In order to obtain a double-resistance strain of CHM1, rifampicin
and streptomycin, at a series of concentrations (5–200 mg ml1),
were added, respectively, into the LB medium, on which CHM1 was
spread and inoculated for 36 h at 28 C. A double-resistance strain
of CHM1 with resistance to rifampicin and streptomycinwas picked
out for a re-inoculating experiment as follows: the double-resistance
strain of CHM1 was incubated in LB culture with 100 mg ml1
rifampicin and streptomycin at 28 C until the bacterial concentration
reached 107 cfu ml1. An aliquot (1 ml) of the culture was
drenched around the roots of rice and cole with three to four pieces
of leaves. One month after drenching, 5 g of stems and leaves were
collected. The strains were re-isolated according to the preceding
method as described in Section 2.2, and then verified by spreading
over a double-resistant selective plate with 100 mg ml1 rifampicin
and streptomycin.