Production of reactive oxygen species (ROS) and hypersensitive cell death are characteristic features of the induced pathogen resistance in plants. Ca²⁺-signals preceding the pathogen-induced cell death have been shown to involve Ca²⁺ entry via the plasma membrane. We therefore analyzed Ca²⁺-signals and Ca²⁺-channels involved in the responses to pathogen attack and ROS. For this purpose, we used the Ca²⁺-reporter protein aequorin as well as the combined single-cell fluorescence imaging and patch clamp technique. Moreover, we compared Ca²⁺-channel activities in Arabidopsis wild type and the pathogen-resistant mutant, dnd1 (Yu et al., 1998) . Cellular responses to ROS were significantly reduced in dnd1. Since DND1 was shown to encode the Ca²⁺-permeable and cyclic nucleotide-gated ion channel CNGC2 (Clough et al., 2000) , this channel represents a possible candidate for H₂O₂-dependent Ca²⁺ entry via the plasma membrane. We thoroughly examined this hypothesis and will present recent results on how CNGC2 transmits cellular responses to reactive oxygen species, a step essential for the control of hypersensitive cell death.

1. Clough SJ, Fengler KA, Yu IC, Lippok B, Smith RK, Jr., Bent AF (2000) The Arabidopsis dnd1 "defense, no death" gene encodes a mutated cyclic nucleotide-gated ion channel. Proc. Natl. Acad. Sci. USA 97: 9323-9328
2. Yu IC, Parker J, Bent AF (1998) Gene-for-gene disease resistance without the hypersensitive response in Arabidopsis dnd1 mutant. Proc. Natl. Acad. Sci. USA 23: 7824