Endophyte: The Misuse of an Old Term

Abstract

In dictionaries of biological terms and phrases, an endophyte is defined as an organism that lives in another organism, in contrast to an epiphyte living on the outside (see e.g. Ainsworth 1971). The term endophyte started to appear in the literature more regularly during the 1980's. However, now it had been made equivalent to fungi in grasses and trees living mostly asymptomatically within the host plant, and giving the host acquired resistance against herbivores (see e.g. Carroll 1988, Clay 1988). Today the term endophyte is commonly defined as a group of fungi which invade the stems and leaves of plants but cause no symptoms of disease (Wilson 1993). To use an already existing general term, such as endophyte, on a specific group of fungi may prove to be difficult. Therefore, the aim of this paper is to show some of the problems associated with the use of the term, and answer questions like: (1) does a specific group of fungi exist that warrant the use of a term? (2) have there been any changes made to the term endophyte to make it more specific and (3) can scientists in one field of research take an already existing term and use it specifically for their organisms? An examination of the so called endophytes reviewed by Clay (1988) and Carroll (1988) shows that they have few features in common. For example, some live asymptomatically within the host while still many do sporulate more or less frequent. They can be systemic or nonsystemic, and some are claimed to be mutualistic, while others are clear parasites and others are borderline cases. This list can be made even longer, and it suggests that there might not exist a specific group to apply the term on. This ambiguity and the confusion it generates is readily found in publications about endophytes (see e.g. Wilson 1993). Furthermore, there has been no changes made to the term to make it fit a specific group of organisms. By leaving the original definition of the term (i.e. living within an organism) and make it specific for a biological group without a redefinition or at least a definition of the boundaries of the new group, will only result in that more and more species will be included, and only chaos will ensure. For example, today researchers have included a vast array of fungi among the endophytes ranging from VAM-mycorrhiza (Clay 1990) to tarspot (Rhytisma punctata) (Wilson 1993). However, even more species will be possible to include. Thus, it will be legitimate to include fungi such as the systemic smuts and rusts, since many of these may live asymptomatically in the plant, from one to several years, often only sporulating for short periods (Wennstrom 1994, Wennstrom and Ericson 1994). Data also show that rusts may enhance growth and the competitive ability of infected hosts (Wennstrom and Ericson 1991). However, I do not believe that systemic rusts and smuts should be included among the so called endophytes, since: (1) it will further complicate things, (2) there is nothing to gain from including them and (3) the term endophyte is already too vague. The term endophyte was first applied to fungi that were found living in the plant. They were believed to live entirely within the host plant. Therefore, the use of the term endophyte seemed to be appropriate at that time. Even if all of these fungi really lived totally asymptomatically within the host, I believe that the usage of an already existing term should be avoided and researchers can not simply adopt a term that is used for something else. This will only result in confusion and uncertainty. I argue that problems associated with the meaning and interpretation of the term endophyte have now become so serious as to question its use altogether. Therefore, I believe it is now time to stop and reconsider the basics. First of all we need to have a clear and sensible definition of the group that focuses on the important features of these fungi. Is it an important feature that they live asymptomatically within the plant, that they produce antiherbivore substances, or that they are mutualistic, or are there other features that are more important? The lack of a definition results in that current discussion about selection pressures and evolutionary consequences among en-