Host Responses to Biological Control Agents

TLDR: A deeper understanding of fruit responses that BCAs provoke of the infection process by necrotrophic pathogens during postharvest and of the accompanying host responses is needed.

Abstract: Host responses in stored fruits induced by biocontrol agents (BCAs) i.e. by non-pathogenic yeasts and bacteria, share many features with the defence mechanisms that are induced in actively growing plant tissues. The perception of a microorganism is accompanied by the production and activation of reactive oxygen species (ROS), antioxidant enzymes, phytoalexins, phenylalanine ammonia lyase and enzymes that degrade fungal cell walls. The responses of harvested fruit to BCAs do not fit with the existing division of induced resistance pathways into Systemic Acquired Resistance (SAR) and rhizobacteria-mediated Induced Systemic Resistance (ISR), nor are the roles of salicylic or jasmonic acid clear. These responses seem to carry elements of both pathways. Moreover, successful BCAs need to be able to resist environments rich in toxic ROS; hydrogen peroxide being the dominant species, generated both during the induction of resistance (as in the defence of citrus fruit against Penicillium digitatum) and during the attack of some necrotrophic pathogens (as in the case of Penicillium expansum invading apples). Application of BCAs to fruits can result in increased production of antioxidant enzymes (by either organism), which protect living cells from the potential damage of ROS. Induction of resistance has usually not been considered an important mechanism in the activity of postharvest biocontrol agents. A deeper understanding of fruit responses that BCAs provoke of the infection process by necrotrophic pathogens during postharvest and of the accompanying host responses is needed. In the following chapter, we present examples from diverse plant-pathogen-BCA systems and suggest approaches for future research.