The vast surface of the plant axis, stretching from root tips occasionally
buried deeply in anoxic sediment, to apical meristems held far aloft, provi
des an extraordinarily diverse habitat for microorganisms. Each zone has to
a greater or lesser extent its own cohort of microorganisms, in aggregate
comprising representatives from all three primary domains of life-Bacteria,
Archaea, and Eucarya. While the plant sets the stage for its microbial inh
abitants, they, in turn, have established varied relationships with their l
arge partner. These associations range from relatively inconsequential (tra
nsient epiphytic saprophytes) to substantial (epiphytic commensals, mutuali
stic symbionts, endophytes, or pathogens). Through recent technological bre
akthroughs, a much better perspective is beginning to emerge on the nature
of these relationships, but still relatively little is known about the role
of epiphytic microbial associations in the life of the plant.