NEW ARABIDOPSIS CUE MUTANTS SUGGEST A CLOSE CONNECTION BETWEEN PLASTID-REGULATION AND PHYTOCHROME REGULATION OF NUCLEAR GENE-EXPRESSION

We searched for new components that are involved in the positive regul ation of nuclear gene expression by light by extending a screen for Ar abidopsis cue (chlorophyll a/b-binding [CAB] protein-underexpressed) m utants (H.-M. Li, K. Culligan, R.A. Dixon, J. Chory [1995] Plant Cell 7: 1599-1610). cue mutants display reduced expression of the CAB3 gene , which encodes light-harvesting chlorophyll protein, the main chlorop last antenna. The new mutants can be divided into (a) phytochrome-defi cient mutants (hy1 and phyB), (b) virescent or delayed-greening mutant s (cue3, cue6, and cue8), and (c) uniformly pale mutants (cue4 and cue 9). For each of the mutants, the reduction in CAB expression correlate s with the visible phenotype, defective chloroplast development, and r educed abundance of the light-harvesting chlorophyll protein. Levels o f protochlorophyllide oxidoreductase (POR) were reduced to varying deg rees in etiolated mutant seedlings. In the dark, whereas the virescent mutants displayed reduced CAB expression and the lowest levels of FOR protein, the other mutants expressed CAB and accumulated FOR at near wild-type levels. All of the mutants, with the exception of cue6, were compromised in their ability to derepress CAB expression in response to phytochrome activation. Based on these results, we propose that the previously postulated plastid-derived signal is closely involved in t he pathway through which phytochrome regulates the expression of nucle ar genes encoding plastid proteins.