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Several genes involved in the regulation of postembryonic organ initiation and growth have been identified. However, it remains largely unclear how developmental cues connect to the cell cycle. RETINOBLASTOMA RELATED (RBR) is a plant homolog of the tumor suppressor Retinoblastoma (pRb), which is a key regulator of the cell cycle. Using inducible RNA interference (RNAi) against Arabidopsis thaliana RBR (RBRi), we reduced RBR expression levels at different stages of plant development. Conditional reduction or loss of RBR function disrupted cell division patterns, promoted context-dependent cell proliferation, and negatively influenced establishment of cell differentiation. Several lineages of toti-and pluripotent cells, including shoot apical meristem stem cells, meristemoid mother cells, and procambial cells, failed to produce appropriately differentiated cells. Meristem activity was altered, leading to a disruption of the CLAVATA-WUSCHEL feedback loop and inhibition of lateral organ formation. Release of RBR from RNAi downregulation restored meristem activity. Gene profiling analyses soon after RBRi induction revealed that a change in RBR homeostasis is perceived as a stress, even before genes regulated by RBR-E2F become deregulated. The results establish RBR as a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis. INTRODUCTIONRetinoblastoma (RB) was the first tumor suppressor gene identified in animals (Friend et al., 1986) and later was associated with cell cycle regulation, thus establishing pRb as a key regulator of cell proliferation. In animals, three members of the RB family of proteins (pRb, p107, and p130), also referred to as pocket proteins (Mulligan and Jacks, 1998), negatively regulate the G1-to-S phase transition during the cell cycle. It is now widely accepted that mitogenic signals activate several cyclin-dependent kinase (CDK)-cyclin complexes during progression through G1 to phosphorylate pRb, thereby releasing it from interactions with E2F/DP transcription factor complexes to facilitate S-phase entry (Weinberg, 1995). Subsequently, pRb was also found to regulate tissue-specific transcription factors that regulate cell differentiation, such as MyoD, which activates muscle-specific genes (De Falco et al., 2006). pRb also regulates germ cell proliferation, differentiation, and survival (Toppari et al., 2003) and is required for osteoblast, keratinocyte, and macrophage differentiation (Nead et al., 1998;Liu et al., 1999;Thomas et al., 2003). Together with the tumor suppressor p53, pRb regulates adipocyte differentiation and function (Hallenborg et al., 2009). The requirement for RB in cell differentiation therefore extends beyond regulation of cell cycle entry. For example, the loss of RB function in cardioblasts affects cell differentiation by modulating the activity of cardiogenic factors (Papadimou et al., 2005).RB-related genes, termed RETINOBLASTOMA-RELATED (RBR; Ach et al., 1997), are also found in monocots (Grafi et al., 1996;Xie et al...

SUMMARYThe role of a subfamily of lipid globule-associated proteins, referred to as plant fibrillins (FIB1a, -1b, -2), was determined using a RNA interference (RNAi) strategy. We show that Arabidopsis plants with reduced levels of these plastid structural proteins are impaired in long-term acclimation to environmental constraint, namely photooxidative stress imposed by high light combined with cold. As a result, their photosynthetic apparatus is inefficiently protected. This leads to the prevalence of an abnormal granal and stromal membrane arrangement, as well as higher photosystem II photoinhibition under stress. The visible phenotype of FIB1-2 RNAi lines also includes retarded shoot growth and a deficit in anthocyanin accumulation under stress. All examined phenotypic effects of lower FIB levels are abolished by jasmonate (JA) treatment. An atypical expression pattern of several JA-induced genes was observed in RNAi plants. A JA-deficient mutant was found to share similar stress phenotypic characteristics with FIB RNAi plants. We conclude a new physiological role for JA, namely acclimation of chloroplasts, and that light/cold stress-related JA biosynthesis is conditioned by the accumulation of plastoglobule-associated FIB1-2 proteins. Consistent correlative data suggest that this FIB effect is mediated by plastoglobule (and triacylglycerol) accumulation as the potential site for initiating the chloroplast stress-related JA biosynthesis.

The yeast Candida zemplinina (Starmerella bacillaris) is frequently isolated from grape and wine environments. Its enological use in mixed fermentation with Saccharomyces cerevisiae has been extensively investigated these last few years, and several interesting features including low ethanol production, fructophily, glycerol and other metabolites production, have been described. In addition, molecular tools allowing the characterization of yeast populations have been developed, both at the inter- and intraspecific levels. However, most of these fingerprinting methods are not compatible with population genetics or ecological studies. In this work, we developed 10 microsatellite markers for the C. zemplinina species that were used for the genotyping of 163 strains from nature or various enological regions (28 vineyards/wineries from seven countries). We show that the genetic diversity of C. zemplinina is shaped by geographical localization. Populations isolated from winemaking environments are quite diverse at the genetic level: neither clonal-like behaviour nor specific genetic signature were associated with the different vineyards/wineries. Altogether, these results suggest that C. zemplinina is not under selective pressure in winemaking environments.