Elongator complex is critical for cell cycle progression and leaf patterning in Arabidopsis

The mitotic cell cycle in higher eukaryotes is of pivotal importance for organ growth and development. Here, we report that Elongator, an evolutionarily conserved histone acetyltransferase complex, acts as an important regulator of mitotic cell cycle to promote leaf patterning in Arabidopsis. Mutations in genes encoding Elongator subunits resulted in aberrant cell cycle progression, and the altered cell division affects leaf polarity formation. The defective cell cycle progression is caused by aberrant DNA replication and increased DNA damage, which activate the DNA replication checkpoint to arrest the cell cycle. Elongator interacts with proliferating cell nuclear antigen (PCNA) and is required for efficient histone 3 (H3) and H4 acetylation coupled with DNA replication. Levels of chromatin-bound H3K56Ac and H4K5Ac known to associate with replicons during DNA replication were reduced in the mutants of both Elongator and chromatin assembly factor 1 (CAF-1), another protein complex that physically interacts with PCNA for DNA replication-coupled chromatin assembly. Disruptions of CAF-1 also led to severe leaf polarity defects, which indicated that Elongator and CAF-1 act, at least partially, in the same pathway to promote cell cycle progression. Collectively, our results demonstrate that Elongator is an important regulator of mitotic cell cycle, and the Elongator pathway plays critical roles in promoting leaf polarity formation.     

Leaf epidermal features of Lithocarpus (Fagaceae) from China and their systematic significance

The leaves of 52 species of Lithocarpus in China were studied. The adaxial leaf epidermis was investigated by light microscopy. Epidermal cells of the adaxial surface were classified into three types on the basis of the outline of their anticlinal walls, i.e. sinuate, straight and curved. The abaxial leaf epidermis was investigated by light microscopy and scanning electron microscopy. The following types of trichome were observed: appressed parallel tuft, stellate, fused stellate, papillae, stipitate fasciculate, solitary unicellular, appressed laterally attached unicellular, curly thin‐walled unicellular, bulbous and thin‐walled peltate. The fused stellate, appressed laterally attached unicellular and curly thin‐walled unicellular trichomes were reported in Lithocarpus for the first time. The appressed parallel tuft trichome, which is recognized as a salient characteristic of Lithocarpus, was not found in 15 species. A cladistic analysis was performed on the basis of the leaf epidermal features. According to the leaf epidermal features and several morphological characteristics, 26 of the 52 species could be divided into seven groups. Similar groups can be found in Barnett's and Camus' systems. The trichomes of four genera in Fagaceae are listed and compared. Lithocarpus had 14 types of trichome, 11 of which were identical to types found in Quercus, more than in Castanopsis and Cyclobalanopsis. The evolutionary trends of trichomes in Fagaceae are discussed and a new point of view is raised. © 2011 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 168 , 216–228.     

Rapid oxygen exchange across the leaves of Littorella uniflora provides tolerance to sediment anoxia

1. Littorella uniflora and Lobelia dortmanna are prominent small rosette species in nutrient‐poor, soft‐water lakes because of efficient root exchange of CO₂ and O₂. We hypothesise that higher gas exchange across the leaves of L. uniflora than of L. dortmanna ensures O₂ uptake from water and underlies its greater tolerance to sediment anoxia following organic enrichment. 2. We studied plant response to varying sediment O₂ demand and biogeochemistry by measuring photosynthesis, gas exchange across leaves and O₂ dynamics in plants during long‐term laboratory and field studies. Frequent non‐destructive sampling of sediment pore water was used to track changes in sediment biogeochemistry. 3. Addition of organic matter triggered O₂ depletion and accumulation of , Fe²⁺ and CO₂ in sediments. Gas exchange across leaf surfaces was 13–16 times higher for L. uniflora than for L. dortmanna. Oxygen in the leaf lacunae of L. uniflora remained above 10 kPa late at night on anoxic sediments despite organic enrichment. Leaf content of N and P of L. uniflora remained sufficient to keep up photosynthesis despite prolonged sediment anoxia, whereas nutrient content was too low for long‐term survival of L. dortmanna. 4. High gas exchange across L. uniflora leaves improves its performance and survival on anoxic sediments compared with L. dortmanna. Lobelia dortmanna uses the same gas‐tight leaves in air and water, which makes it highly susceptible to sediment anoxia but more cost‐effective in ultra‐oligotrophic environments because of slow leaf turnover.     

Key canopy traits drive forest productivity

Quantifying the mechanistic links between carbon fluxes and forest canopy attributes will advance understanding of leaf-to-ecosystem scaling and its potential application to assessing terrestrial ecosystem metabolism. Important advances have been made, but prior studies that related carbon fluxes to multiple canopy traits are scarce. Herein, presenting data for 128 cold temperate and boreal forests across a regional gradient of 600 km and 5.4°C (from 2.4°C to 7.8°C) in mean annual temperature, I show that stand-scale productivity is a function of the capacity to harvest light (represented by leaf area index, LAI), and to biochemically fix carbon (represented by canopy nitrogen concentration, %N). In combination, LAI and canopy %N explain greater than 75 per cent of variation in above-ground net primary productivity among forests, expressed per year or per day of growing season. After accounting for growing season length and climate effects, less than 10 per cent of the variance remained unexplained. These results mirror similar relations of leaf-scale and canopy-scale (eddy covariance) maximum photosynthetic rates to LAI and %N. Collectively, these findings indicate that canopy structure and chemistry translate from instantaneous physiology to annual carbon fluxes. Given the increasing capacity to remotely sense canopy LAI, %N and phenology, these results support the idea that physiologically based scaling relations can be useful tools for global modelling.     

Evaluation of the potential for sexual reproduction in field populations of Cercospora beticola from USA

Cercospora leaf spot, caused by the hemibiotrophic fungal pathogen Cercospora beticola, is the most economically damaging foliar disease of sugarbeet worldwide. Although most C. beticola populations display characteristics reminiscent of sexual recombination, no teleomorph has been described. To assess whether populations in northern United States have characteristics consistent with sexual reproduction, 1024 isolates collected over a 3-y period were analyzed for frequency and distribution of mating type genes. After clone correction, an approximately equal distribution of mating types was found for each sampling year. Mating type frequency was also assessed in individual lesions. Lesions always consisted of isolates with a single mating type and microsatellite haplotype, but both mating types and up to five microsatellite haplotypes could be found on an individual leaf. The MAT1-1-1 and MAT1-2-1 genes were sequenced from 28 MAT1-1 and 28 MAT1-2 isolates, respectively. Three MAT1-1-1 nucleotide haplotypes were identified that encoded a single amino acid sequence. For MAT1-2-1, five nucleotide haplotypes were identified that encoded four protein variants. MAT1-1-1 and MAT1-2-1 gene expression analyses were conducted on plants inoculated with either or both mating types. MAT1-1-1 expression remained low, but MAT1-2-1 spiked during late stages of colonization. A segment of the MAT1-2-1 coding sequence was also found in MAT1-1 isolates. Taken together, these results suggest that C. beticola has the potential for sexual reproduction.     

Experimental warming alters spring phenology of certain plant functional groups in an early successional forest community

Experimental study of the effects of projected climate change on plant phenology allows us to isolate effects of warming on life‐history events such as leaf out. We simulated a 2 °C temperature increase and 20% precipitation increase in a recently harvested temperate deciduous forest community in central Pennsylvania, USA, and observed the leaf out phenology of all species in 2009 and 2010. Over 130 plant species were monitored weekly in study plots, but due to high variability in species composition among plots, species were grouped into five functional groups: short forbs, tall forbs, shrubs, small trees, and large trees. Tall forbs and large trees, which usually emerge in the late spring, advanced leaf out 14–18 days in response to warming. Short forbs, shrubs, and small trees emerge early in spring and did not alter their phenology in response to warming or increased precipitation treatments. Earlier leaf out of tall forbs and large trees coincided with almost 3 weeks of increased community‐level leaf area index, indicating greater competition and a condensed spring green‐up period. While phenology of large trees and tall forbs appears to be strongly influenced by temperature‐based growth cues, our results suggest that photoperiod and chilling cues more strongly influence the leaf out of other functional groups. Reduced freeze events and warmer temperatures from predicted climate change will interact with nontemperature growth cues to have cascading consequences throughout the ecosystem.     

Large herbivores limit CO2 uptake and suppress carbon cycle responses to warming in West Greenland

Changes in the terrestrial carbon cycle may ameliorate or exacerbate future climatic warming. Research on this topic has focused almost exclusively on abiotic drivers, whereas biotic factors, including trophic interactions, have received comparatively little attention. We quantified the singular and interactive effects of herbivore exclusion and simulated warming on ecosystem CO₂ exchange over two consecutive growing seasons in West Greenland. Exclusion of caribou and muskoxen over the past 8 years has led to dramatic increases in shrub cover, leaf area, ecosystem photosynthesis, and a nearly threefold increase in net C uptake. These responses were accentuated by warming, but only in the absence of herbivores. Carbon cycle responses to herbivore exclusion alone and combined with warming were driven by changes in gross ecosystem photosynthesis, as limited differences in ecosystem respiration were observed. Our results show that large herbivores can be of critical importance as mediators of arctic ecosystem responses to climate change.