Phytochrome A-mediated inhibition of seed germination in tomato

Far-red light (FR) inhibition of seed germination of tomato (Solanum lycopersicum L.) was studied with the phytochrome (phy)-hypersensitive mutants, hp-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 readily 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 was no longer effective, prolonged FR exposure or hourly FR pulses suppressed germination in hp-1(w) and whereas in the suppressive effect of FR was almost absent. The effect of the prolonged FR was greater than that of the hourly 3-min FR pulses having equal photon fluence, and was fluence-rate dependent. Thus we conclude that the germination inhibition by FR in tomato seed 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 indication of phyA being involved in the HIR of seed germination inhibition.