Plant tissue culture contamination: challenges and opportunities

Researchers have striven, often without success, to eliminate all microorganisms from in vitro plant cultures, assuming that the presence of any bacterium or fungus is harmful and undesirable. Although contamination remains a serious challenge for in vitro plant cell, tissue or organ culture researchers, over the past 25 years a large body of evidence has accumulated that the presence of microorganisms in plant in vitro cultures, while often harmful, may also, depending on the specific organism, have no effect on cultures or even be beneficial, presenting new opportunities for micropropagation or secondary metabolite production. Furthermore, in recent years, evidence has been accumulating that calls into question the entire possibility of a microorganism-free (axenic) in vitro plant culture. This presentation will review recent developments in identifying and controlling in vitro contaminants, will describe the effects of different types of microorganisms and will suggest new ways of describing and evaluating the presence of microorganisms. Although the beneficial effects of microorganisms on plants has been known for more than 90 years, it has only been during the last 25 years that such effects have been described in plant tissue cultures. These studies, and the opportunities afforded by them, are reviewed. Also, recent evidence suggesting that there is no such thing as an axenic or sterile plant in vitro culture is presented and its implications discussed.