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Plant secondary metabolites as an alternative in pest management. Background, research approaches and trends.

Among the topics considered in this review are the general aspects and approaches to study plant secondary metabolites from the pest management perspective, including the progress achieved in the discovery process of new potential biopesticides. A background and the present situation of the development and use of these metabolites in pest management are covered emphasizing their perspectives and challenges.

For a successful research and development process leading to a commercial product, a wide range of criteria biological, environmental, toxicological, regulatory, and commercial must be satisfied from the beginning.

Among the major challenges to be faced by the candidate products to reach the market are the sustainable use of raw materials, the standardization of chemically complex extracts, and the regulatory requirements and approval. The potential of plants and their secondary metabolites for plant health could be used in different strategies: El potencial de las plantas y sus metabolitos para la sanidad vegetal puede ser utilizado en diferentes estrategias: This will further increase the demand for higher crop yields per unit area, requiring chemicals used in crop production to be even more sophisticated 1.

To a large extent, agriculture development has been due to the use of synthetic pesticides to reduce the losses caused by pests. At the same time, some of these products have affected human health and have created environmental and pest-resistance problems 2. In order to contribute to programmes of integrated crop management, products for plant protection are required to display high effectiveness and specificity, demonstrate benign environmental and toxicological profiles, and be biodegradable 1.

In the search for alternative solutions to crop protection problems, the interest in plants and their chemo-biodiversity as a source of bioactive substances has increased. Plants are capable of synthesizing an overwhelming variety of small organic molecules called secondary metabolites, usually with very complex and unique carbon skeleton structures 3. These subtances have been used for the benefit of humankind for many years as crop protection agents 4.

Plants release chemical compounds into the environment and when they are used as cover crops, mulch, smother crops, intercrops or green manures, or grown in rotational sequences, can combat insect pests and disease pathogens and improve farm yields 5. Botanicals include crude or semirefined extracts and isolated or purified compounds from various plants species and commercial products 3.

Where a physiological effect on a pest was required, early compounds were simply extracted from a source and used as an impure mixture of chemicals, one or more of which gave the required response. The science of natural products has advanced significantly in recent times, and these compounds are being used as products in their own right as pure or at least characterised compounds, as new chemical skeletons that can be modified by the ingenious synthesis chemist or as indicators of new, effective biochemical modes of action increasingly important in a world of high-throughput in vitro screening 6.

A background and the present situation of the development and use of these metabolites in pest management are covered discussing their perspectives and challenges. General aspects and approaches to the study of plant secondary metabolites from the pest management perspective A characteristic feature of plants and other sessile organisms, which cannot run away in case of danger and do not have an immune system to combat pathogens, is their capacity to synthesize an enormous variety of low molecular weight compounds, the so-called secondary metabolites 7.

To date, the number of described structures exceeds8,9.

By definition, these compounds are not essential for the growth and development of a plant but rather are required for the biopestciides of plants with their environment 3. The biosynthesis of several secondary metabolites is constitutive, whereas in many plants it can be induced and enhanced by biological stress biopedticides, such as wounding or infection fegetal, The simplest functional definitions recognize phytoalexins as compounds that are synthesised de novo as opposed to being released by, for example, hydrolytic activity and phytoanticipins as pre-formed infectional inhibitors.

These definitions are based on the dynamic of the synthesis of the molecule, not on its chemical structure; so, the distinction between phytoalexin and phytoanticipin is not always obvious. Some compounds may be phytoalexins in one species and phytoanticipins in others. A good example is the methylated flavanone sakuranetin, which accumulates constitutively in leaf glands of blackcurrant Ribes nigrum L.

[Prospect of biopesticide on organic farming] [1999]

In cases where a constitutive metabolite is produced in larger amounts after infection, its status as a phytoalexin would oirgen on whether or not the constitutive concentrations were sufficient to be antimicrobial Secondary metabolites have been studied using the approach of classical phytochemistry, focused on knowledge of the chemical components of a plant.

Often, plant secondary metabolites may be referred to as fe natural products, in which case they illicit effects on other organisms.

There are three broad categories of plant secondary metabolites as natural products: Related plant families generally make use of related chemical structures for defence for example, isoflavonoids in the Fabaceaesesquiterpenes in the Solanaceaealthough some chemical classes are used for defensive functions across taxa for example, phenylpropanoid derivatives When the role bjopesticides secondary metabolites in natural interactions between organisms is considered, they are called infochemicals or semiochemicals, terms commonly used in studies of these substances by chemical ecology.


According to the definition of the Organization for Economic Cooperation and Development OECD 14semiochemicals are chemicals emitted by plants, animals, and other organisms that evoke a behavioural or physiological response in individuals of the same or other species.

Biopesticides de origen vegetal pdf free

They include pheromones and allelochemicals Allelochemicals are semiochemicals produced by individuals of one species that modify the behaviour of individuals of a different species i. They include allomones emitting species benefitskairomones receptor species benefits and synomones both species benefit.

Pheromones are semiochemicals produced by individuals of a species that modify the behaviour of other individuals of the same species i. In all these relations, the living organisms exert their effects by orign production of biologically active secondary metabolites. Most semiochemicals are volatile because of their low molecular weight. The volatility gives to this chemical signal an advantage for biopesticidds because it can travel long distances in the wind Allelopathy is an ecological phenomenon whereby secondary metabolites synthesised by plants and microorganisms too influence biological biopestivides agricultural systems; they may be either stimulatory or inhibitory.

Secondary metabolite studies conducted using the chemical ecology approach, have shown that substances released by plants are also involved in tritrophic oorigen in ecosystems. Myrcene, along with TMTT and E – ocimene, has previously been implicated in the change of the behavioural response of the stink bug egg parasitoid, Telenomus podisi Ash. By applying a natural plant defence activator such as cis -jasmone to the vrgetal, not only the parasitoid efficiency was increased, but aphid colonisation could also be reduced, providing a more effective control strategy The ecological impacts of secondary metabolites extend beyond plant-insect coevolution.

Their roles above and below ground include the attraction of predatory species upon herbivory attack, but additionally, these chemical compounds may act as chemical messengers which influence the expression of genes involved in plant defense mechanisms or even influence gene expression of neighboring plants As results of the origdn of other researches, natural products have been used to protect plants from pathogens indirectly by induction of systemic acquired resistance SARincluding phytoalexins.


These SAR-inducing compounds and preparations are termed elicitors Since such activity is indirect, the pathogen origej evolve resistance directly to the elicitor, making such bipesticides excellent candidates for the integrated disease management. Plant inducers act on a very broad spectrum of plant species and fungal and viral pathogens as well, whilst the expression of their efficacy is influenced by environmental conditions, genotype, the physiological stage of the treated plants, and crop nutrition The plant kingdom represents a huge reservoir of new molecules to be discovered; the plants produce enormous varieties of chemicals which are believed to be important in mediating the interaction between plants and their environment.

Veetal is plethora of scientific and ethnobotanical literature listing plants with known pest control properties 2. Over plant species are known to have pesticidal properties, and many of these plants are used by farmers in developing countries 18, Within a single species, to 20 individual primary and secondary compounds may be produced, although most of them as trace amounts which usually are overlooked in a phytochemical analysis 7.

Only a small percentage of plants has been screened for pesticidal activity, and in addition, many such studies are not complete and often bioassay procedures used have been inadequate or inappropriate 20, Potentially useful biological compounds remain undiscovered, uninvestigated, undeveloped, or underutilized from this reservoir of plant material 7,8, Discovery process of new potential biopesticides Initially, biopesticieds and error experiments led to the discovery of uses of secondary metabolites and to the development of procedures for their extraction and use.

The botanical extracts are obtained from plant fractionation by various processes and their composition varies depending on the botanical sample, the experimental conditions, and the physicochemical properties of the compounds The complexity of the plant metabolism results in a large number of molecules, and the extracts from the same plant are not only complex, moreover their molecular composition is very variable from one biopestiicdes to another The modern chemistry has discovered the structures of many of these biologically active compounds, and systematic studies of natural products for plant protection became recognized within the field of chemistry 9.

The approaches employed when studying secondary metabolites, to achieve applied significance, must combine three readily available technologies: Extracts must be screened for biological activity, the active extracts selected, fractionated by directed bioassays and the bioactive compounds identified and then exploited Figure 1 biopesticidee Bioassay-guided fractionation has proven successful as a well-established platform to isolate and characterize active constituents present in natural product extracts; however, sometimes such an approach requires multiple chromatographic steps and large amounts of biological material Recent technological improvements in the area of chromatographic separation methods have nevertheless provided new possibilities to accelerate the overall process of bioassay-guided fractionation.

The increasing sophistication of such techniques by linking them directly on-line or indirectly by adding an additional step of sample concentration at-line with analytical assays allows the more rapid dereplication of extracts identifying known natural products prior to thorough characterization thereby focusing resources on novel molecules Improvements in high-throughput metabolite profiling, using gas chromatography or liquid chromatography linked to mass spectrometry, make it possible to screen for changes in the levels of several hundred plant metabolites in a single sample Metabolomics the study of global metabolite profiles in a system, e.


Craige Trenerry and Rochfor 26 presented an overview of the instrumentation and data management tools required for metabolomics. The analysis of the metabolites synthesised by a biological system the metabolome is particularly challenging due to the diverse chemical nature of the metabolites, and uses techniques such as high-field nuclear magnetic resonance NMR spectroscopy and mass spectrometry MSeither stand alone or hyphenated with gas chromatography GCliquid chromatography LCand capillary electrophoresis CEto measure populations of low-molecular-weight metabolites in biological systems.

Advanced statistical and bioinformatics tools are then used to maximize the recovery of information and interpret the large data sets generated. Nowadays, small amounts of samples are required to complete the discovery process due oigen the miniaturisation of the bioassay designs microinmersion method, use of 96 well plates as dispositive for assaying, bioautography, etc.

The computer-assisted structure elucidation of natural products and the dereplication process allow concentrating research efforts on new bioactive compounds and significantly reducing the time taken to determine the structure of complex secondary metabolites 28, Although active constituents present in extracts can now be identified more quickly as less time is expended on the purification of inactive constituents, still appreciable amount of time is invested if the bioactive compounds need to be isolated for the determination of their structure and in-depth biological testing At the different stages of the process for discovering and developing products for plant protection, the combination of in vitro and in vivo bioassays is recommended.

In vivo screens give an early realistic read-out vgetal efficacy in the practical context and in vitro tests have particular utility in unearthing new mode of action targets 1. The isolation, which still remains a crucial step in each phytochemical study, has to be guided by an appropriate bioassay. Criteria that distinguish good bioassays are: Several reliable and very sensitive bioassay techniques which are indicative of toxicity are known.

These bioassay techniques, with exception of perhaps a few, cannot be used as a rapid, general screening procedure for the detection of toxic secondary metabolites because of cost, specificity, sophistication or objection by animal rights activists. Simple biological test systems are of prime importance to identify the active principles; the brine shrimp lethality test BST is a general bioassay, in this regard considered particularly useful It easily utilizes a large number of organisms for statistical validation and requires no special equipment and a relatively small amount of sample mg or less In this way, the novel bioactive compounds can be rapidly detected and isolated through bioactivity-guided screening and fractionations of the plant extracts.

Conventional biological screenings of chemicals have been focused on acute toxicity and many biossays are aimed to select substances that are the most potent and have rapid action 9. In contrast, insect-plant chemical re in nature are more subtle and most plant defensive chemicals discourage insect herbivore, either by deterring feeding and oviposition or by impairing larval growth, rather than killing outright 9,21, For the past years there has been an increased interest in the behavioural manipulation of insect pests for their management, as an vegteal to broad-spectrum insecticides.

Specific bioassays are designed to detect repellent, antifeedant and deterrent effects However, the demonstration of bioactivity in the laboratory is simply the first step in the development of a commercial product, and numerous other criteria must be satisfied before the true commercial potential can be realised Among the advancements in the discovery process for new molecules for crop protection, in which the vefetal is guided by experiments that indicate the properties which constitute premium products contributing to agricultural sustainability, tests that provide early information on environmental and toxicological properties as well as the spectrum of biological activity are also included 1.

Empirical tests are needed to confirm low nontarget toxicity especially low mammalian toxicityand persistence under field conditions needs to be assessed The mode of action is among the features of a bioactive compound that largely determine whether the odigen mentioned issues are addressed or not, and subsequently whether its commercial development will be addressed.

The early discovery of the mode of action of bioactive compounds could accelerate pesticide research and development by reducing the required time and costs. The screening of such compounds with respect to their mode of action requires accurate and robust bioanalytical tools. Metabolomics is a powerful bioanalytical tool that will likely play a significant role in the acceleration of the discovery of mode of action of bioactive compounds Plant defense chemicals or combinations thereof that exhibit more than one mode of action should be especially suitable for crop protection These will determine the overall efficacy of the chosen substance and its applicability for different spatial and temporal scales as well as cropping systems Persistence and other aspects of field performance can be partly addressed through proper formulation, provided that solvents and adjuvants used are compatible with conventional application equipment and can maintain a cost to the enduser that is competitive with that of other pest management products Current trends in formulation involves the development of green pesticide technology, using oil-in-water micro-emulsions as a nano-pesticide delivery system to replace the traditional orgen concentrates oilin order to reduce the use of organic veegtal and increase the dispersity, wettability and penetration properties of the droplets