The BON/CPN gene family represses cell death and promotes cell growth in Arabidopsis.

Copines are calcium-dependent membrane-binding proteins that are highly conserved among protozoa, plants, nematodes and mammals. Although they are implicated in membrane trafficking and signal transduction, the functions of these proteins are not well understood. The Arabidopsis copine gene BON1/CPN1 was previously shown to negatively regulate a disease resistance (R) gene SNC1. Here we report that in Arabidopsis, as in other organisms, there is a family of copine genes, BON1, 2 and 3. Using double and triple mutant combinations we show that these three copine genes have overlapping functions essential for the viability of plants. The loss of function of BON1 combined with that of BON2 or BON3 leads to extensive cell death phenotypes resembling the hypersensitive response (HR) in defense responses. The resulting lethality can be suppressed by mutations in PAD4 or EDS1 which are required for R gene signaling and cell death control. Accession-dependent phenotypes of the mutant combinations suggest that the BON/CPN genes may together repress several R genes other than SNC1. Moreover, the mutant combinations exhibit developmental defects when R-gene-mediated defense responses are largely suppressed in pad4 and eds1 mutants. Thus, the copine family in Arabidopsis may have effects in promoting growth and development in addition to repressing cell death, and these two processes might be intricately intertwined.