Dicamba

Brief update on geno- and cytotoxicity of dicamba, an auxinic herbicide

1. Pesticide and herbicide definition.
2. Auxinic herbicides.
3. Characterization of dicamba.
4. Prior investigations on genotoxicity and cytotoxicity of dicamba.
5. New findings on the genotoxic and cytotoxic capabilities of dicamba.

Some words before starting

The aim for the present article is to present updated data on the geno- and cytotoxicity of dicamba, an auxinic herbicide commonly used worldwide. Definitions of pesticides and herbicides are given for contextualizing the topic. The main features of auxinic herbicides are presented as well as a characterization of dicamba taking into account its role as a xenobiotic molecule. Finally, the genotoxic and cytotoxic capabilities will be developed, dealing with previous data and new findings.

What is a pesticide? And a herbicide?

A pesticide, as defined by EPA, one of the most worldwide recognized regulatory agencies, is “any substance or mixture of substances intended for preventing, destroying, repelling or mitigating any pest”. For further clarification of the term pesticide, the following is added: “Though often misunderstood to refer only to insecticides, the term pesticide also applies to herbicides, fungicides, and various other substances used to control pests.” In order to acquire a more complete notion of the term pesticide, a revision of what is actually a pest is needed. Again, EPA provides a definition: “Pests are living organisms that occur where they are not wanted or that cause damage to crops or humans or other animals. Examples include insects, mice and other animals, unwanted plants (weeds), fungi, microorganisms such as bacteria and viruses, and prions.” . However, as Ware and Whitacre (2004) call to our attention, pests are organisms that are competitive to mankind or its interests in some manner. In this way, pests are defined by human beings and their life-style and preferences rather than by nature: i.e. in an agricultural environment pests may include diverse animal, plant, bacterial and fungal species that compete with or damage crops whereas in an urban environment pests would be represented by another set of species that carry diseases, eat fabric wools, leather and wood, deteriorate or infest packaged foods and garden plants or even infest pets. In short, every person has an intuitive appreciation of what a pest is . In this line of thought, a herbicide is then, a type of pesticide used to kill unwanted plants . Herbicides are employed mainly to protect crops that are affected by weeds in various ways. Unwanted plants compete with crop plants for sunlight and also for water and nutritive substances in the soil. Harvested grain may be contaminated with weed seeds that decrease the quality of crops and may be toxic to humans and animals. Complete loss of the crop is also a result of competitive effect of unwanted plants Herbicides have been used to protect crops since ancient times. The first recorded use of a herbicide can be traced back to 1200 BC when the Biblical armies spread the conquered fields with salt and ashes in order to make them unproductive . Ever since this nonselective herbicide application, mankind has employed herbicides. Many substances have been applied to fight unwanted plants, for instance pouring olive oil on their roots . The discovery of the first chemical weed killer is attributed to Marco Terentius Varro, a Roman scholar (116 BC – 27 BC). Varro recommended the application of amurca to control weeds . Amurca is the watery liquid obtained from pressed olives under light pressure, before olive oil is drained under greater pressure . An herbicide mixture was prepared by boiling the amurca with salt in copper . For a long time farmers protected their crops by the application of few chemicals available such as petroleum oils, diluted sulfuric acid, iron sulfate, copper sulfate, copper nitrate, and sodium arsenate to control broadleaf weeds in cereal crops . This situation changed after the Second World War with the advent of synthetic organic pesticides, starting with DDT .

Auxinic herbicides

There are multiple schemes of herbicide classification according to several criteria, such as their chemical similarity, site of action, mode of action, timing and method of application. the classification which will be developed in this article is the categorization proposed by Kappler and Namuth (2004) based on the mode of action of herbicides. These authors list seven possible modes of action for herbicides and include an eighth group for those compounds which lack a known mode of action or have not been classified. Herbicides act as inhibitors on 1) amino acid synthesis, 2) seedling growth, 3) photosynthesis, 4) lipid synthesis, and 5) pigment metabolism. Herbicides also act as cell membrane disrupters, and growth regulators. Growth regulators are of particular interest for dicamba is a member of this group of herbicides. This class of herbicides are usually termed as auxinic herbicides for they mimic auxins action in the coordination of several growth and behavioral processes in the plant’s life cycle . The molecules of synthetic auxinic herbicides resemble the structure of the natural plant hormone auxin (indole-3-acetic acid, also known as IAA) and cause physiological effects in sensitive plants similar to those caused by high doses of IAA . Other similarities to IAA include:

1. at low doses they act as plant-growth regulators and have a stimulatory effect on plant-cell growth, while at high doses they exert phytotoxic effects;
2. some of them can replace IAA as the hormone supplement used for proper cell development in plant-cell culture medium;
3. differential sensitivity has been observed among different tissues as well as between tissues at different physiological stages of growth; and
4. auxin and auxinic herbicides induce growth by cell elongation as opposed to cell division.

Herbicide families within this mode of action include phenoxies, picolinic acids, carboxylic acids, and the benzoic acids . The chemical families, active ingredients, some common trade names and major uses of auxinic herbicides can be accessed at http://plantandsoil.unl.edu/croptechnology2005/crop_tech/?what=topicsD&informationModuleId=1022008824&topicOrder=2&max=7&min=1&

Benzoic acid derivatives are mostly applied as dimethylamine salts. The majority of the compounds in this group are employed as herbicides but some of them have fungicide, insecticide and microbicide properties. Benzoic herbicides are slightly toxic and can cause dermatitis through dermal exposure. Besides dicamba, clorphenac, chlorambem, and tricamba are members of the benzoic acid .