomato

In the present study, 43 isolates of Phosphate solubilizing rhizobacteria (PSRB) were isolated from 37 rhizospheric soil samples of tomato collected from tomato growing regions of Karnataka. Among the 43 isolates, 33 were found to be positive for solubilizing both inorganic and organic forms of phosphorous. The isolates were analyzed for their ability to colonize roots of tomato and to increase the seed quality parameters under laboratory conditions. On the basis of above criteria, 16 isolates were selected for further studies. Organic acids from PSRB isolates were analyzed and phytase zymogram for two isolates viz., PSRB21 and 31 was prepared. Under greenhouse conditions, all selected isolates showed increased shoot length, root length, fresh weight, dry weight and phosphorous content of tomato seedlings to various extent with respect to control. Analysis of pH and available phosphorous in rhizosphere soil samples of 30 day-old-seedlings revealed that the available phosphorous content was high in rhizospheric soil samples of plants raised from seeds bacterized with PSRB isolates over control. Even though all selected PSRB’s were able to increase the plant growth, only few of them showed protection against fusarium wilt and none of them against early blight.     

Use of synthetic signal sequences to explore the protein export machinery

The information for correct localization of newly synthesized proteins in both prokaryotes and eukaryotes resides in self-contained, often transportable targeting sequences. Of these, signal sequences specify that a protein should be secreted from a cell or incorporated into the cytoplasmic membrane. A central puzzle is presented by the lack of primary structural homology among signal sequences, although they share common features in their sequences. Synthetic signal peptides have enabled a wide range of studies of how these "zipcodes" for protein secretion are decoded and used to target proteins to the protein machinery that facilitates their translocation across and integration into membranes. We review research on how the information in signal sequences enables their passenger proteins to be correctly and efficiently localized. Synthetic signal peptides have made possible binding and crosslinking studies to explore how selectivity is achieved in recognition by the signal sequence-binding receptors, signal recognition particle, or SRP, which functions in all organisms, and SecA, which functions in prokaryotes and some organelles of prokaryotic origins. While progress has been made, the absence of atomic resolution structures for complexes of signal peptides and their receptors has definitely left many questions to be answered in the future.     

Molecular biological approaches to plant nutrition

In the last decade, understanding of ion transport has grown sufficiently to pose sensible questions about the molecular nature of the processes and their regulation. Techniques for identifying and cloning genes and for genetic transformation provide the means for answering these questions.Transport of ions across membranes is obviously a major aspect of mineral nutrition since it occurs during initial absorption, compartmentation and mobilisation of nutrients. Here, we will briefly review the types of transport protein involved and show how molecular biology and recombinant DNA technology have revealed something of their structure. Strategies used to identify the genes for transporters are discussed and reference is made to areas in which the availability of cloned genes will facilitate future studies.Mineral nutrition involves, however, more than membrane transport. The absorption rates of major nutrients are quite strictly regulated by biochemical factors which vary with the rate at which nutrients are used in growth. Nitrogen, sulphur and phosphate nutrition in micro-organisms are regulated by the interaction of various DNA-binding proteins with the promoter regions of genes for key enzymes in the assimilatory pathways and the specific ion permeases. The expression of the regulatory protein or its activity can be modified by metabolites, such as glutamine. Some evidence supports the idea that higher plants also have groups of genes with a common regulation of expression.An attempt is made to identify some reasonable objectives, which should increase understanding of the regulation of nutrient transport.Abbreviations and conventions cDNA complementary strand of DNA prepared from a messenger RNA - NR nitrate reductase - NiR nitrite reductase - P_i inorganic phosphate - PPase pyrophosphate - RUBISCO rubulose bisphosphate carboxylase oxygenase. Genes are referred to as follows e.g. cys-3 and scon-1. The protein products of these genes, where they have no familiar name are referred to as CYS3 and SCON-1 respectively     

Genetic diversity of cultivable plant growth-promoting rhizobacteria in Korea

To elucidate the biodiversity of plant growth-promoting rhizobacteria (PGPR) in Korea, 7,638 bacteria isolated from the rhizosphere of plant species growing in many different regions were screened. A large number of PGPR were identified by testing the ability of each isolate to promote the growth of cucumber seedlings. After redundant rhizobacteria were removed via amplified rDNA restriction analysis, 90 strains were finally selected as PGPR. On the basis of 16S ribosomal RNA sequences, 68 Gram-positive (76%) and 22 Gram-negative (24%) isolates were assigned to 21 genera and 47 species. Of these genera, Bacillus (32 species) made up the largest complement, followed by Paenibacillus (19) and Pseudomonas (11). Phylogenetic analysis showed that most of the Grampositive PGPR fell into two categories: low- and high- G+C (Actinobacteria) strains. The Gram-negative PGPR were distributed in three categories: alpha-proteobacteria, beta- proteobacteria, and gamma-proteobacteria. To our knowledge, this is the largest screening study designed to isolate diverse PGPR. The enlarged understanding of PGPR genetic diversity provided herein will expand the knowledge base regarding beneficial plant-microbe interactions. The outcome of this research may have a practical effect on crop production methodologies.