Regulation of phytochrome B signaling by phytochrome A and FHY1 in Arabidopsis thaliana

Phytochrome A (phyA) and phytochrome B (phyB) share the control of many pro cesses but little is known about mutual signaling regulation. Here, we repo rt on the interactions between phyA and phyB in the control of the activity of an Lhcb1*2 gene fused to a reporter, hypocotyl growth and cotyledon unf olding in etiolated Arabidopsis thaliana. The very-low fluence responses (V LFR) induced by pulsed far-red light and the high-irradiance responses (HIR ) observed under continuous far-red light were absent in the phyA and phyA phyB mutants, normal in the phyB mutant, and reduced in the fhy1 mutant tha t is defective in phyA signaling. VLFR were also impaired in Columbia compa red to Landsberg erecta. The low-fluence responses (LFR) induced by red-lig ht pulses and reversed by subsequent far-red light pulses were small in the wild type, absent in phyB and phyA phyB mutants but strong in the phyA and fhy1 mutants. This indicates a negative effect of phyA and FHY1 on phyB-me diated responses. However, a pre-treatment with continuous far-red light en hanced the LFR induced by a subsequent red-light pulse. This enhancement wa s absent in phyA, phyB, or phyA phyB and partial in fhy1. The levels of phy B were not affected by the phyA or fhy1 mutations or by far-red light pre-t reatments. We conclude that phyA acting in the VLFR mode (i.e. under light pulses) is antagonistic to phyB signaling whereas phyA acting in the HIR mo de (i.e. under continuous far-red light) operates synergistically with phyB signaling, and that both types of interaction require FHY1.