Plant Surface Micro-Organisms as Sources of Compounds Toxic for Humans and Domestic Animals

Abstract

At first sight this topic may evoke a variety of expectations, or even cause surprise that toxinogenic micro-organisms might be of any consequence on the surface of plants. Generally most non-senescent plant surfaces do not support populations of potentially saprophytic micro-organisms that are significant in the local generation of toxins. Sometimes pathogenic fungi may establish sub-cuticular, or deeper, infections in particular host plant leaves, stems, inflorescences and fruits, but most of these fungi are not recognised sources of toxins for human and/or domestic animals. Populations of bacteria and yeasts may be numerically large on plant surfaces such as fruits, through which sugars and amino acids exude or where exudates from insect damage may accumulate. Again, these micro-organisms are not notably toxinogenic except indirectly as sources of acids and alcohol in fermentation processes. However, microbial propagules, often in the form of spores, readily impact on aerial plant surfaces, so that plant material used as human food or animal feed is often inoculated with an array of micro-organisms that may proliferate in appropriate conditions of moisture and temperature if optimal storage conditions are not maintained. Then, superficial latent micro-organisms, mainly fungi, will proliferate superficially on, or penetrate deeply into, food commodities and cause spoilage. More moisture and heat will be generated through such metabolic activity and an even more complex environment, nutritionally and physically, will arise within which regulatory influences on fungal metabolism may switch on pathways for secondary metabolite biosynthesis. These pathways are widespread in their distribution amongst opportunist fungi and are extremely diverse in their expression, leading to a vast array of end-product compounds.