AN ARABIDOPSIS MUTANT HYPERSENSITIVE TO RED AND FAR-RED LIGHT SIGNALS

A new mutant called psi2 (for phytochrome signaling) was isolated by s creening for elevated activity of a chlorophyll a/b binding protein-lu ciferase (CAB2-LUC) transgene in Arabidopsis. This mutant exhibited hy persensitive induction of CAB1, CAB2, and the small subunit of ribulos e-l,5-bisphosphate carboxylase (RBCS) promoters in the very low fluenc e range of red light and a hypersensitive response in hypocotyl growth in continuous red light of-higher fluences. In addition, at high- but not low-light fluence rates, the mutant showed light-dependent superi nduction of the pathogen-related protein gene PR-la and developed spon taneous necrotic lesions in the absence of any pathogen. Expression of genes responding to various hormone and environmental stress pathways in the mutant was not significantly different from that of the wild t ype. Analysis of double mutants demonstrated that the effects of the p si2 mutation are dependent on both phytochromes phyA and phyB. The mut ation is recessive and maps to the bottom of chromosome 5. Together, o ur results suggest that PSI2 specifically and negatively regulates bot h phyA and phyB phototransduction pathways. The induction of cell deat h by deregulated signaling pathways observed in psi2 is reminiscent of retinal degenerative diseases in animals and humans.