Plant disease resistance inducers, so-called plant
activators, are agrochemicals that protect crops from
pathogens. They confer long-lasting resistance against a
broad range of diseases by activating their immune system.
Since plant activators impinge on host plants, unlike
commonly-used pesticides which directly target pathogens,
no drug-resistant microbes for plant activators have been
found so far in the field. They originated from probenazole
(Oryzemate ) and have been widely used over 30 years for
the protection of paddy-field rice from blast fungus and
bacterial leaf blight in East Asia. In spite of the advantages
of plant activators, their application is still limited. The lack
of both knowledge about their modes of action and an
appropriate high-throughput screening system restrict the
isolation of novel compounds. We established a highthroughput
chemical screening procedure to identify plant
immune-priming compounds which increase but do not
directly induce immune responses in Arabidopsis
suspension cells upon infection of the bacterial pathogen,
Pseudomonas syringae pv. tomato DC3000 avrRpm1. From
the screening of a commercially available chemical library
of 10,000 diverse small organic molecules, we identified
seven compounds that prime the immune response and we
designated them ‘imprimatins’ for immune-priming
chemicals. The isolated compound, imprimatin C1 activates
the expression of defense-related genes independent of
pathogen and functions as a weak analog of salicylic acid
(SA). Those originally-isolated three compounds and their
four derivatives were classified into two groups with distinct
molecular structures and they weve named imprimatins A
and B. We found that they conferred disease resistance in
plants by inhibiting both a known and a previously
unknown SA glucosyltransferase (SAGT). Imprimatins and
their targets are useful for the development of practical
plant activators and also their modes of action might give a
clue for novel crop protection technology
