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Arabidopsis CAD1 negatively controls plant immunity mediated by both salicylic acid-dependent and -independent signaling pathways

Tsutsui, Tomokazu, Asada, Yutaka, Tamaoki, Masanori, Ikeda, Akira, Yamaguchi, Junji
Plant science 2008 v.175 no.4 pp. 604-611
Arabidopsis thaliana, resistance mechanisms, signal transduction, salicylic acid, hypersensitive response, apoptosis, host-pathogen relationships, plant diseases and disorders
The Arabidopsis cad1 (constitutively activated cell death 1) mutant shows a phenotype of hypersensitive response (HR)-like cell death and increased levels of endogenous salicylic acid (SA), indicating that the CAD1 protein negatively controls the SA-mediated pathway in plant immunity. To clarify the relationship between the CAD1 protein and SA-mediated defense pathways, a molecular genetic analysis was carried out using mutants related to SA biosynthesis. A genetic cross between the cad1 mutant and the pad4 and sid2 mutants only partially suppressed the cad1 phenotypes. These double mutants still showed acquired resistance to Pst DC3000 infection similar to the cad1 single mutant. These results indicate that the CAD1 negatively controls not only the SA-dependent defense pathway but also the SA-independent pathway. To clarify the CAD1-mediated SA-independent pathway, a revertant was isolated from an M2 population of the sid2 cad1 mutant. The revertant can1t mutant did not show cell death phenotype and the plant and leaf size recovered to those of the wild-type. The can1t mutant also showed pathogen susceptibility comparable to wild type, indicating that the can1t mutant looses the pathogen-resistance that the cad1 mutant acquires. There results suggested that CAN1 is a positive regulator of the novel plant immunity under the control of CAD1.