**1. Introduction**

180 Fungicides for Plant and Animal Diseases

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The rapid emergence of fungicide resistance has brought a strong demand for crop protection agents with a new mode of action. One of the challenges for modern plant pathology research is the discovery of new modes of action that provide improved activity of fungicides against commercially important target, combined with assures environmental and public safety, is a critical step in safeguarding food security. This chapter reviews biochemical and molecular biological approaches that have revealed new insights into fungal growth and morphogenesis and offer potential target sites for the development of new fungicides. It also discusses prospects for exploiting these modern technologies for the development of fungicides.

Agriculture will always face crop losses caused by microorganisms. Since bible the first pages talk about pest, nowadays, in the XXI century must be added to pest, the losses caused by the effect of global climate change (Gustafson, 2011). Particularly, fungal plant pathogen that comprises an important group of microorganisms that causes significant economic losses in agriculture around the world. They are able to infect any tissue at any stage of plant growth (Garrido *et al*., 2010). Control of plant diseases typically depends upon the application of chemical fungicides, despite their potentially toxic effects on non-target organisms and the environment (Santos *et al,* 2008; Ferrer-Alcón, *et al*, 2009). Although effective, their extensive use for several decades has disrupted biological control by natural enemies and has led to new pathogen races that are resistant to fungicides (Fernandez-Acero *et al.,* 2006). In spite of the incredible amount of biological information about fungal plant pathogens, there is a scarce commercial fungicide developed from a new knowledge approach. The absence of fungicides that are capable of acting in more than one site of action is a direct consequence of resistance to fungicides, which is common among currently used agrochemicals (Brent & Hollomon, 2007). For example, chlorhexidine, quaternary ammonium compounds, organic acids, esters and alcohols that acts by interfering with the structure or permeability of the cell membrane, subsequently altering its barrier function (Russell, 2003). Pyribencarb cause an inhibition of the electron transport system in fungi, it has been suggested that pyribencarb inhibited succinate-cytocrome C reductase in *Botrytis cinerea* and *Corynespora cassicola* and decylubiquinol-cytocrome C reductase in *B. cinerea* in the same way as strobilurin fungicides . Benzimidazole fungicides, such as benomyl, act

Target-Site-Specific Screening System for Antifungal Compounds 183

natural products may be suitably modified to obtain designer molecules for fungicide. Pharmacological testing, modifying, derivatising and research on these natural products represent a good strategy for discovering and developing new fungicides. The combinatorial chemistry has helped in the development of a series of similar but

At present, structure based-drug design and ligand-based drug design are two great strategies that can be applied for the discovery and/or development of new fungicides. Structure based-drug design relies on a knowledge of the three dimensional structure of the biological receptor, obtained through experimental methods such as X-ray crystallography or NMR spectroscopy. When the experimental structure of a target is not available, it may be possible to create a homology model of the biological receptor on the basis of the experimental structure of another known material (mostly a related protein). The use of various tools like automated computational procedures has provided a means of suggesting new drug candidates and optimizing time and resources. Sometimes the information about the three dimensional structure of the receptor is not available. In this sense, ligand-based drug design is focused on the knowleage of other molecules can be used to derive the minimum necessary structural characteristics that a molecule must present in order to bind to the receptor. Ligand-based drugs design can be applied in cases where the structure of the receptors is uknown but a series of compouds have been identified that exert the fungicide activity. It is necessary to have several compounds structurally similar with high activity, with no activity and with a range of intermediate activities. These other compounds that bind to the biological receptor of interest provides us information the minimum necessary structural characteristics that a molecule must present in order to bind to the receptor (Speck-Planche *et al.,* 2011). Both strategies of drug discovery can be extended to and applied in the design of more effective agrochemicals, and specifically fungicides. Example of applying new technologies towards the rational design of fungicides to control phytopathogenic fungi of commercial crops was used by Fernández-Acero *et al*, (2006). They found substrates with antifungal properties against oomycetes, they screened compounds analogous to various phytoalexins and to flavanes derivatives which display antifungal

The use of bioinformatics techniques to biological systems was demonstrated in the Structural Proteomics In Europe (SPINE) project, which was established to develop new methods and technology for high throughput structural biology. Developments covers target selection, target registration, wet and dry laboratory data management and structure annotation as they pertain to high throughput study (Albeck *et al.,* 2006). How this program, there are now many databases which is constantly being updated with the latest data of groups to seek new targets, new fungicides and relevant information like new virulence factors of some fungi, some of these pages are: (www.broadinstitute.org/science/projects/fungal-genomeinitiative, http://cogeme.ex.ac.uk. www.phi-base.org, http://www.expasy.org/ and

In the post-genomic era, new terms related with chemical "-omics" have appeared. The term "genetic chemical" describes the use of small molecules to selectively perturb gene function. When this concept is applied on a genome-wide scale it is named "chemogenomics". The application of chemogenomics to protein targets is named "chemoproteomics"; although a more explicit definition is TRAP (targeted related affinity profiling) defined as the use of

homologous structural compounds for testing (Singh & Sharma, 2011).

activity against *Phytophthora* fungi.

http://bioinformatics.charite.de/supernatural2/.

through specific binding of the β-tubulin subunit of fungal tubulin, which consequently interferes with microtubules assembly, which in turn is essential for numerous cellular processes, such as mitosis and cytoskeleton formation. Metal ions such copper and silver have been proposed to interact strongly with thiol groups in fungal enzymes and proteins. The inhibitory activity of these compounds may be caused by enzyme damage through binding to key functional groups, particularly sulphydril groups in plasma membrane and cytosol. Flucytosine (pyrimidine analog) the sites of action are nucleic acids, this agent is taken up by fungal cells via the enzyme cytosine permease. Biocides exhibit a multiplicity of antifungal mechanisms. The knowledge of their mechanism of action, combined with an understanding of quantitative structure activity relationships, provides an important platform from which novel biocides may emerge, offering enhanced activity and environmental acceptability (Fernandez- Acero *et al.,* 2011).

Nowadays, new approaches based on graph-theoretical descriptors have emerged as powerful tools for the design of bioactive agents (Marrero –Ponce et al., 2008). The purpose of these approaches is to perform a massive screening of databases of heterogeneous series of compounds and to extract as much structural information as possible at different levels of chemical diversity. So, the use of methodologies and promising approaches may enable the discovery and identification of new candidates as potential fungicides. The new agrochemicals that can be designed will have a wide range of action against different species. Also, they will be able to act by different mechanisms of action and thus avoid the problems of cross-resistance (Speck-Planche *et al,* 2011).
