The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.

Carbon flow in the rhizosphere: carbon trading at the soil–root interface

In view of rising prices of crude oil due to increasing fuel demands, the need for alternative sources of bioenergy is expected to increase sharply in the coming years. Among potential alternative bioenergy resources, lignocellulosics have been identified as the prime source of biofuels and other value-added products. Lignocelluloses as agricultural, industrial and forest residuals account for the majority of the total biomass present in the world. To initiate the production of industrially important products from cellulosic biomass, bioconversion of the cellulosic components into fermentable sugars is necessary. A variety of microorganisms including bacteria and fungi may have the ability to degrade the cellulosic biomass to glucose monomers. Bacterial cellulases exist as discrete multi-enzyme complexes, called cellulosomes that consist of multiple subunits. Cellulolytic enzyme systems from the filamentous fungi, especially Trichoderma reesei, contain two exoglucanases or cellobiohydrolases (CBH1 and CBH2), at least four endoglucanases (EG1, EG2, EG3, EG5), and one β-glucosidase. These enzymes act synergistically to catalyse the hydrolysis of cellulose. Different physical parameters such as pH, temperature, adsorption, chemical factors like nitrogen, phosphorus, presence of phenolic compounds and other inhibitors can critically influence the bioconversion of lignocellulose. The production of cellulases by microbial cells is governed by genetic and biochemical controls including induction, catabolite repression, or end product inhibition. Several efforts have been made to increase the production of cellulases through strain improvement by mutagenesis. Various physical and chemical methods have been used to develop bacterial and fungal strains producing higher amounts of cellulase, all with limited success. Cellulosic bioconversion is a complex process and requires the synergistic action of the three enzymatic components consisting of endoglucanases, exoglucanases and β-glucosidases. The co-cultivation of microbes in fermentation can increase the quantity of the desirable components of the cellulase complex. An understanding of the molecular mechanism leading to biodegradation of lignocelluloses and the development of the bioprocessing potential of cellulolytic microorganisms might effectively be accomplished with recombinant DNA technology. For instance, cloning and sequencing of the various cellulolytic genes could economize the cellulase production process. Apart from that, metabolic engineering and genomics approaches have great potential for enhancing our understanding of the molecular mechanism of bioconversion of lignocelluloses to value added economically significant products in the future.

Bioconversion of lignocellulosic biomass: biochemical and molecular perspectives

In the growth condition(s) of plants, numerous secondary metabolites (SMs) are produced by them to serve variety of cellular functions essential for physiological processes, and recent increasing evidences have implicated stress and defense response signaling in their production. The type and concentration(s) of secondary molecule(s) produced by a plant are determined by the species, genotype, physiology, developmental stage and environmental factors during growth. This suggests the physiological adaptive responses employed by various plant taxonomic groups in coping with the stress and defensive stimuli. The past recent decades had witnessed renewed interest to study abiotic factors that influence secondary metabolism during in vitro and in vivo growth of plants. Application of molecular biology tools and techniques are facilitating understanding the signaling processes and pathways involved in the SMs production at subcellular, cellular, organ and whole plant systems during in vivo and in vitro growth, with application in metabolic engineering of biosynthetic pathways intermediates.

/pdf/stress-and-defense-responses-in-plant-secondary-metabolites-29akr1xmji.pdf

Stress and defense responses in plant secondary metabolites production

Feasibility of industrial-scale treatment of dye wastewater via bio-adsorption technology

Plant tissue culture as an important tool for the continuous production of active compounds including secondary metabolites and engineered molecules. Novel methods (gene editing, abiotic stress) can improve the technique. Humans have a long history of reliance on plants for a supply of food, shelter and, most importantly, medicine. Current-day pharmaceuticals are typically based on plant-derived metabolites, with new products being discovered constantly. Nevertheless, the consistent and uniform supply of plant pharmaceuticals has often been compromised. One alternative for the production of important plant active compounds is in vitro plant tissue culture, as it assures independence from geographical conditions by eliminating the need to rely on wild plants. Plant transformation also allows the further use of plants for the production of engineered compounds, such as vaccines and multiple pharmaceuticals. This review summarizes the important bioactive compounds currently produced by plant tissue culture and the fundamental methods and plants employed for their production.

/pdf/in-vitro-plant-tissue-culture-means-for-production-of-5cvlep1j9n.pdf

In vitro plant tissue culture: means for production of biological active compounds

Rational metabolic engineering for enhanced alpha-tocopherol production in Helianthus annuus cell culture

Over the years, plant cells and hairy roots have been established as a successful and viable alternative for production of bioactive secondary metabolites and recombinant proteins, replacing the use of whole plants. Bioreactors are used for continuous and consistent in vitro production of these low-volume high-value bioactive/therapeutic molecules from plant cells and hairy roots at large scale. The design and operation of bioreactors for plant cell and hairy root cultivation differs from well-established microbial cultivation due to their size, aggregation, sensitivity to hydrodynamic stress, and viscous nature of the culture broth. The choice of bioreactor and nutrient feeding strategies to overcome substrate limitation and inhibition can be instrumental in enhancing the biomass and product productivity in plant cell and hairy root cultivations at large scale. Hence, this chapter deals briefly with the design and development of bioreactors to achieve maximum productivity in plant cell and hairy root cultivations. The overview of reactor operating parameters considered while designing bioreactors for plant cells and hairy roots are discussed. The chapter also includes application of mathematical modeling to optimize the design of bioreactors and in silico prediction of nutrient feeding strategies during fed-batch and continuous mode of bioreactor cultivation.

Bioreactor Design and Analysis for Large-Scale Plant Cell and Hairy Root Cultivation

Optimization of clinical uricase production by Bacillus cereus under submerged fermentation, its purification and structure characterization

Biological phosphate removal by model based fed-batch cultivation of Acinetobacter calcoaceticus

The secondary metabolite azadirachtin (C35H44O16) is a tetranortriterpenoid obtained from the neem tree (Azadirachta indica). It has long been investigated for its biopesticidal properties. It is a natural insecticide, known to affect feeding, growth, reproduction and metamorphosis of the insect pests. Because of the broad-spectrum control of insects and the relatively low nontarget toxicity it has been widely used in agriculture. To add to its advantage the biopesticidal property of azadirachtin is not only limited to phytophagous insects, but is also known to affect the other pathogenous organisms like nematodes, fungi and micro-organisms. It is a highly oxygenated and complex molecule, which makes its chemical synthesis difficult as well as uneconomical. Studies are still in progress to make its chemical synthesis successful and practically feasible for the mass production of azadirachtin. Currently, azadirachtin is isolated by solvent extraction of the seeds of the Azadirachta indica tree. There are various limitations in extracting azadirachtin from plant sources, majorly due to its limited availability/short shelf-life of seeds, degradation during storage and considerable genotypic/environmental variation in its content from different sources. At present, the demand for azadirachtin is greater than the supply. However, due to variability of azadirachtin available in seeds (0.2–0.6 %) it is difficult to base its mass production on natural sources. A significantly larger amount of material (seeds) would need to be processed to yield a reasonable amount of azadirachtin. Instead, it would be better to rely on a rather stable parent cell line that can be cultivated in vitro (in bioreactors) with a faster doubling time. A biotechnological approach can be very useful for reaching longterm goals. A deeper understanding of different aspects of large-scale azadirachtin production is therefore very important. In recent years, a considerable amount of information has been obtained on the production of azadirachtin by cell and tissue cultures of Azadirachta indica. This approach has an added potential of increasing yield by culture selection and manipulation using elicitors, precursors, permeabilising agents and growth regulators. Azadirachtin is extremely liable to atmospheric degradation in the presence of sunlight. Although few investigations regarding enhancement in its atmospheric stability have been done, more detailed analysis is required to select appropriate stabilisers against the photo-degradation of azadirachtin. A great deal of work with Azadirachta indica has been focused on the extraction and quantification of azadirachtin. Purification of azadirachtin is difficult to accomplish, especially on a preparative scale due to its complexity and similarity in structure of the chemicals found in the seeds, foliage and cell culture. Reverse-phase high-performance liquid chromatography is widely used for the qualitative and quantitative estimation of azadirachtin. Use of other methods like super-critical fluid chromatography (SFC) and liquid chromatography-mass spectrometry combined with flash chromatography, thin-layer chromatography and SFC have also been documented for authentic quantification. Even though azadirachtin has long been recognised as a potent biopesticide, no reports are available to date on the commercial production of azadirachtin by plant cell/tissue culture. This chapter provides a deeper insight with respect to the origin, chemical nature, application, mode of action and prevalent technologies for the production of this high-value bioactive molecule.

Smita Srivastava

Papers

Rational metabolic engineering for enhanced alpha-tocopherol production in Helianthus annuus cell culture

Bioreactor Design and Analysis for Large-Scale Plant Cell and Hairy Root Cultivation

Optimization of clinical uricase production by Bacillus cereus under submerged fermentation, its purification and structure characterization

Biological phosphate removal by model based fed-batch cultivation of Acinetobacter calcoaceticus

In Vitro Azadirachtin Production