Deciphering endophyte behaviour: the link between endophyte biology and efficacious biological control agents

Fungal and bacterial endophytes associate with the majority of plant species found in natural and managed ecosystems. They are regarded as extremely important plant partners that provide improved stress tolerance to the host compared to plants that lack this symbiosis. Fossil records of endophytes date back more than 400 million years, implicating these microorganisms in host plant adaptation to habitat transitions. However, it is only recently that endophytes, and their bioactive products, have received meaningful attention from the scientific community. The benefits they can confer to their plant hosts include the promotion of plant growth and survival through the inhibition of pathogenic microorganisms and invertebrate pests, the removal of soil contaminants including heavy metals, improved tolerance of low fertility soils, and increased tolerance to extreme temperatures and low water availability. These endophytes, by their definition, require a suitable plant host for some or all of their lifecycle and cause no apparent disease symptoms. However, this class of microorganisms is also extremely diverse and can exhibit many different biological behaviours with respect to their class of biological interaction (mutualistic vs. commensalistic); 2) the level of symbiotic intimacy (obligate vs. facultative); 3) their in planta colonisation patterns (systemic vs localised); 4) degree of host specificity (low vs. high); 5) their means of propagation (vertical vs. horizontal) and 6) the mechanism of biological control by which they operate. Not all microbial endophyte technologies have been successfully commercialised for a range of reasons. Of interest in the development of the next generation of plant protection products is how much of this is due to the biology of the particular endophytic microorganism. In this review, we highlight selected case studies of endophytes and discuss their lifestyles and behavioural traits, and discuss how these factors contribute towards their effectiveness as biological control agents.