2.2.2.2 Furocoumarins
Furocoumarins are found in many plant species, such as celery, citrus plants, and parsnips. In plants, they are released in the presence of stressful events, such as drastic climatic changes and physical damage. They may cause skin reactions and, in susceptible people, gastrointestinal problems may follow.
2.2.2.3 Lectins
Lectins are common phytotoxins in beans, with the highest concentration in red kidney beans. This is the reason why even a few beans eaten raw can cause nausea, severe stomachache, and sometimes diarrhea. At high temperatures, lectins are destroyed. In local settings, boiling beans for over 10 minutes helps to eliminate these phytotoxins.
2.2.2.4 Solanines and Chalcones
Solanines and chalcones are glycoalkaloid phytotoxins present mostly in the Solanaceae family. This is a family that includes potatoes, tomatoes, and eggplants. Generally, the concentration of these toxins is low but they are found to be high in some plants depending on the stage of growth and the environment. Unripe tomatoes and potato sprouts have relatively high levels of solanines and chalcones. Stressful conditions such as ultraviolet light, high temperatures, biochemical attack from microorganisms, and physical damage such as bruising provide a good environment for the release of these toxins.
2.2.2.5 Pyrrolizidine Alkaloids
Pyrrolizidine alkaloids are a group of toxins produced by about 600 plant species. They are most commonly found in the Asteraceae, Boraginaceae, and Fabaceae families. They generally grow as weeds that, in the course of growth, tend to contaminate food plants. Pyrrolizidine alkaloids have high stability during processing and they have been detected in honey, herbs and spices, herbal teas, and wheat products. The overall risk to health is yet to be explored [13]. This demonstrates the convenience and importance associated with the chemical characteristic classification of phytotoxins.
2.2.2.6 Pharmacological Characteristics
These are related to biological characterization, but with the addition of relevance in therapeutic, pharmacokinetic, and pharmacodynamics activities. Although endophytes usually cause diseases to the host plants, they sometimes have beneficial effects to the plants as well as the animal kingdom that consume the attacked plants. Endophytic fungi has been known to have damaging properties on the leaves of c. Papaya for example but some toxic compounds isolated from these endophytes possess very good cytotoxic properties which can be used to develop drug compounds to control, manage and possibly cure cancers in its various specific stages. In the same understanding, these toxins can be instrumental in the pharmaceutical industry focusing on what they do as a means of their interaction with biological targets. Some can act better on fungi, others on bacteria, viruses and protozoa. Their interactions could span from cytotoxicity, metabolic modulation, imunal modulation, cell growth arrest or enhancement, transportation efficiency and excretion among other pharmacokinetic and physiological proccesses. All phytotoxins that have similar properties as described in the sections above can be categorised based on their pharmacological relevance and studied together.
2.3 Currently Available Classification Tools
Apart from some textbooks [14, 15], there is also software available to researchers to help them to make collections and classifications of phytotoxins that are uploaded in their respective libraries. These include the Aggregate Computational Toxicology Online Resource database [16], which is managed by the US Environmental Protection Agency; the Clinical Toxicology (CliniTox) database [17, 18]; the Toxic Plants–PhytoToxins (TPPT) database [19]; the SuperToxic database [20]; the compendium of the European Food Safety Authority [21]; the Super Natural II database [22]; and KNApSAcK‐3D [23–25]. Despite the existence of these databases, there are still gaps in systematically clustering phytotoxins because of the limitations of the tools. For example, the ACToR database fails to link toxins and toxin metabolites to some of their effects on, for example, environmental management [16] and the CliniTox database falls short in providing details of chemical characterization [19].
The TPPT database has made efforts to be better than most; however, it is primarily focused on European plant phytotoxins, in particular Swiss plants, apart from a few of the most commonly known toxins from elsewhere. The justification for this is that Swiss vegetation is a good representation of central Europe, with several altitude zones, giving it the advantage of having a wide range of plant species [26]. This being the case, there remains a need for a more improved version of a tool or a non‐computer‐based standard that can incorporate regional databases and standards into one that can give a wider picture in one resource. The databases that are available can be useful in modeling a standard that can be used in the classification of phytotoxins. In this way, phytotoxins can be optimized using the databases and/or the standards developed thereof to suit the needs of researchers, government agencies, and industries.
2.4 Role of Phytotoxin Classification
The classification of phytotoxins plays a role mainly in human survival and economic development [1]. This is evident in agricultural management, studies of natural medicines, the discovery of novel drugs and their pro‐drug metabolites, the preservation and protection of the quality of water in various water bodies, the security of societies, and the proper management of the environment to ensure people’s safety. The sections below briefly detail how important the classification of phytotoxins is to society.
2.4.1 Drug Discovery
The classification of phytotoxins by their biological activities is helpful in drug discovery as researchers can then easily target a metabolite and study it just because it has similar bioactivities to some already known standard drugs. Ascochytin, a phytotoxin from Ascochyta pisi that induces spotting disease on the leaves of peas, is structurally similar to citrinin, a mycotoxin with antifungal activities. As such, ascochytin was regarded as a potential hit for an antifungal [1, 27]. The same is true for griseofulvin, an antibiotic produced by Penicillium griseofulvum as one of its natural products, which is also a known phytotoxin [28].
2.4.2 Environmental Monitoring
Understanding the type and nature of phytotoxins in an environment is essential to the safety of the community. Among many other potentially harmful and toxic effects of phytotoxins, some can be fatal if ingested in very small amounts; others are irritants to the skin, eyes, and the respiratory tract and can cause more significant physiological harm with time; and yet other phytotoxins are carcinogenic. Understanding these phytotoxins can help in devising ways through which the community can avoid or effectively minimize the risks associated with each one of them or, at least, know which category they may fall in. It is imperative for environmental monitors to include phytotoxins in their various assessments, including environmental impact assessments for community projects. The incorporation of phytotoxins in environmental monitoring is said to be hampered by the high diversity of phytotoxin structures and the unavailability of reliable and well‐validated standard analytical methods [19]. This challenge can be overcome by devising standard protocols that are only feasible with the development of a sound systematic categorization of the toxins.
2.4.3 Phytotoxins, Aquatic Life, and Water Quality
Phytotoxins that are produced by algae in bodies of water, both fresh and oceans, are known as algal toxins [13]. They contaminate fish and other aquatic animals to different extents; they also contaminate drinking water and cannot be eliminated by freezing or cooking [13].
Phytotoxins can also potentially contaminate bodies of water, particularly in situations where leaching of chemicals from plants into the water is possible. This happens particularly for phytochemicals