Phytochrome-mediated growth inhibition of seminal roots in rice seedlings

In rice (Oryza sativa) seedlings, continuous white-light irradiation inhibited the growth of seminal roots but promoted the growth of crown roots. Here, we examined the mechanisms of photoinhibition of seminal root growth. Photoinhibition occurred in the absence of nitrogen, but increased with increasing nitrogen concentrations. In the presence of nitrogen, photoinhibition was correlated with coiling of the root tips. The seminal roots were most photosensitive 48- 72 h after germination during the 7-d period after germination. White-light irradiation for at least 6 h was required for photoinhibition, and the Bunsen-Roscoe law of reciprocity was not observed. Experiments with phytochrome mutants showed that far-red light was perceived exclusively by phyA, that red light was perceived by both phyA and phyB, and that phyC had little or no role in growth inhibition or coiling of the seminal roots. Fluence-response curve analyses also showed that phyA and phyB control ver