Introduction
Tomato yellow leaf curl virus (TYLCV) is one of the major and serious diseases of tomato which causes considerable amount of yield loss in Egypt [
1‐
3]. One hundred twenty five million tons of tomatoes were produced in the world in 2007. China, the largest producer, accounted for about one quarter of the global output, followed by the United States, Turkey, India and Egypt.
http://www.fas.usda.gov/htp/2009%20Tomato%20Article.pdf. Losses from plant diseases can have a significant economic impact, causing a reduction in income for crop producers, distributors, and higher prices for consumers.
In order to control TYLC-disease, it was found that frequent spray (at 7 days interval) of insecticide, like Cypermethrin (0.01%) or Dimethoate (0.1%) is effective to minimize the disease by controlling its vector whitefly (
Bemisia tabaci) [
4,
5].
Researches focused on the use of alternative method to avoid the undesirable effects of the insecticides. In 1940s several investigators suggested the use of milk as spraying or dipping of seedlings for reducing the incidence of virus infections. Recent studies demonstrated the effectiveness of milk in reducing infection of tobacco mosaic virus (TMV) in pepper, tomato, and tobacco [
6,
7].
Whey represents a rich and heterogeneous mixture of secreted proteins with wide ranging nutritional, biological and food functional attributes. The main constituents of whey are α-lactalbumin (ALA), β-lactoglobulin (BLG) and two small globular proteins that account for approximately 70-80% of total whey protein. Historically, whey has been considered a waste product and disposed of in the most cost-effective manner, or processed into relatively low value commodities such as whey powder and various grades of whey protein concentrate/isolate (WPC, WPI). Nowadays, whey proteins and their derivatives are widely used in the food industry due to the excellent functional and nutritive properties adding to the commercial value of the processed foods [
8]. The biological components of whey proteins, including β-lactoglobulin, α-lactalbumin, lactoferrin, lactoperoxidase, immunoglobulins and glycomacropeptides, demonstrate a wide range of immune enhancing properties, and act as antioxidant, antihypertensive, antitumer, antiviral, antimicrobial and chelating agent. They also improve muscle strength and body composition and prevent cardiovascular, cancer diseases and osteoporosis [
9].
In spite of their high biological properties, native whey proteins are not hydrolyzed easily by means of digestion enzymes as pepsin and trypsin, due to disulfide bonds in the protein molecules. The poor digestibility of whey proteins is considered to be the reason for their allergenicity [
10]. Therefore, modification of whey proteins to enhance or alter their biological and functional properties may increase its applications. Whey protein modification can be accomplished by chemical, enzymatic, or physical techniques [
11,
12]. Acetylation, succinylation, esterification, amidation, phosphorylation, and thiolation are chemical modifications that induce significant alterations of the structure and functional behavior of whey proteins.
Relatively small alterations of structure, brought about through chemical derivatization, often can be reflected in significant changes of physical and biological properties [
13,
14].
Many studies concerned with the antiviral activity of native and modified whey proteins in human [
15,
16]. Other studies focused on the use of milk or milk components to control plant viruses [
17]. Therefore the objective of this work was to find and study possible antiviral compounds that would provide effective disease control under practical conditions, while also minimizing environmental impacts using native and modified whey proteins fractions (α-lactalbumin, β-lactoglobuline and lactoferrine) to control TYLCV.
Discussion
Esterification is an important and easy tool of protein modification. Esterification blocks free carboxyl groups raising thus the net positive charge and rendering more basic the modified protein. It has been recently reported that increased basicity of dairy proteins after their esterification endow them with DNA-binding properties [
12,
14,
28].
Early studies led to several hypotheses about milk's mode of action. The first one was in the 1930s suggested that milk inhibited infection by somehow reducing the plant's susceptibility to the virus [
7]. The second one in the 1940s suggested that the milk "inactivated" the virus by forming a loose "molecular union" which, if broken, results in re-activation of the virus. That is, the inhibiting effects were reversible and the effect was on the virus and not the plant. The studies of an Australian scientist in the 1950s supported the earlier hypothesis that milk contains a substance that inhibits virus infection due to its effect on the plant, by supposedly inducing some type of resistance. It was also found that the inhibitory effects were restricted only in the treated part of the plant. Furthermore, investigations suggested that the active substance in the milk was a protein. The conclusion that the active substance is a protein component or number of such components is supported by recent work carried out by USDA scientists. But the answer to how exactly milk inhibits or reduces virus infection is still unknown [
6,
7].
Milk is rather heterogeneous suspension of oil (butter-fat), protein (cassein), sugar (lactose) and a multitude of possibly bioactive trace ingredients, including minerals, enzymes and vitamins. Possible modes of action of milk-based sprays were provided by [
29]. These include an increase in the pH of the leaf surface [
30], the establishment of a protective barrier, the establishment of possibly antagonistic organisms [
31,
32] the direct induction of systemic resistance [
33] and/or the production of biocidal compounds [
34]. All of the above processes will probably be highly dependent on the environmental conditions and the timing of the epidemic with respect to the phenology of the crop.
Milk contains several salts and amino-acids. These substances have been shown to be effective in controlling powdery mildew and other diseases [
31‐
33,
35‐
37].
The obtained results reveal that the antiviral activity of lactoferrin (either native or purified form) is greater than α-lactalbumin or β-lactoglobulin.
Milk whey proteins acquire net positive charges after esterification with methanol or ethanol enabling them to interact with negatively charged macromolecules such as nucleic acids or some proteins [
12]. Consequently, these basic proteins may interact with viral DNA or RNA. Esterification not only increases the gross positive charge of the protein but also its hydrophobicity by grafting hydrophobic methyl or ethyl groups on the carboxyl groups of aspartyl and glutamyl residues. Enhanced hydrophobicity may also promote hydrophobic interactions with the hydrophobic binding sites formed by viral capsid proteins. Some antiviral inhibitory effects were already explained by the entry of hydrophobic inhibitory molecules in the hydrophobic binding cavities on the viral surface [
38‐
40].
The interaction of antiviral proteins such as LF with receptors on cell surface and/or with viral envelope proteins is critical to blocking viral entry to target cells. The charge on the antiviral protein plays an indispensable role in this interaction. Chemical modifications lead to changes in the charges on milk proteins which can enhance their antiviral properties [
41,
42].
The results indicate that the inhibition of TYLCV may be related to the degree of cationisation of esterified whey proteins as well as to the size of the backbone protein which could be due to:1) Saturating binding to viral DNA by purely coulombic interactions, inhibiting its replication and transcription; 2) Hydrophobic interactions with viral capsid proteins; 3) Perturbation of viral DNA-protein interactions, hence inhibition of the translation of viral proteins; 4) Interference with/saturation of viral entry sites on the cellular membranes.
Many researchers recommend the use of milk to reduce the spread of virus particles between plants. Techniques using milk are frequently used in nurseries to stop the spread of virus between susceptible hosts when people touch the plant, during pruning. They reported milk proteins inactivated the capsid protein of the virus. Milk is not a potential environmental or food contaminant; consequently it can be used in organic agriculture.
Also, the data of [
43‐
45] indicated that whey was effectively used to control powdery mildew in cucumber and zucchini and they recommended further studies to optimize the concentration and timing of whey applications for mildew management in commercial crops.
The antiviral effect of the used whey protein fractions can be arranged in descending order as follows: lactoferrin (native or modified form) > native α-lactalbumine > modified β-lactoglobulin > modified α-lactalbumin = native β-lactoglobulin. More studies are needed to improve the antiviral activity of both of α-lactalbumin and β-lactoglobulin.
In future experiments, we will examine combined regimen of alternating milk-based and chemical sprays and also using different concentration of whey, whey protein fractions and skim milk. These strategies may provide adequate protection against this disease, while reducing the chemical load on the environment and forestalling the development of resistant strains.
Finally the use of alternative "green" methods would have its advantage in the market, as many consumers are ready to pay more for pesticide-free products. This point could be of enough interest to justify the present work.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AMA conceived the research, performed the experiments, and wrote the manuscript; SHT developed the conceptual aspects of the work and edited the manuscript; MIS conceived of the study, and participated in its design and coordination; AZA participated in the design of the study; MMA conceived the research, performed the experiments, and edited the manuscript; AAR carried out the molecular genetic studies. All authors read and approved the final manuscript.