At different times after inoculation the plants and soil were
removed from the containers, the soil was carefully separated from the roots, and
the roots were gently shaken. Each whole plant was placed in 5 ml (tomato) or
2 ml (A. thaliana) of washing buffer in a vial. The vial was sealed and inverted 10
times. The washing buffer was removed, and the procedure was repeated. The
number of viable bacteria per milliliter of washing buffer was determined by
viable cell counts using Luria agar containing 50 mg of rifampin per ml. The
viable cell counts in the first two washes were combined to give the number of
bacteria loosely bound to the roots. Each plant was then placed in 5 or 2 ml of
washing buffer and sonicated with a Branson ultrasonic cleaner (model B220) for
60 s (tomato) or 30 s (A. thaliana). The viable cell counts for the bacteria released
into the washing buffer were determined, and the resulting data are reported
below as the number of tightly bound bacteria. The plants were then removed
from the buffer, placed on Luria agar plates containing rifampin, and covered
with soft (0.7%) agar. The root lengths were measured. The plates were incubated
until colonies were apparent. The number of colonies per centimeter of