Chlorophyll degradation is an aspect of leaf senescence, which is an active process to salvage nutrients from old tissues. non-yellow coloring1 (nyc1) is a rice (Oryza sativa) stay-green mutant in which chlorophyll degradation during senescence is impaired. Pigment analysis revealed that degradation of not only chlorophylls but also light-harvesting complex II (LHCII)-bound carotenoids was repressed in nyc1, in which most LHCII isoforms were selectively retained during senescence. Ultrastructural analysis of nyc1 chloroplasts revealed that large and thick grana were present even in the late stage of senescence, suggesting that degradation of LHCII is required for the proper degeneration of thylakoid membranes. Mapbased cloning of NYC1 revealed that it encodes a chloroplast-localized short-chain dehydrogenase/reductase (SDR) with three transmembrane domains. The predicted structure of the NYC1 protein and the phenotype of the nyc1 mutant suggest the possibility that NYC1 is a chlorophyll b reductase. Although we were unable to detect the chlorophyll b reductase activity of NYC1, NOL (for NYC1-like), a protein closely related to NYC1 in rice, showed chlorophyll b reductase activity in vitro. We suggest that NYC1 and NOL encode chlorophyll b reductases with divergent functions. Our data collectively suggest that the identified SDR protein NYC1 plays essential roles in the regulation of LHCII and thylakoid membrane degradation during senescence. INTRODUCTIONThe final step of leaf development is senescence, which is an active process to salvage nutrients from old leaves. Leaf yellowing, which is caused by unmasking of preexisting carotenoids by chlorophyll degradation, is a good indicator of senescence (Matile, 2000). Most chlorophyll exists in protein complexes in leaves, because free chlorophyll photooxidatively damages cells. Chlorophyll a is a component of several protein complexes, including the photosystem I (PSI) and photosystem II (PSII) reaction center complexes and the cytochrome b 6 f complex. Chlorophyll b exists only in the light-harvesting chlorophyll a/b-protein complex (LHCP). LHCP binds chlorophyll a, chlorophyll b, and carotenoids (neoxanthin, violaxanthin, and lutein) (Liu et al., 2004). Chlorophyll b is thought to be important for the stability of LHCP (Bellemare et al., 1982). PSI-associated light-harvesting complex I (LHCI) and PSII-associated LHCII proteins are encoded by the Lhca and Lhcb gene families, respectively. LHCPs are localized in the thylakoid membrane. Lhcb1, -2, and -3 are major LHCII proteins and form trimers, but Lhcb4, -5, and -6 occur as monomers. LHCII is localized predominantly in grana, the stacking region of the thylakoid membrane. LHCII has been thought to play an important role in the formation of grana (Allen and Forsberg, 2001).The chlorophyll synthesis pathway has been well characterized, and most, if not all, genes encoding enzymes involved in chlorophyll synthesis have been isolated (Nagata et al., 2005). On the other hand, the chlorophyll degradation pathway is less...

The regenerative process in the pancreas is of particular interest because diabetes results from an inadequate number of insulinproducing beta cells and pancreatic cancer may arise from the uncontrolled growth of progenitor/stem cells. Continued and substantial growth of islet tissue occurs after birth in rodents and humans, with additional compensatory growth in response to increased demand. In rodents there is clear evidence of pancreatic regeneration after some types of injury, with proliferation of preexisting differentiated cell types accounting for some replacement. Additionally, neogenesis or the budding of new islet cells from pancreatic ducts has been reported, but the existence and identity of a progenitor cell have been debated. We hypothesized that the progenitor cells are duct epithelial cells that after replication undergo a regression to a less differentiated state and then can form new endocrine and exocrine pancreas. To directly test whether ductal cells serve as pancreatic progenitors after birth and give rise to new islets, we generated transgenic mice expressing human carbonic anhydrase II (CAII) promoter: Cre recombinase (Cre) or inducible CreER TM to cross with ROSA26 loxP-Stop-loxP LacZ reporter mice. We show that CAII-expressing cells within the pancreas act as progenitors that give rise to both new islets and acini normally after birth and after injury (ductal ligation). This identification of a differentiated pancreatic cell type as an in vivo progenitor of all differentiated pancreatic cell types has implications for a potential expandable source for new islets for replenishment therapy for diabetes.diabetes ͉ islets of Langerhans ͉ lineage tracing

SummaryYellowing, which is related to the degradation of chlorophyll and chlorophyll-protein complexes, is a notable phenomenon during leaf senescence. NON-YELLOW COLORING1 (NYC1) in rice encodes a membrane-localized short-chain dehydrogenase/reductase (SDR) that is thought to represent a chlorophyll b reductase necessary for catalyzing the first step of chlorophyll b degradation. Analysis of the nyc1 mutant, which shows the staygreen phenotype, revealed that chlorophyll b degradation is required for the degradation of light-harvesting complex II and thylakoid grana in leaf senescence. Phylogenetic analysis further revealed the existence of NYC1-LIKE (NOL) as the most closely related protein to NYC1. In the present paper, the nol mutant in rice was also found to show a stay-green phenotype very similar to that of the nyc1 mutant, i.e. the degradation of chlorophyll b was severely inhibited and light-harvesting complex II was selectively retained during senescence, resulting in the retention of thylakoid grana even at a late stage of senescence. The nyc1 nol double mutant did not show prominent enhancement of inhibition of chlorophyll degradation. NOL was localized on the stromal side of the thylakoid membrane despite the lack of a transmembrane domain. Immunoprecipitation analysis revealed that NOL and NYC1 interact physically in vitro. These observations suggest that NOL and NYC1 are co-localized in the thylakoid membrane and act in the form of a complex as a chlorophyll b reductase in rice.