Building a safe space for stem cells
The meristem, the collection of stem cells that builds plants, is resistant to viral infection. Wu
et al.
now show that WUSCHEL, a transcription factor that helps ...to sustain stem cell production in the shoot apical meristem of
Arabidopsis
, also protects that stem cell domain from viruses. WUSCHEL inhibited viral protein synthesis by repressing methyltransferases that regulate ribosomal RNA processing and ribosome stability.
Science
, this issue p.
227
Manipulation of protein synthesis restricts viral invasion in stem cells of the
Arabidopsis
shoot apical meristem.
Stem cells in plants constantly supply daughter cells to form new organs and are expected to safeguard the integrity of the cells from biological invasion. Here, we show how stem cells of the
Arabidopsis
shoot apical meristem and their nascent daughter cells suppress infection by cucumber mosaic virus (CMV). The stem cell regulator WUSCHEL responds to CMV infection and represses virus accumulation in the meristem central and peripheral zones. WUSCHEL inhibits viral protein synthesis by repressing the expression of plant
S
-adenosyl-
l
-methionine–dependent methyltransferases, which are involved in ribosomal RNA processing and ribosome stability. Our results reveal a conserved strategy in plants to protect stem cells against viral intrusion and provide a molecular basis for WUSCHEL-mediated broad-spectrum innate antiviral immunity in plants.
Complex multicellular organisms evolved on Earth in an oxygen-rich atmosphere
; their tissues, including stem-cell niches, require continuous oxygen provision for efficient energy metabolism
. ...Notably, the maintenance of the pluripotent state of animal stem cells requires hypoxic conditions, whereas higher oxygen tension promotes cell differentiation
. Here we demonstrate, using a combination of genetic reporters and in vivo oxygen measurements, that plant shoot meristems develop embedded in a low-oxygen niche, and that hypoxic conditions are required to regulate the production of new leaves. We show that hypoxia localized to the shoot meristem inhibits the proteolysis of an N-degron-pathway
substrate known as LITTLE ZIPPER 2 (ZPR2)-which evolved to control the activity of the class-III homeodomain-leucine zipper transcription factors
-and thereby regulates the activity of shoot meristems. Our results reveal oxygen as a diffusible signal that is involved in the control of stem-cell activity in plants grown under aerobic conditions, which suggests that the spatially distinct distribution of oxygen affects plant development. In molecular terms, this signal is translated into transcriptional regulation by the N-degron pathway, thereby linking the control of metabolic activity to the regulation of development in plants.
New parameterizations of contact freezing and immersion freezing in stratiform mixed-phase clouds (with temperatures between 0' and -35'C) for black carbon and mineral dust assumed to be composed of ...either kaolinite (simulation KAO) or montmorillonite (simulation MON) are introduced into the ECHAM4 general circulation model. The effectiveness of black carbon and dust as ice nuclei as a function of temperature is parameterized from a compilation of laboratory studies. This is the first time that freezing parameterizations take the chemical composition of ice nuclei into account. The rather subtle differences between these sensitivity simulations in the present-day climate have significant implications for the anthropogenic indirect aerosol effect. The decrease in net radiation in these sensitivity simulations at the top of the atmosphere varies from 1 c 0.3 to 2.1 c 0.1 W m-2 depending on whether dust is assumed to be composed of kaolinite or montmorillonite. In simulation KAO, black carbon has a higher relevancy as an ice nucleus than in simulation MON, because kaolinite is not freezing as effectively as montmorillonite. In simulation KAO, the addition of anthropogenic aerosols results in a larger ice water path, a slightly higher precipitation rate, and a reduced total cloud cover. On the contrary, in simulation MON the increase in ice water path is much smaller and globally the decrease in precipitation is dominated by the reduction in warm-phase precipitation due to the indirect cloud lifetime effect.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In multicellular organisms, not all cells are created equal. Instead, organismal complexity is achieved by specialisation and division of labour between distinct cell types. Therefore, the organism ...depends on the presence, correct proportion and function of all cell types. It follows that early development is geared towards setting up the basic body plan and to specify cell lineages. Since plants employ a post-embryonic mode of development, the continuous growth and addition of new organs require a source of new cells, as well as a strict regulation of cellular composition throughout the entire life-cycle. To meet these demands, evolution has brought about complex regulatory systems to maintain and control continuously active stem cell systems. Here, we review recent work on the mechanisms of non cell-autonomous control of shoot stem cells in the model plant
Arabidopsis thaliana
with a strong focus on the cell-to-cell mobility and function of the WUSCHEL homeodomain transcription factor.
The quiescent center (QC) maintains the activity of the surrounding stem cells within the root stem cell niche, yet specific molecular players sustaining the low rate of QC cell division remain ...poorly understood. Here, we identified a R2R3-MYB transcription factor, BRAVO (BRASSINOSTEROIDS AT VASCULAR AND ORGANIZING CENTER), acting as a cell-specific repressor of QC divisions in the primary root of Arabidopsis. Ectopic BRAVO expression restricts overall root growth and ceases root regeneration upon damage of the stem cells, demonstrating the role of BRAVO in counteracting Brassinosteroid (BR)-mediated cell division in the QC cells. Interestingly, BR-regulated transcription factor BES1 (BRI1-EMS SUPRESSOR 1) directly represses and physically interacts with BRAVO in vivo, creating a switch that modulates QC divisions at the root stem cell niche. Together, our results define a mechanism for BR-mediated regulation of stem cell quiescence in plants.
Display omitted
•BRAVO is a central stem cell specific component of the Brassinosteroid pathway•BRAVO counteracts BR-mediated cell division to protect the root stem cell niche•BRAVO/BES1 interaction creates a switch to control stem cell quiescence
Maintenance of the stem cell niche is essential for proper root growth, yet the players maintaining quiescence remain poorly characterized. Vilarrasa-Blasi et al. characterize BRAVO, a stem cell specific brassinosteroid-regulated transcription factor that is a negative regulator of quiescence in the root apex. BES1/BRAVO mutual regulation regulates stem cell maintenance.
This manuscript compiles both theoretical and experimental information on contact freezing with the aim to better understand this potentially important but still not well quantified heterogeneous ...freezing mode. There is no complete theory that describes contact freezing and how the energy barrier has to be overcome to nucleate an ice crystal by contact freezing. Experiments on contact freezing conducted using the cold plate technique indicate that it can initiate ice formation at warmer temperatures than immersion freezing. Additionally, a qualitative difference in the freezing temperatures between contact and immersion freezing has been found using different instrumentation and different ice nuclei. There is a lack of data on collision rates in most of the reported data, which inhibits a quantitative calculation of the freezing efficiencies. Thus, new or modified instrumentation to study contact nucleation in the laboratory and in the field are needed to identify the conditions at which contact nucleation could occur in the atmosphere. Important questions concerning contact freezing and its potential role for ice cloud formation and climate are also summarized.
Aerosols interact with radiation and clouds. Substantial progress made over the past 40 years in observing, understanding, and modeling these processes helped quantify the imbalance in the Earth's ...radiation budget caused by anthropogenic aerosols, called aerosol radiative forcing, but uncertainties remain large. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable, and arguable lines of evidence, including modeling approaches, theoretical considerations, and observations. Improved understanding of aerosol absorption and the causes of trends in surface radiative fluxes constrain the forcing from aerosol‐radiation interactions. A robust theoretical foundation and convincing evidence constrain the forcing caused by aerosol‐driven increases in liquid cloud droplet number concentration. However, the influence of anthropogenic aerosols on cloud liquid water content and cloud fraction is less clear, and the influence on mixed‐phase and ice clouds remains poorly constrained. Observed changes in surface temperature and radiative fluxes provide additional constraints. These multiple lines of evidence lead to a 68% confidence interval for the total aerosol effective radiative forcing of ‐1.6 to ‐0.6 W m−2, or ‐2.0 to ‐0.4 W m−2 with a 90% likelihood. Those intervals are of similar width to the last Intergovernmental Panel on Climate Change assessment but shifted toward more negative values. The uncertainty will narrow in the future by continuing to critically combine multiple lines of evidence, especially those addressing industrial‐era changes in aerosol sources and aerosol effects on liquid cloud amount and on ice clouds.
Plain Language Summary
Human activities emit into the atmosphere small liquid and solid particles called aerosols. Those aerosols change the energy budget of the Earth and trigger climate changes, by scattering and absorbing solar and terrestrial radiation and playing important roles in the formation of cloud droplets and ice crystals. But because aerosols are much more varied in their chemical composition and much more heterogeneous in their spatial and temporal distributions than greenhouse gases, their perturbation to the energy budget, called radiative forcing, is much more uncertain. This review uses traceable and arguable lines of evidence, supported by aerosol studies published over the past 40 years, to quantify that uncertainty. It finds that there are two chances out of three that aerosols from human activities have increased scattering and absorption of solar radiation by 14% to 29% and cloud droplet number concentration by 5 to 17% in the period 2005–2015 compared to the year 1850. Those increases exert a radiative forcing that offsets between a fifth and a half of the radiative forcing by greenhouse gases. The degree to which human activities affect natural aerosol levels, and the response of clouds, and especially ice clouds, to aerosol perturbations remain particularly uncertain.
Key Points
An assessment of multiple lines of evidence supported by a conceptual model provides ranges for aerosol radiative forcing of climate change
Aerosol effective radiative forcing is assessed to be between ‐1.6 and ‐0.6 W m−2 at the 16–84% confidence level
Although key uncertainties remain, new ways of using observations provide stronger constraints for models
Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei and may have an influence on the hydrological cycle. Here we ...investigate aerosol effects on convective clouds by extending the double-moment cloud microphysics scheme developed for stratiform clouds, which is coupled to the HAM double-moment aerosol scheme, to convective clouds in the ECHAM5 general circulation model. This enables us to investigate whether more, and smaller cloud droplets suppress the warm rain formation in the lower parts of convective clouds and thus release more latent heat upon freezing, which would then result in more vigorous convection and more precipitation. In ECHAM5, including aerosol effects in large-scale and convective clouds (simulation ECHAM5-conv) reduces the sensitivity of the liquid water path increase with increasing aerosol optical depth in better agreement with observations and large-eddy simulation studies. In simulation ECHAM5-conv with increases in greenhouse gas and aerosol emissions since pre-industrial times, the geographical distribution of the changes in precipitation better matches the observed increase in precipitation than neglecting microphysics in convective clouds. In this simulation the convective precipitation increases the most suggesting that the convection has indeed become more vigorous.
This paper introduces and evaluates the second version of the global aerosol-climate model ECHAM-HAM. Major changes have been brought into the model, including new parameterizations for aerosol ...nucleation and water uptake, an explicit treatment of secondary organic aerosols, modified emission calculations for sea salt and mineral dust, the coupling of aerosol microphysics to a two-moment stratiform cloud microphysics scheme, and alternative wet scavenging parameterizations. These revisions extend the model's capability to represent details of the aerosol lifecycle and its interaction with climate. Nudged simulations of the year 2000 are carried out to compare the aerosol properties and global distribution in HAM1 and HAM2, and to evaluate them against various observations. Sensitivity experiments are performed to help identify the impact of each individual update in model formulation. Results indicate that from HAM1 to HAM2 there is a marked weakening of aerosol water uptake in the lower troposphere, reducing the total aerosol water burden from 75 Tg to 51 Tg. The main reason is the newly introduced κ-Köhler-theory-based water uptake scheme uses a lower value for the maximum relative humidity cutoff. Particulate organic matter loading in HAM2 is considerably higher in the upper troposphere, because the explicit treatment of secondary organic aerosols allows highly volatile oxidation products of the precursors to be vertically transported to regions of very low temperature and to form aerosols there. Sulfate, black carbon, particulate organic matter and mineral dust in HAM2 have longer lifetimes than in HAM1 because of weaker in-cloud scavenging, which is in turn related to lower autoconversion efficiency in the newly introduced two-moment cloud microphysics scheme. Modification in the sea salt emission scheme causes a significant increase in the ratio (from 1.6 to 7.7) between accumulation mode and coarse mode emission fluxes of aerosol number concentration. This leads to a general increase in the number concentration of smaller particles over the oceans in HAM2, as reflected by the higher Ångström parameters. Evaluation against observation reveals that in terms of model performance, main improvements in HAM2 include a marked decrease of the systematic negative bias in the absorption aerosol optical depth, as well as smaller biases over the oceans in Ångström parameter and in the accumulation mode number concentration. The simulated geographical distribution of aerosol optical depth (AOD) is better correlated with the MODIS data, while the surface aerosol mass concentrations are very similar to those in the old version. The total aerosol water content in HAM2 is considerably closer to the multi-model average from Phase I of the AeroCom intercomparison project. Model deficiencies that require further efforts in the future include (i) positive biases in AOD over the ocean, (ii) negative biases in AOD and aerosol mass concentration in high-latitude regions, and (iii) negative biases in particle number concentration, especially that of the Aitken mode, in the lower troposphere in heavily polluted regions.
Anthropogenic aerosols can influence the climate indirectly by changing the optical properties and precipitation formation of water clouds. An indirect effect that has not been considered involves ...the subset of anthropogenic aerosols that act as ice nuclei and thereby determines the lifetime of ice and mixed‐phase clouds. If, in addition to mineral dust, a fraction of the hydrophilic soot aerosol particles is assumed to act as contact ice nuclei as evident from recent laboratory studies, then increases in aerosol concentration from pre‐industrial times to present‐day pose a new indirect effect, a “glaciation indirect effect”, on clouds. Here increases in contact ice nuclei in the present‐day climate result in more frequent glaciation of clouds and increase the amount of precipitation via the ice phase. This effect can at least partly offset the solar indirect aerosol effect on water clouds.
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