Active oxygen species (AOS) are believed to have important roles in plants in general and in plant—pathogen interactions in particular. They are believed to be involved in signal transduction, cell wall reinforcement, hypersensitive response (HR) and phytoalexin production, and to have direct antimicrobial effects. Since current methods are inadequate for localizing AOS in intact plant tissue, most studies have been conducted using cell suspension culture/elicitors systems. 3,3‐diaminobenzidine (DAB) polymerizes instantly and locally as soon as it comes into contact with H₂O₂ in the presence of peroxidase, and it was found that, by allowing the leaf to take up this substrate, in‐vivo and in‐situ detection of H₂O₂ can be made at subcellular levels. This method was successfully used to detect H₂O₂ in developing papillae and surrounding haloes (cell wall appositions) and whole cells of barley leaves interacting with the powdery mildew fungus. Thus, H₂O₂ can be detected in the epidermal cell wall subjacent to the primary germ tube from 6 h after inoculation, and subjacent to the appressorium from 15 h. The earliest time point for observation of H₂O₂ in relation to epidermal cells undergoing HR is 15 h after inoculation, first appearing in the zones of attachment to the mesophyll cells underneath, and eventually in the entire epidermal cell. Furthermore, it was observed that proteins in papillae and HR cells are cross‐linked, a process believed to be fuelled by H₂O₂. This cross‐linking reinforces the apposition, presumably assisting the arrest of the pathogen.

Background: Fungal endophytes occur ubiquitously in plants and are being increasingly studied for their ability to support plant health and protect the host from diseases. Using endophytes in disease control provides potential advantages compared to other biocontrol agents since they colonise the plant internally and thereby are protected by the host, thus potentially reducing environmental sensitivity. A thorough understanding of their mechanisms is required in their mutualistic association with plants; both to optimise their efficacy and for registration as plant protection products. Aims: To provide a critical review on the mechanisms employed by endophytic fungi in biological disease control. Furthermore, we draw attention to gaps in our knowledge of the complex interactions between plant, pathogen and endophyte and discuss implications for future research. Methods: Review of literature where endophytic colonisation during the specific interaction has been confirmed. Results: Known disease-reducing mechanisms include direct inhibition of pathogen activity by competition, antibiosis and mycoparasitism and indirect inhibition by induced resistance, where the plant’s own defence system is activated to combat the diseases. Relying on in vitro studies of alone can result in misleading conclusions. Conclusions: We need to investigate nature and requirements for establishment of successful plant-endophyte interactions, for development of efficient biocontrol agents.