Fig. 5.
SAFE1 is a suppressor of 1O2-induced, EX1-independent signaling. (A) Possible modes of SAFE1 function. SAFE1 might operate as a negative regulator downstream of EX1 (I), negatively regulate an unknown downstream component (II), or act in an EX1-independent pathway (III). In the latter case, flu safe1 plants should display a very strong 1O2-induced stress response, whereas safe1 plants should behave like WT. (B–D) 1O2-induced stress responses are enhanced in flu safe1 and absent in safe1 as indicated by levels of cotyledon bleaching (B), chlorophyll autofluorescence (C), and Fv/Fm values (D). Different levels of 1O2 generation were achieved by incubating seedlings in the D as indicated. Mean values ± SDs (n > 30) are provided. (E) Direct visualization of Pchlide accumulation in 4-d-old etiolated seedlings. While safe1 plants did not accumulate Pchlide, flu safe1 and flu ex1 safe1 accumulated similar amounts of Pchlide. (F) Detection of 1O2 generation by using SOSG. (G) Cell-death response after moderate 1O2 generation. Note that flu shows a weak response, while flu ex1 safe1 and flu safe1 exhibit stronger and strongest effects, respectively. LDD, light/dark/dim-light. (H) 1O2-induced cell death responses in mature leaves of safe1, flu safe1, and flu ex1 safe1. (I) A heatmap illustrates and summarizes Pchlide accumulation, 1O2 generation, and corresponding 1O2-induced stress strengths based on the decrease in Fv/Fm in all mutants examined in this study.