Mutations in mitochondrial DNA (mtDNA) accumulate in tissues of mammalian species and have been hypothesized to contribute to aging. We show that mice expressing a proofreading-deficient version of the mitochondrial DNA polymerase g (POLG) accumulate mtDNA mutations and display features of accelerated aging. Accumulation of mtDNA mutations was not associated with increased markers of oxidative stress or a defect in cellular proliferation, but was correlated with the induction of apoptotic markers, particularly in tissues characterized by rapid cellular turnover. The levels of apoptotic markers were also found to increase during aging in normal mice. Thus, accumulation of mtDNA mutations that promote apoptosis may be a central mechanism driving mammalian aging.

SUMMARY Caloric restriction (CR) extends the lifespan and healthspan of a variety of species, and slows the progression of age-related hearing loss (AHL), a common age-related disorder associated with oxidative stress. Here we report that CR reduces oxidative DNA damage in multiple tissues and prevents AHL in wild-type mice, but fails to modify these phenotypes in mice lacking the mitochondrial deacetylase Sirt3, a member of the sirtuin family. In response to CR, Sirt3 directly deacetylates and activates mitochondrial isocitrate dehydrogenase 2 (Idh2), leading to increased NADPH levels and an increased ratio of reduced to oxidized glutathione in mitochondria. In cultured cells, overexpression of Sirt3 and/or Idh2 increases NADPH levels and protects from oxidative stress-induced cell death. Therefore, our findings identify Sirt3 as an essential player in enhancing the mitochondrial glutathione antioxidant defense system during CR, and suggest that Sirt3-dependent mitochondrial adaptations may be a central mechanism of aging retardation in mammals.

ABA is a major phytohormone that regulates a broad range of plant traits and is especially important for adaptation to environmental conditions. Our understanding of the molecular basis of ABA responses in plants improved dramatically in 2009 and 2010, banner years for ABA research. There are three major components; PYR/PYL/ RCAR (an ABA receptor), type 2C protein phosphatase (PP2C; a negative regulator) and SNF1-related protein kinase 2 (SnRK2; a positive regulator), and they offer a double negative regulatory system, [PYR/PYL/RCAR—| PP2C—| SnRK2]. In the absence of ABA, PP2C inactivates SnRK2 by direct dephosphorylation. In response to environmental or developmental cues, ABA promotes the interaction of PYR/PYL/RCAR and PP2C, resulting in PP2C inhibition and SnRK2 activation. This signaling complex can work in both the nucleus and cytosol, as it has been shown that SnRK2 phosphorylates basic-domain leucine zipper (bZIP) transcription factors or membrane proteins. Several structural analyses of PYR/PYL/RCAR have provided the mechanistic basis for this ‘core signaling’ model, by elucidating the mechanism of ABA binding of receptors, or the ‘gate–latch–lock’ mechanism of interaction with PP2C in inhibiting activity. On the other hand, intercellular ABA transport had remained a major issue, as had intracellular ABA signaling. Recently, two plasma membrane-type ABC transporters were identified and shed light on the influx/efflux system of ABA, resolving how ABA is transported from cell to cell in plants. Our knowledge of ABA responses in plants has been greatly expanded from intracellular signaling to intercellular transport of ABA.

Strigolactones (SLs) are phytohormones that inhibit shoot branching and function in the rhizospheric communication with symbiotic fungi and parasitic weeds. An a/b-hydrolase protein, DWARF14 (D14), has been recognized to be an essential component of plant SL signalling, although its precise function remains unknown. Here we present the SL-dependent interaction of D14 with a gibberellin signalling repressor SLR1 and a possible mechanism of phytohormone perception in D14-mediated SL signalling. D14 functions as a cleavage enzyme of SLs, and the cleavage reaction induces the interaction with SLR1. The crystal structure of D14 shows that 5-hydroxy-3-methylbutenolide (D-OH), which is a reaction product of SLs, is trapped in the catalytic cavity of D14 to form an altered surface. The D14 residues recognizing D-OH are critical for the SL-dependent D14 À SLR1 interaction. These results provide new insight into crosstalk between gibberellin and SL signalling pathways.

Plant respiratory burst oxidase homolog (rboh) proteins, which are homologous to the mammalian 91-kDa glycoprotein subunit of the phagocyte oxidase (gp91 phox ) or NADPH oxidase 2 (NOX2), have been implicated in the production of reactive oxygen species (ROS) both in stress responses and during development. Unlike mammalian gp91 phox /NOX2 protein, plant rboh proteins have hydrophilic N-terminal regions containing two EF-hand motifs, suggesting that their activation is dependent on Ca 2؉ . However, the significance of Ca 2؉ binding to the EF-hand motifs on ROS production has been unclear. By employing a heterologous expression system, we showed that ROS production by Arabidopsis thaliana rbohD (AtrbohD) was induced by ionomycin, which is a Ca 2؉ ionophore that induces Ca 2؉ influx into the cell. This activation required a conformational change in the EF-hand region, as a result of Ca 2؉ binding to the EF-hand motifs. We also showed that AtrbohD was directly phosphorylated in vivo, and that this was enhanced by the protein phosphatase inhibitor calyculin A (CA). Moreover, CA itself induced ROS production and dramatically enhanced the ionomycin-induced ROS production of AtrbohD. Our results suggest that Ca 2؉ binding and phosphorylation synergistically activate the ROS-producing enzyme activity of AtrbohD.Photosynthetic plants have developed various mechanisms to cope with oxidative stress, such as the production of antioxidants and enzymes that scavenge reactive oxygen species (ROS).3 Plants are also equipped with mechanisms for producing ROS in response to internal and external stimuli. ROS production is induced during many physiological processes, including stress responses, cell growth, hormonal responses, stomatal closure, and disease resistance (see Refs. 1-4 and references therein).ROS production is induced in plants in response to recognition of pathogenic signals, such as pathogen/microbe-associated molecular patterns (PAMPs/MAMPs) or elicitors. Elicitor-induced ROS production is preceded by a rapid increase in the cytosolic free Ca 2ϩ concentration ([Ca 2ϩ ] cyt ) (5-7) and is inhibited both by Ca 2ϩ chelators such as EGTA and BAPTA, and by Ca 2ϩ channel blockers such as La 3ϩ (6,8). The overexpression of rice two-pore channel 1 (OsTPC1), which is a putative voltage-gated Ca 2ϩ channel, enhanced elicitor-induced ROS production (9). Elicitor-induced ROS production is also inhibited by diphenylene iodonium (DPI), which is known to inhibit NADPH oxidase activity (6, 10). NADPH oxidase activity in the microsomal membrane fraction from tomato and tobacco was activated by adding Ca 2ϩ in vitro (11), suggesting that elicitor-induced ROS production by plant NADPH oxidase might be dependent on Ca 2ϩ . In mammalian phagocytes, ROS production is mediated by the NADPH-dependent phagocytic oxidase (phox) complex, which consists of the catalytic subunit gp91 phox /NADPH oxidase (NOX) 2, together with the regulatory subunits p22 phox , p40 phox , p47 phox , p67 phox , and the small GTP-binding protein Rac (12). In...