The neoplastic diseases crown gall and hairy root are incited
by the phytopathogenic bacteria Agrobacterium tumefaciens and
Agrobacterium rhizogenes, respectively. Although the molecular
mechanism of T-DNA transfer to the plant most likely is the same
for both species, the physiological basis of tumorigenesis is
fundamentally different. Crown gall tumors result from the overproduction
of the phytohormones auxin and cytokinin specified
by A. tumefaciens T-DNA genes. Although the T-DNA of some Riplasmids
of A. rhizogenes contains auxin biosynthetic genes,
these loci are not always necessary for hairy root formation.
Recent experiments suggest that hairy root tumors result from
the increased sensitivity of transformed cells to endogenous
auxin levels. An understanding of hairy root tumorigenesis will
likely result in an increased knowledge of plant developmental
processes.