When resistance sources are not available in germplasm,
one strategy is to introduce inheritable changes or mutations
in crop plants and to select rare mutants having resistance
to particular stresses. Various mutagenic agents are used
for the purpose and they are divided into two main groups,
i.e. physical mutagens (gamma, UV, X-ray irradiation) and
chemical mutagens (EMS, MMS, colchicine etc.). Some
novel chemicals like benzothiadiazole (BTH) have also
been used in wheat and tobacco for this purpose. In such
an experiment, BTH was used as an activator of systemic
acquired resistance (SAR) in Arabidopsis thaliana.
Mutants showed resistance against several pathogens
including Pseudomonas syringae pv tomato, turnip
crinkle virus, and Peronospora parasitica. Mutants showed
accumulation of mRNAs of SAR related PR-1, PR-2, and
PR-5 genes, which made the plant resistant to the abovementioned
pathogens (Lawton et al., 1996).