Phytochromes have been reported to play important roles in seedling de-etiolation and flowering in rice.To identify the roles of phytochromes in regulating root growth and accumulation of dry substances,the lengths of seminal roots and the dry weights of seedlings were measured in the wild type as well as the phytochrome A(phyA) and phytochrome B(phyB) mutants grown under different conditions.When the whole seedlings were exposed to white light,the elongation of the seminal roots was significantly photoinhibited in the wild type,whereas this inhibitory effect was clearly reduced in the phyA and phyB mutants.When the roots of the seedlings were blocked from white light,the phyA and phyB mutants exhibited significantly longer seminal roots than the wild type.These results suggest that both the root-localized and shoot-localized PHYA and PHYB are involved in the photoinhibition of seminal root elongation in rice seedlings.By measuring the dry weights of roots and shoots,it is revealed that PHYB positively regulates the accumulation of dry substances in shoots,however,PHYA exerts the contrary effects on the accumulation of dry substances in roots and shoots of rice seedlings. Phytochromes have been reported to play important roles in seedling de-etiolation and flowering in rice. To identify the roles of phytochromes in regulating root growth and accumulation of dry substances, the lengths of seminal roots and the dry weights of seedlings were measured in the wild type as well as the phytochrome A (phyA) and phytochrome B (phyB) mutants grown under different conditions. WHEN the whole seedlings were exposed to white light, the elongation of the seminal roots was significant photoinhibited in the wild type, and this inhibitory effect was clearly reduced in the phyA and phyB mutants.When the roots of the seedlings were blocked from white light, the phyA and phyB mutants exhibited significantly longer seminal roots than the wild type.These results suggest that both the root-localized and shoot-localized PHYA and PHYB are involved in the photoinhibition of seminal root elongation in rice seedlings.By measuring the dry weights of roots and shoots, it is found tha t PHYB positively regulates the accumulation of dry substances in shoots, however, PHYA exerts the contrary effects on the accumulation of dry substances in roots and shoots of rice seedlings.