Phytochrome A-mediated inhibition of seed germination in tomato

Far-red light (FR) inhibition of seed germination of tomato (Solanum lycope rsicum L.) was studied with the phytochrome (phy)-hypersensitive mutants, h p-1(w), hp-1(w), fri(1), a phyA-deficient double mutant, and hp-1(w) , tri( 1), a phyB1-deficient double mutant. Seeds of all mutants germinated readil y in the dark at 25 degreesC, and the germination was retarded by a single 100-s FR pulse given 1-3 It after sowing. The effect of an FR pulse was red -light reversible in all mutants used. After 24 h where a single FR pulse w as no longer effective, prolonged FR exposure or hourly FR pulses suppresse d germination in hp-1(w) and whereas in the suppressive effect of FR was al most absent. The effect of the prolonged FR was greater than that of the ho urly 3-min FR pulses having equal photon fluence, and was fluence-rate depe ndent. Thus we conclude that the germination inhibition by FR in tomato see d consists of a low-fluence response and a high irradiance response (HIR); the latter is controlled by phyA, but not phyB1. This is the first indicati on of phyA being involved in the HIR of seed germination inhibition.