Yeast killer toxins: from ecological significance to application

A linear model of interspecific competition with separate parameters for exploitation and interference is deduced. Interference is assumed to have a cost and an effect. The interfering species realizes a "profit" if some resources, which the species interfered against would have utilized, are made available as a result of the interference. Interference is favored when its cost is small, its effect is high, and the resource overlap with the species interfered against is high. Interference is likely to be an alternative strategy to high exploitation efficiency. The incorporation of interference into niche theory clarifies the competitive phenomenon of unstable equilibrium points, excess density compensation on islands, competitive avoidance by escape in time and space, the persistence of the "prudent predator," and the magnitude of the difference between the size of a species' fundamental niche and its realized niche.

Significant losses in harvested fruit can be directly attributable to decay fungi. Some of these pathogenic fungi are also the source of mycotoxins that are harmful to humans. Biological control of postharvest decay of fruits, vegetables and grains using antagonistic yeasts has been explored as one of several promising alternatives to chemical fungicides, the use of which is facing increasingly more stringent regulation. Yeast species have been isolated over the past two decades from a variety of sources, including fruit surfaces, the phyllosphere, soil and sea water, and their potential as postharvest biocontrol agents has been investigated. Several mechanisms have been proposed as responsible for their antagonistic activity, including competition for nutrients and space, parasitism of the pathogen, secretion of antifungal compounds, induction of host resistance, biofilm formation, and most recently, the involvement of reactive oxygen species (ROS) in defense response. It has been recognized that a biocontrol system is composed of a three-way interaction between the host (commodity), the pathogen and the yeast, all of which are affected by environmental factors. Efficacy and consistent performance in controlling postharvest diseases are the hurdles that must be overcome if the use of yeast biocontrol agents and other alternatives are to be widely used commercially. Therefore, attempts have been made to combine alternative treatments in order improve their overall performance. The current review provides a brief overview of the topic of the use of yeasts as postharvest biocontrol agents and includes information on the sources from which yeast antagonists have been isolated, their mode of action, and abiotic stress resistance in yeast as it relates to biocontrol performance. Areas in need of future research are also highlighted.

Since the discovery of toxin-secreting killer yeasts more than 40 years ago, research into this phenomenon has provided insights into eukaryotic cell biology and virus-host-cell interactions. This review focuses on the most recent advances in our understanding of the basic biology of virus-carrying killer yeasts, in particular the toxin-encoding killer viruses, and the intracellular processing, maturation and toxicity of the viral protein toxins. The strategy of using eukaryotic viral toxins to effectively penetrate and eventually kill a eukaryotic target cell will be discussed, and the cellular mechanisms of self-defence and protective immunity will also be addressed.