Interorganelle communication is essential for the eukaryotic cell. We are using the model system. We are using the model system, Arabidopsis thaliana, to study crosstalk between the chloroplast and the nucleus. We have recently discovered that early light-inducible protein (Elip) genes are induced by photobleaching, and therefore represent a new area for understanding plastid-nuclear communication. Elip genes are also induced by very high light intensity, but it is not known whether the same signaling cascade is operating in response to very high light treatment and photobleaching. There are other in vivo situations where the chloroplast and nucleus must communicate. We have isolated a mutant named conditional chlorina (cch) that is unable to synthesize adequate amounts of Chl, and the light harvest complex of photosystem II is dismantled. In this line, interoganelle communication, which occurs during the acclimation to Ch1 starvation, can be studied. The specific aims of this proposal are 1) Determine if Elip induction in response to photobleaching and/or very high light treatment is altered in available mutants. 2) Identify regions of the Elip promoters that are essential for induction by photobleaching and/or very high light intensity. 3) Design and begin screening for mutants that have altered expression of Elip genes. 4) Determine the in vivo half-lives of the Lhcb proteins under normal and Chi limited conditions. 5) Use microarray analysis to identify genes that are induced or repressed during Ch1-limited conditions. 6) Map and clone suppressors of the conditional chlorina mutant that retain more Lhcb proteins during Chl limitations.