L-ascorbic acid (vitamin C) is a powerful reducing agent found in millimolar concentrations in plants, and is proposed to play an important role in scavenging free radicals in plants and animals. However, surprisingly little is known about the role of this antioxidant in plant environmental stress adaptation or ascorbate biosynthesis. We report the isolation of sozi, a semi-dominant ozone-sensitive mutant that accumulates only 30% of the normal ascorbate concentration. The results of genetic approaches and feeding studies show that the ascorbate concentration affects foliar resistance to the oxidizing gas ozone. Consistent with the proposed role for ascorbate in reactive oxygen species detoxification, lipid peroxides are elevated in sozi, but not in wild type following ozone fumigation. We show that the sozi mutant is hypersensitive to both sulfur dioxide and ultraviolet B irradiation, thus implicating ascorbate in defense against varied environmental stresses. In addition to defining the first ascorbate deficient mutant in plants, these Free radicals can damage macromolecules by oxidative processes, leading to cancer and other diseases associated with aging (1). Antioxidants act to detoxify reactive oxygen species (ROS) such as hydrogen peroxide (H202), superoxide (02-), hydroxyl radical (-OH-), and organic hydroperoxides. Enzymes active in ROS removal include superoxide dismutases, catalases, peroxidases, and glutathione S-transferases (1). Plants also synthesize abundant small molecule antioxidants including AsA (L-ascorbic acid or Vitamin C), glutathione, a-tocopherol (Vitamin E), and carotenoids (2). Although there is increasing evidence that these plant-derived antioxidants are important components of the human diet, relatively little is known about their specific functions in plants.Microbial and animal mutants altered in antioxidant enzymes and signal transduction processes played a pivotal role in testing the importance of specific antioxidant detoxification pathways, evaluating the proposed role of ROS in cellular processes and dissecting ROS signal transduction. For example, bacterial mutants altered in response to growth under enhanced active oxygen conditions identified key antioxidant enzymes and proteins that regulate their synthesis (3). Yeast mutants deficient in glutathione are hypersensitive to H202, implicating glutathione in ROS detoxification (4). A mammalian cell line expressing a mutant p2lras was recently used to demonstrate a key role for this G protein in ROS signal transduction (5).A collection of oxidative stress-sensitive plant mutants would permit a critical assessment of the roles of antioxidant systems and elucidation of ROS signal transduction pathways. With this goal in mind, Arabidopsis thaliana mutants with altered sensitivity to the anthropogenic oxidizing air pollutant 03 (ozone) (6) are being identified. We describe here a semi-dominant monogenic 03 sensitive mutant (sozl, sensitive to ozone), which is deficient in ascorbic acid (AsA). This deficiency also causes...

Vitamin C (L-ascorbic acid; AsA) acts as a potent antioxidant and cellular reductant in plants and animals. AsA has long been known to have many critical physiological roles in plants, yet its biosynthesis is only currently being defined. A pathway for AsA biosynthesis that features GDP-mannose and L-galactose has recently been proposed for plants. We have isolated a collection of AsAdeficient mutants of Arabidopsis thaliana that are valuable tools for testing of an AsA biosynthetic pathway. The bestcharacterized of these mutants (vtc1) contains Ϸ25% of wildtype AsA and is defective in AsA biosynthesis. By using a combination of biochemical, molecular, and genetic techniques, we have demonstrated that the VTC1 locus encodes a GDP-mannose pyrophosphorylase (mannose-1-P guanyltransferase). This enzyme provides GDP-mannose, which is used for cell wall carbohydrate biosynthesis and protein glycosylation as well as for AsA biosynthesis. In addition to genetically defining the first locus involved in AsA biosynthesis, this work highlights the power of using traditional mutagenesis techniques coupled with the Arabidopsis Genome Initiative to rapidly clone physiologically important genes.

The Library of Integrated Network-Based Cellular Signatures (LINCS) is an NIH Common Fund program that catalogs how human cells globally respond to chemical, genetic, and disease perturbations. Resources generated by LINCS include experimental and computational methods, visualization tools, molecular and imaging data, and signatures. By assembling an integrated picture of the range of responses of human cells exposed to many perturbations, the LINCS program aims to better understand human disease and to advance the development of new therapies. Perturbations under study include drugs, genetic perturbations, tissue micro-environments, antibodies, and disease-causing mutations. Responses to perturbations are measured by transcript profiling, mass spectrometry, cell imaging, and biochemical methods, among other assays. The LINCS program focuses on cellular physiology shared among tissues and cell types relevant to an array of diseases, including cancer, heart disease, and neurodegenerative disorders. This Perspective describes LINCS technologies, datasets, tools, and approaches to data accessibility and reusability.

Crucial transitions in cancer-including tumor initiation, local expansion, metastasis, and therapeutic resistance-involve complex interactions between cells within the dynamic tumor ecosystem. Transformative single-cell genomics technologies and spatial multiplex in situ methods now provide an opportunity to interrogate this complexity at unprecedented resolution. The Human Tumor Atlas Network (HTAN), part of the National Cancer Institute (NCI) Cancer Moonshot Initiative, will establish a clinical, experimental, computational, and organizational framework to generate informative and accessible three-dimensional atlases of cancer transitions for a diverse set of tumor types. This effort complements both ongoing efforts to map healthy organs and previous largescale cancer genomics approaches focused on bulk sequencing at a single point in time. Generating single-cell, multiparametric, longitudinal atlases and integrating them with clinical outcomes should help identify novel predictive biomarkers and features as well as therapeutically relevant cell types, cell states, and cellular interactions across transitions. The resulting tumor atlases should have a profound impact on our understanding of cancer biology and have the potential to improve cancer detection, prevention, and therapeutic discovery for better precision-medicine treatments of cancer patients and those at risk for cancer.Cancer forms and progresses through a series of critical transitions-from pre-malignant to malignant states, from locally contained to metastatic disease, and from treatment-responsive to treatment-resistant tumors (Figure 1). Although specifics differ across tumor types and patients, all transitions involve complex dynamic interactions between diverse pre-malignant, malignant, and non-malignant cells (e.g., stroma cells and immune cells), often organized in specific patterns within the tumor