Plant antifungal proteins and their applications in agriculture.
Abstract
Fungi are far more complex organisms than viruses or bacteria and can develop numerous diseases in plants that cause loss of a substantial portion of the crop every year. Plants have developed various mechanisms to defend themselves against these fungi which include the production of low-molecular-weight secondary metabolites and proteins and peptides with antifungal activity. In this review, families of plant antifungal proteins (AFPs) including defensins, lectins, and several others will be summarized. Moreover, the application of AFPs in agriculture will also be analyzed.
An antifungal medication, also known as an antimycotic medication, is apharmaceutical fungicide or fungistaticused to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis(thrush), serious systemic infections such as cryptococcal meningitis, and others. Such drugs are usually obtained by a doctor's prescription, but a few are available OTC (over-the-counter).
Highlights
► Mammals, insects, plants and fungi produce various antimicrobial proteins (AMPs). ► This review focuses on small, cysteine-rich antifungal proteins. ► Their antifungal spectrum, structure and mode of action are described. ► Additional biological functions based on their signalling activity are highlighted. ► The multitude of functions endows AMPs with great medical/biotechnological potential.
1. Introduction
Small proteins with antimicrobial activity, so called antimicrobial proteins (AMPs), are produced by organisms throughout all kingdoms comprising prokaryotes, lower and higher eukaryotes. AMPs are secreted proteins that efficiently inhibit the growth of viruses, bacteria, fungi and parasites. In unicellular organisms, AMPs might provide their hosts the advantage to successfully compete with organisms that possess similar nutritional and ecological requirements. In multicellular organisms, AMPs constitute a primitive mechanism of innate immunity and form the first line of defence to protect their hosts from microbial attack. The innate immunity represents an evolutionarily ancient and widespread defence mechanism found in plants, insects and vertebrates. In addition, vertebrates developed the adaptive immune system – a sophisticated mechanism that uses antibodies and killer cells to recognize and eliminate invading microorganisms and allows immunological memory and self versus non-self recognition. The innate immune response is fast, and complements the adaptive immunity. Thus, both mechanisms combine to form an optimal and efficient defence system that supports the fitness of the host. The fact that closely related AMPs are widely distributed over different eukaryotic kingdoms, i.e. the class of defensins, suggests that ancestral AMP genes existed in basal eukaryotes even before fungal and insect lineages diverged (Lehrer and Ganz, 1999;Lehrer, 2007; Zhu, 2008).
AMPs are gene-encoded and they are either constitutively expressed or rapidly transcribed upon induction. In higher eukaryotes invading microbes and their products, e.g. lipopolysaccharides (Mendez-Samperio et al., 2007;Amlie-Lefond et al., 2005), or host cellular compounds, such as butyrate (Murakami et al., 2002), cytokines (Wolk et al., 2004; Wilson et al., 2007; Lai and Gallo, 2009), and vitamins (Schauber et al., 2006) stimulate AMP production.
Due to the vast variety in function, structure, expression pattern, target organisms and producing hosts, the classification of AMPs is difficult and somewhat arbitrary to date. Mostly, AMPs are classified according to their functional and/or structural properties. Both characteristics are determined by the primary sequence of the protein which very often shows a high number of certain amino acids such as glycine, cysteine, histidine, proline, tyrosine, arginine, lysine and serine.
Although only a defined number of AMPs has been structurally analysed by nuclear magnetic resonance (NMR), calorimetric dichroism (CD) or X-ray crystallography, the secondary and tertiary structure of numerous AMPs has been predicted from primary sequence homologies. The most common classes contain proteins with α-helical, β-sheet or mixed α-helical/β-sheet structures (Zhu, 2008;Dimarcq et al., 1998;Giangaspero et al., 2001; Tossiet al., 2000).
The best functionally characterized AMPs are bactericidal, whereas the properties of antifungal AMPs and their mode-of-action are less well studied. In the online database of AMPs athttp://aps.unmc.edu/AP/main.php around 1900 AMPs of different origin are registered. From these, more than 1500 AMPs (79 %) have been assigned antibacterial activity compared to 648 antifungal AMPs (34 %). This classification, however, is redundant and antibacterial AMPs may also show antifungal activity that has not been investigated so far.
Most interestingly, the number of reports that document new additional functions of AMPs beyond their antimicrobial activity is constantly increasing. These features might arise from signalling functions of AMPs that accompany the activity of AMPs to interfere with the cell proliferation of microbes. For example, antimicrobial peptides from bacteria are part of the quorum-sensing mechanism that helps microorganisms to communicate and co-ordinate their behaviour by
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