Biotrophy and the cytokinin conundrum

Cytokinin biosynthesis in plants has been studied for well over 40 years, yet despite intensive research we are still no closer to understanding the molecular steps involved in this pathway. In addition to well established effects in plants, it is clear that cytokinin fur also plays an important role in the interactions of some microorganisms with plants, thus stimulating further questions about the origins of cytokinins and raising the possibility of the involvement of micro-organisms in cytokinin biosynthesis and distribution. More recently with research into PPFMs and other intricate plant-microbe interactions, the cytokinin conundrum seems likely to become even more complex. This review highlights these issues and proposes a hypothetical model for cytokinin biosynthesis and distribution during plant-microbe association. (C) 2000 Academic Press.