Patricia Leon-Mejia - Chloroplastsignals that impact leaf development

Chloroplast signals that impactleaf developmentThe most conspicuousfunction of chloroplasts is photosynthesis. However, these organelles alsofunction as factories that synthesized a pletora of essential compounds forplant life. Chloroplasts are thus central an enviromental sensor of the cell,and this information is transmitted to the ncucleus through retrogradesignaling, modulating the expression of nuclear-genes to fine-tune the generalplant developmental and external responses. Recent advances have demonstratedthe existence of multiple retrograde signaling pathways, but only few signalsresponsible for this regulation have been identified.  In recent years,evidence has supported that apocarotenoids are important signals that regulatediverse aspects of plant and animal development. The characterization of an Arabidopsisclb5 mutant, impaired in early chloroplast development, provided geneticand molecular evidences that the accumulation of apocarotenoid profoundlyaffects nuclear and plastid gene expression as well as leaf development. Thealbino clb5 mutant of Arabidopsis displays a unique defect in themorphology of the leaf with radial shape and defects in leaf polarity, that isnot observed in other albino mutants supporting the idea that chloroplaststatus is tightly monitored and strongly correlated with leaf development.These phenotypes are a consequence of the defect in the zeta carotenedesaturase (ZDS) activity that results in the accumulation of phytofluene andz-carotenoid intermediates. Our work also demonstrates that the signal responsiblefor the clb5 phenotypes is produced through specific cleavage ofphytofluene or z-carotenoids by the CCD4 enzyme. Furthermore, theanalysis of gene expression for ZDS and CCD4 shows an correlation acrossmultiple tissues and developmental stages. Using wide genome analysis, we havefound that this signal affects the expression of key nuclear-encoded genesimportant for leaf and plastid development as well as genes from thechloroplast genome in comparison to the carotenoid-deficient pds3 mutantplant, supporting the unique role that these carotenoids have over plantdevelopment. Finally, we found that the regulation on gene expression dependson specific cis-acting sequences located in the promoter region of thetarget genes.