2.9.1S
WHAT ARE ENDOPHYTES ANYWAY?
O Petrini

Comano, Switzerland

Endophytes were originally defined as those organisms that live within a plant tissue. Some researchers, however, have started to consider endophytes to be nonaggressive organisms, often even postulating a mutualistic role within their hosts. In its most conservative definition the term 'endophyte' now includes all organisms that, at some time of their life cycle, live symptomlessly within plant tissues. Many people tend to think of endophytes as being exclusively fungi, yet a large number of bacteria are also known to live endophytically.

Endophyte associations may range from intimate contact, where the organism inhabits the intercellular spaces and xylem vessels in the plant, to more or less superficial colonization of peripheral tissues such as bark layers. For endophytes colonising bark and phellem tissues the specific term 'phellophytes' has been proposed. The mainly clavicipitaceous grass endophytes live systemically in a mutualistic symbiosis with their hosts. The endophytes of trees and shrubs, together with the non-clavicipitaceous grass endophytes are apparently ubiquitous taxa as well as host-specific taxa. It can be expected that investigations employing biochemical detection and differentiation methods will probably reveal a large biodiversity among both types of symbionts.

A considerable research effort has been directed towards the clavicipitaceous grass endophytes. There has been a sustained emphasis on the potential benefits derived by the host from the symbiosis and on the use of grass endophytes in the biocontrol of plant pests. Studies on the clavicipitaceous endophytes may, however, be of limited relevance for other research on endophytes, as these fungi form a particular group of closely related species with ecological requirements and adaptations distinct from that of other endophytes.

Symptomless endophytes of plants other than grasses have been known for more than 70 ;years, yet most investigations on endophytic fungi of trees and shrubs date only from the last 20 ;years. It is now assumed that phanerogams almost always harbour endophytes, but the mode of colonization by endophytes has only been thorougly investigated in grasses. At present, the grass endophytes remain the only group of endophytic fungi which have been shown to colonise their hosts systemically.

Endophytes are ideally suited for studies in population biology. As they are extremely common in the plant kingdom, the endophytic assemblages of various host taxa can serve as models for comparative investigations. In addition, a large number of endophytic fungi can be easily cultivated, thus easily allowing investigations of the physiology, biochemistry and molecular biology. Finally, they have rarely been subjected to any artificial selection pressure and are therefore excellent objects for population studies.

Endophytic fungi can be regarded as specialized symbionts of higher plants. Many of them appear to have evolved mechanisms for coordinated communication and regulation with their hosts. They are often characterized by a phase of interrupted growth following infection, or at least by a cryptic phase where asymptomatic colonisation of living host tissue occurs. Their presence on host tissue often becomes apparent only after the onset of natural senescence or induced necrosis. Endophytic fungi have also been described as playing a protective role against insect herbivory not only in grasses but also in conifers and many are potential producers of novel antimicrobial secondary metabolites.

The early occupation of host tissue prior to either natural senescence or induced necrosis gives endophytes an advantage over saprobes normally excluded from healthy tissue. Metabolites produced by some endophytes can modify host growth responses and accelerate or delay senescence. Endophytes might therefore prove interesting subjects for the study of coordinated control of plant growth regulators and anti-fungal metabolites.

Considering the variability of interactions seen among endophytes, it would be wise to define them in a neutral way that does not imply any advantage or disadvantage for the individual partners.