Showing papers in "New Phytologist in 2011"

Sources of variability in canopy reflectance and the convergent properties of plants

Abstract: How plants interact with sunlight is central to the existence of life and provides a window to the functioning of ecosystems. Although the basic properties of leaf spectra have been known for decades, interpreting canopy-level spectra is more challenging because leaf-level effects are complicated by a host of stem- and canopy-level traits. Progress has been made through empirical analyses and models, although both methods have been hampered by a series of persistent challenges. Here, I review current understanding of plant spectral properties with respect to sources of uncertainty at leaf to canopy scales. I also discuss the role of evolutionary convergence in plant functioning and the difficulty of identifying individual properties among a suite of interrelated traits. A pattern that emerges suggests a synergy among the scattering effects of leaf-, stem- and canopy-level traits that becomes most apparent in the near-infrared (NIR) region. This explains the widespread and well-known importance of the NIR region in vegetation remote sensing, but presents an interesting paradox that has yet to be fully explored: that we can often gain more insight about the functioning of plants by examining wavelengths that are not used in photosynthesis than by examining those that are.

A meta-analysis of plant responses to dark septate root endophytes

Abstract: Summary •Dark septate endophytes (DSE) frequently colonize roots in the natural environment, but the effects of these fungi on plants are obscure, with previous studies indicating negative, neutral or positive effects on plant performance •In order to reach a consensus for how DSE influence plant performance, meta-analyses were performed on data from 18 research articles, in which plants had been inoculated with DSE in sterile substrates •Negative effects of DSE on plant performance were not recorded Positive effects were identified on total, shoot and root biomass, and on shoot nitrogen (N) and phosphorus contents, with increases of 26–103% in these parameters for plants inoculated with DSE, relative to uninoculated controls Inoculation increased total, shoot and root biomass by 52–138% when plants had not been supplied with additional inorganic N, or when all, or the majority, of N was supplied in organic form Inoculation with the DSE Phialocephala fortinii was found to increase shoot and root biomass, shoot P concentration and shoot N content by 44–116%, relative to uninoculated controls •The analyses here suggest that DSE enhance plant performance under controlled conditions, particularly when all, or the majority, of N is available in organic form

OsHMA3, a P1B‐type of ATPase affects root‐to‐shoot cadmium translocation in rice by mediating efflux into vacuoles

Abstract: • The cadmium (Cd) over-accumulating rice (Oryza sativa) cv Cho-Ko-Koku was previously shown to have an enhanced rate of root-to-shoot Cd translocation. This trait is controlled by a single recessive allele located at qCdT7. • In this study, using positional cloning and transgenic strategies, heavy metal ATPase 3 (OsHMA3) was identified as the gene that controls root-to-shoot Cd translocation rates. The subcellular localization and Cd-transporting activity of the gene products were also investigated. • The allele of OsHMA3 that confers high root-to-shoot Cd translocation rates (OsHMA3mc) encodes a defective P(1B) -ATPase transporter. OsHMA3 fused to green fluorescent protein was localized to vacuolar membranes in plants and yeast. An OsHMA3 transgene complemented Cd sensitivity in a yeast mutant that lacks the ability to transport Cd into vacuoles. By contrast, OsHMA3mc did not complement the Cd sensitivity of this yeast mutant, indicating that the OsHMA3mc transport function was lost. • We propose that the root cell cytoplasm of Cd-overaccumulating rice plants has more Cd available for loading into the xylem as a result of the lack of OsHMA3-mediated transportation of Cd to the vacuoles. This defect results in Cd translocation to the shoots in higher concentrations. These data demonstrate the importance of vacuolar sequestration for Cd accumulation in rice.

Leaf‐out phenology of temperate woody plants: from trees to ecosystems

Abstract: Leafing-out of woody plants begins the growing season in temperate forests and is one of the most important drivers of ecosystem processes. There is substantial variation in the timing of leaf-out, both within and among species, but the leaf development of almost all temperate tree and shrub species is highly sensitive to temperature. As a result, leaf-out times of temperate forests are valuable for observing the effects of climate change. Analysis of phenology data from around the world indicates that leaf-out is generally earlier in warmer years than in cooler years and that the onset of leaf-out has advanced in many locations. Changes in the timing of leaf-out will affect carbon sequestration, plant-animal interactions, and other essential ecosystem processes. The development of remote sensing methods has expanded the scope of leaf-out monitoring from the level of an individual plant or forest to an entire region. Meanwhile, historical data have informed modeling and experimental studies addressing questions about leaf-out timing. For most species, onset of leaf-out will continue to advance, although advancement may be slowed for some species because of unmet chilling requirements. More information is needed to reduce the uncertainty in predicting the timing of future spring onset.

Sensitivity of leaf size and shape to climate: Global patterns and paleoclimatic applications

Abstract: Summary • Paleobotanists have long used models based on leaf size and shape to reconstruct paleoclimate. However, most models incorporate a single variable or use traits that are not physiologically or functionally linked to climate, limiting their predictive power. Further, they often underestimate paleotemperature relative to other proxies. • Here we quantify leaf–climate correlations from 92 globally distributed, climatically diverse sites, and explore potential confounding factors. Multiple linear regression models for mean annual temperature (MAT) and mean annual precipitation (MAP) are developed and applied to nine well-studied fossil floras. • We find that leaves in cold climates typically have larger, more numerous teeth, and are more highly dissected. Leaf habit (deciduous vs evergreen), local water availability, and phylogenetic history all affect these relationships. Leaves in wet climates are larger and have fewer, smaller teeth. Our multivariate MAT and MAP models offer moderate improvements in precision over univariate approaches (± 4.0 vs 4.8� C for MAT) and strong improvements in accuracy. For example, our provisional MAT estimates for most North American fossil floras are considerably warmer and in better agreement with independent paleoclimate evidence. • Our study demonstrates that the inclusion of additional leaf traits that are functionally linked to climate improves paleoclimate reconstructions. This work also illustrates the need for better understanding of the impact of phylogeny and leaf habit on leaf–climate relationships.

Deciphering the distribution of the savanna biome

Abstract: Summary •We aimed to identify the limits of savanna across Africa, Australia and South America. We based our investigation on the rich history of hypotheses previously examined: that the limits of savanna are variously determined by rainfall, rainfall seasonality, soil fertility and disturbance. •We categorized vegetation on all continents as ‘savanna’ (open habitats with a C4 grass layer) or ‘not-savanna’ (closed habitats with no C4 grass layer) and used a combination of statistical approaches to examine how the presence of savanna varied as a function of five environmental correlates. •The presence of savanna is constrained by effective rainfall and rainfall seasonality. Soil fertility is regionally important, although the direction of its effect changes relative to rainfall. We identified three continental divergences in the limits of savanna that could not be explained by environment. •Climate and soils do not have a deterministic effect on the distribution of savanna. Over the range of savanna, some proportion of the land is always ‘not-savanna’. We reconciled previous contradictory views of savanna limits by developing a new conceptual framework for understanding these limits by categorizing environmental factors into whether they had a positive or negative effect on woody growth and the frequency of disturbance.

Distinct seasonal assemblages of arbuscular mycorrhizal fungi revealed by massively parallel pyrosequencing

Abstract: • Understanding the dynamics of rhizosphere microbial communities is essential for predicting future ecosystem function, yet most research focuses on either spatial or temporal processes, ignoring combined spatio-temporal effects. • Using pyrosequencing, we examined the spatio-temporal dynamics of a functionally important community of rhizosphere microbes, the arbuscular mycorrhizal (AM) fungi. We sampled AM fungi from plant roots growing in a temperate grassland in a spatially explicit manner throughout a year. • Ordination analysis of the AM fungal assemblages revealed significant temporal changes in composition and structure. Alpha and beta diversity tended to be negatively correlated with the climate variables temperature and sunshine hours. Higher alpha diversity during colder periods probably reflects more even competitive interactions among AM fungal species under limited carbon availability, a conclusion supported by analysis of beta diversity which highlights how resource limitation may change localized spatial dynamics. • Results reveal distinct AM fungal assemblages in winter and summer at this grassland site. A seasonally changing supply of host-plant carbon, reflecting changes in temperature and sunshine hours, may be the driving force in regulating the temporal dynamics of AM fungal communities. Climate change effects on seasonal temperatures may therefore substantially alter future AM fungal community dynamics and ecosystem functioning.

Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus Acer

Abstract: • Vulnerability to cavitation and conductive efficiency depend on xylem anatomy. We tested a large range of structure-function hypotheses, some for the first time, within a single genus to minimize phylogenetic 'noise' and maximize detection of functionally relevant variation. • This integrative study combined in-depth anatomical observations using light, scanning and transmission electron microscopy of seven Acer taxa, and compared these observations with empirical measures of xylem hydraulics. • Our results reveal a 2 MPa range in species' mean cavitation pressure (MCP). MCP was strongly correlated with intervessel pit structure (membrane thickness and porosity, chamber depth), weakly correlated with pit number per vessel, and not related to pit area per vessel. At the tissue level, there was a strong correlation between MCP and mechanical strength parameters, and some of the first evidence is provided for the functional significance of vessel grouping and thickenings on inner vessel walls. In addition, a strong trade-off was observed between xylem-specific conductivity and MCP. Vessel length and intervessel wall characteristics were implicated in this safety-efficiency trade-off. • Cavitation resistance and hydraulic conductivity in Acer appear to be controlled by a very complex interaction between tissue, vessel network and pit characteristics.

Sodium transport in plants: a critical review

Abstract: Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na(+) entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na(+) partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na(+) fluxes and cytosolic Na(+) concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na(+) transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis.

Responses of ecosystem nitrogen cycle to nitrogen addition: a meta-analysis

Abstract: Summary •Anthropogenic nitrogen (N) addition may substantially alter the terrestrial N cycle. However, a comprehensive understanding of how the ecosystem N cycle responds to external N input remains elusive. •Here, we evaluated the central tendencies of the responses of 15 variables associated with the ecosystem N cycle to N addition, using data extracted from 206 peer-reviewed papers. •Our results showed that the largest changes in the ecosystem N cycle caused by N addition were increases in soil inorganic N leaching (461%), soil NO3− concentration (429%), nitrification (154%), nitrous oxide emission (134%), and denitrification (84%). N addition also substantially increased soil NH4+ concentration (47%), and the N content in belowground (53%) and aboveground (44%) plant pools, leaves (24%), litter (24%) and dissolved organic N (21%). Total N content in the organic horizon (6.1%) and mineral soil (6.2%) slightly increased in response to N addition. However, N addition induced a decrease in microbial biomass N by 5.8%. •The increases in N effluxes caused by N addition were much greater than those in plant and soil pools except soil NO3−, suggesting a leaky terrestrial N system.

Carbon reserves and canopy defoliation determine the recovery of Scots pine 4 yr after a drought episode

Abstract: • Severe drought may increase physiological stress on long-lived woody vegetation, occasionally leading to mortality of overstory trees. Little is known about the factors determining tree survival and subsequent recovery after drought. • We used structural equation modeling to analyse the recovery of Scots pine (Pinus sylvestris) trees 4 yr after an extreme drought episode occurred in 2004-2005 in north-east Spain. Measured variables included the amount of green foliage, carbon reserves in the stem, mistletoe (Viscum album) infection, needle physiological performance and stem radial growth before, during and after the drought event. • The amount of green leaves and the levels of carbon reserves were related to the impact of drought on radial growth, and mutually correlated. However, our most likely path model indicated that current depletion of carbon reserves was a result of reduced photosynthetic tissue. This relationship potentially constitutes a feedback limiting tree recovery. In addition, mistletoe infection reduced leaf nitrogen content, negatively affecting growth. Finally, successive surveys in 2009-2010 showed a direct association between carbon reserves depletion and drought-induced mortality. • Severe drought events may induce long-term physiological disorders associated with canopy defoliation and depletion of carbon reserves, leading to prolonged recovery of surviving individuals and, eventually, to delayed tree death.

Establishing a time-scale for plant evolution

Abstract: • Plants have utterly transformed the planet, but testing hypotheses of causality requires a reliable time-scale for plant evolution. While clock methods have been extensively developed, less attention has been paid to the correct interpretation and appropriate implementation of fossil data. • We constructed 17 calibrations, consisting of minimum constraints and soft maximum constraints, for divergences between model representatives of the major land plant lineages. Using a data set of seven plastid genes, we performed a cross-validation analysis to determine the consistency of the calibrations. Six molecular clock analyses were then conducted, one with the original calibrations, and others exploring the impact on divergence estimates of changing maxima at basal nodes, and prior probability densities within calibrations. • Cross-validation highlighted Tracheophyta and Euphyllophyta calibrations as inconsistent, either because their soft maxima were overly conservative or because of undetected rate variation. Molecular clock analyses yielded estimates ranging from 568-815 million yr before present (Ma) for crown embryophytes and from 175-240 Ma for crown angiosperms. • We reject both a post-Jurassic origin of angiosperms and a post-Cambrian origin of land plants. Our analyses also suggest that the establishment of the major embryophyte lineages occurred at a much slower tempo than suggested in most previous studies. These conclusions are entirely compatible with current palaeobotanical data, although not necessarily with their interpretation by palaeobotanists.

Involvement of miR169 in the nitrogen-starvation responses in Arabidopsis

Abstract: Recent studies have revealed that microRNAs (miRNAs) regulate plant adaptive responses to nutrient deprivation. However, the functional significance of miRNAs in adaptive responses to nitrogen (N) limitation remains to be explored. The Arabidopsis miR169 was strongly down-regulated, whereas its targets, NFYA (Nuclear Factor Y, subunit A) family members, were strongly induced by nitrogen N starvation. Analysis of the expression of miR169 precursors showed that MIR169a was substantially down-regulated in both roots and shoots by N starvation. Accumulation of the NFYA family members was suppressed in transgenic Arabidopsis with constitutive expression of MIR169a. Transgenic Arabidopsis plants overexpressing MIR169a accumulated less N and were more sensitive to N stress than the wild type. N sensitivity of 35S::MIR169a might be attributable to impaired uptake systems. These results provide evidence that miRNAs have functional roles in helping plants to cope with fluctuations in N availability in the soil.

Evolutionary ecology of plant–microbe interactions: soil microbial structure alters selection on plant traits

Abstract: Summary • Below-ground microbial communities influence plant diversity, plant productivity, and plant community composition. Given these strong ecological effects, are interactions with below-ground microbes also important for understanding natural selection on plant traits? • Here, we manipulated below-ground microbial communities and the soil moisture environment on replicated populations of Brassica rapa to examine how microbial community structure influences selection on plant traits and mediates plant responses to abiotic environmental stress. • In soils with experimentally simplified microbial communities, plants were smaller, had reduced chlorophyll content, produced fewer flowers, and were less fecund when compared with plant populations grown in association with more complex soil microbial communities. Selection on plant growth and phenological traits also was stronger when plants were grown in simplified, less diverse soil microbial communities, and these effects typically were consistent across soil moisture treatments. • Our results suggest that microbial community structure affects patterns of natural selection on plant traits. Thus, the below-ground microbial community can influence evolutionary processes, just as recent studies have demonstrated that microbial diversity can influence plant community and ecosystem processes.

Association genetics of complex traits in plants.

Abstract: Association mapping is rapidly becoming the main method for dissecting the genetic architecture of complex traits in plants. Currently most association mapping studies in plants are preformed using sets of genes selected to be putative candidates for the trait of interest, but rapid developments in genomics will allow for genome-wide mapping in virtually any plant species in the near future. As the costs for genotyping are decreasing, the focus has shifted towards phenotyping. In plants, clonal replication and/or inbred lines allows for replicated phenotyping under many different environmental conditions. Reduced sequencing costs will increase the number of studies that use RNA sequencing data to perform expression quantitative trait locus (eQTL) mapping, which will increase our knowledge of how gene expression variation contributes to phenotypic variation. Current population sizes used in association mapping studies are modest in size and need to be greatly increased if mutations explaining less than a few per cent of the phenotypic variation are to be detected. Association mapping has started to yield insights into the genetic architecture of complex traits in plants, and future studies with greater genome coverage will help to elucidate how plants have managed to adapt to a wide variety of environmental conditions.

The biophysical design of plant cuticles: an overview

Abstract: The outer surfaces of epidermal cell walls are impregnated with an extracellular matrix called the cuticle. This composite matrix provides several functions at the interface level that enable plants to thrive in different habitats and withstand adverse environmental conditions. The lipid polymer cutin, which is the main constituent of the plant cuticle, has some unique biophysical properties resulting from its composition and structure. This review summarizes the progress made towards understanding the biophysical significance of this biopolymer with special focus on its structural, thermal, biomechanical, and hydric properties and relationships. The physiological relevance of such biophysical properties is discussed in light of existing knowledge on the plant cuticle.

Multifunctional fungal plant symbionts: new tools to enhance plant growth and productivity.

Abstract: A number of fungi are known to colonize plant roots but do not cause disease. These include mycorrhizas, binucleate Rhizoctonia spp., Piriformaspora indica, various plant growth-promoting rhizobacteria and, the subject of this commentary, Trichoderma spp. (Shoresh et al., 2010). Many of these organisms have been known for decades as agents that biocontrol plant diseases, but recent studies have demonstrated that they have many other useful attributes. These organisms are very clearly endophytic plant symbionts; this was, only recently, first described for Trichoderma strains (Harman et al., 2004). The first step for any of these microorganisms is for them to colonize roots. In this issue of New Phytologist, Vargas and colleagues (pp. 777–789), together with another recent paper (Vargas et al., 2009), demonstrate that plant-derived sucrose is an important resource and is critical for plant root colonization by Trichoderma virens. A fungal invertase is key to initiation of the mechanisms of root colonization (Vargas et al., 2009). The fungal genome includes a plant-like sucrose transporter: the specific sucrose ⁄ H2 symporter is induced during the early stages of root colonization. Furthermore, the results suggested the presence of a sucrose-independent network in the fungal cells that regulates the symbiotic association (Vargas et al., 2010).

The evolution of RNA editing and pentatricopeptide repeat genes.

Abstract: The pentatricopeptide repeat (PPR) is a degenerate 35-amino-acid structural motif identified from analysis of the sequenced genome of the model plant Arabidopsis thaliana. From the wealth of sequence information now available from plant genomes, the PPR protein family is now known to be one of the largest families in angiosperm species, as most genomes encode 400-600 members. As the number of PPR genes is generally only c. 10-20 in other eukaryotic organisms, including green algae, the family has obviously greatly expanded during land plant evolution. This provides a rare opportunity to study selection pressures driving a 50-fold expansion of a single gene family. PPR proteins are sequence-specific RNA-binding proteins involved in many aspects of RNA processing in organelles. In this review, we will summarize our current knowledge about the evolution of PPR genes, and will discuss the relevance of the dramatic expansion in the family to the functional diversification of plant organelles, focusing primarily on RNA editing.

Hydraulics and life history of tropical dry forest tree species: coordination of species' drought and shade tolerance.

Abstract: Summary • Plant hydraulic architecture has been studied extensively, yet we know little about how hydraulic properties relate to species’ life history strategies, such as drought and shade tolerance. The prevailing theories seem contradictory. • We measured the sapwood (Ks) and leaf (Kl) hydraulic conductivities of 40 coexisting tree species in a Bolivian dry forest, and examined associations with functional stem and leaf traits and indices of species’ drought (dry-season leaf water potential) and shade (juvenile crown exposure) tolerance. • Hydraulic properties varied across species and between life-history groups (pioneers vs shade-tolerant, and deciduous vs evergreen species). In addition to the expected negative correlation of Kl with drought tolerance, we found a strong, negative correlation between Kl and species’ shade tolerance. Across species, Ks and Kl were negatively correlated with wood density and positively with maximum vessel length. Consequently, drought and shade tolerance scaled similarly with hydraulic properties, wood density and leaf dry matter content. We found that deciduous species also had traits conferring efficient water transport relative to evergreen species. • Hydraulic properties varied across species, corresponding to the classical tradeoff between hydraulic efficiency and safety, which for these dry forest trees resulted in coordinated drought and shade tolerance across species rather than the frequently hypothesized trade-off.

Plasticity and evolution in drought avoidance and escape in the annual plant Brassica rapa.

Abstract: A key question in ecological genetics is to what extent do plants adapt to changes in climatic conditions, such as drought, through plasticity or evolution. To address this question, seeds of 140 maternal families of Brassica rapa were generated from collections made before (1997) and after (2004) a natural drought. These seeds were planted in the glasshouse and grown under low-water and high-water conditions. Post-drought lines flowered earlier than pre-drought lines, showing an evolutionary shift to earlier flowering. There was significant genetic variation and genotype by environment (G × E) interactions in flowering time, indicating genetic variation in plasticity in this trait. Plants that flowered earlier had fewer leaf nodes and lower instantaneous (A/g) and integrated (δ(13)C) water use efficiency than late-flowering plants. These results suggest that B. rapa plants escape drought through early flowering rather than avoid drought through increased water use efficiency. The mechanism of this response appears to be high transpiration and inefficient water use, leading to rapid development. These findings demonstrate a trade-off between drought avoidance and escape, and indicate that, in this system, where drought acts to shorten the growing season, selection for drought escape through earlier flowering is more important than phenotypic plasticity.

Global vegetation and terrestrial carbon cycle changes after the last ice age.

Abstract: • In current models, the ecophysiological effects of CO₂ create both woody thickening and terrestrial carbon uptake, as observed now, and forest cover and terrestrial carbon storage increases that took place after the last glacial maximum (LGM). Here, we aimed to assess the realism of modelled vegetation and carbon storage changes between LGM and the pre-industrial Holocene (PIH). • We applied Land Processes and eXchanges (LPX), a dynamic global vegetation model (DGVM), with lowered CO₂ and LGM climate anomalies from the Palaeoclimate Modelling Intercomparison Project (PMIP II), and compared the model results with palaeodata. • Modelled global gross primary production was reduced by 27-36% and carbon storage by 550-694 Pg C compared with PIH. Comparable reductions have been estimated from stable isotopes. The modelled areal reduction of forests is broadly consistent with pollen records. Despite reduced productivity and biomass, tropical forests accounted for a greater proportion of modelled land carbon storage at LGM (28-32%) than at PIH (25%). • The agreement between palaeodata and model results for LGM is consistent with the hypothesis that the ecophysiological effects of CO₂ influence tree-grass competition and vegetation productivity, and suggests that these effects are also at work today.

Epidermis: the formation and functions of a fundamental plant tissue

Abstract: Epidermis differentiation and maintenance are essential for plant survival. Constant cross-talk between epidermal cells and their immediate environment is at the heart of epidermal cell fate, and regulates epidermis-specific transcription factors. These factors in turn direct epidermal differentiation involving a whole array of epidermis-specific pathways including specialized lipid metabolism necessary to build the protective cuticle layer. An intact epidermis is crucial for certain key processes in plant development, shoot growth and plant defence. Here, we discuss the control of epidermal cell fate and the function of the epidermal cell layer in the light of recent advances in the field.

Silencing of 4-coumarate:coenzyme A ligase in switchgrass leads to reduced lignin content and improved fermentable sugar yields for biofuel production.

Abstract: Summary • The lignin content of feedstock has been proposed as one key agronomic trait impacting biofuel production from lignocellulosic biomass. 4-Coumarate:coenzyme A ligase (4CL) is one of the key enzymes involved in the monolignol biosynthethic pathway. • Two homologous 4CL genes, Pv4CL1 and Pv4CL2, were identified in switchgrass (Panicum virgatum) through phylogenetic analysis. Gene expression patterns and enzymatic activity assays suggested that Pv4CL1 is involved in monolignol biosynthesis. Stable transgenic plants were obtained with Pv4CL1 downregulated. • RNA interference of Pv4CL1 reduced extractable 4CL activity by 80%, leading to a reduction in lignin content with decreased guaiacyl unit composition. Altered lignification patterns in the stems of RNAi transgenic plants were observed with phloroglucinol-HCl staining. The transgenic plants also had uncompromised biomass yields. After dilute acid pretreatment, the low lignin transgenic biomass had significantly increased cellulose hydrolysis (saccharification) efficiency. • The results demonstrate that Pv4CL1, but not Pv4CL2, is the key 4CL isozyme involved in lignin biosynthesis, and reducing lignin content in switchgrass biomass by silencing Pv4CL1 can remarkably increase the efficiency of fermentable sugar release for biofuel production.

Whole‐system responses of experimental plant communities to climate extremes imposed in different seasons

Abstract: • Discrete climate events such as heat waves and droughts can have a disproportionate impact on ecosystems relative to the temporal scale over which they occur. Research oriented towards (extreme) events rather than (gradual) trends is therefore urgently needed. • Here, we imposed heat waves and droughts (50-yr return time) in a full factorial design on experimental plant communities in spring, summer or autumn. Droughts were created by removing the controlled water table (rainout shelters prevented precipitation), while heat waves were imposed with infrared heaters. • Measurements of whole-system CO(2) exchange, growth and biomass production revealed multiple interactions between treatments and the season in which they occurred. Heat waves had only small and transient effects, with infrared imaging showing little heat stress because of transpirational cooling. If heat waves were combined with drought, negative effects observed in single factor drought treatments were exacerbated through intensified soil drying, and heat stress in summer. Plant recovery from stress differed, affecting the biomass yield. • In conclusion, the timing of extreme events is critical regarding their impact, and synergisms between heat waves and drought aggravate the negative effects of these extremes on plant growth and functioning.

Improved nitrogen nutrition enhances root uptake, root-to-shoot translocation and remobilization of zinc (65Zn) in wheat

Abstract: This study focussed on the effect of increasing nitrogen (N) supply on root uptake and root-to-shoot translocation of zinc (Zn) as well as retranslocation of foliar-applied Zn in durum wheat (Triticum durum). Nutrient solution experiments were conducted to examine the root uptake and root-to-shoot translocation of (65) Zn in seedlings precultured with different N supplies. In additional experiments, the effect of varied N nutrition on retranslocation of foliar-applied (65) Zn was tested at both the vegetative and generative stages. When N supply was increased, the (65) Zn uptake by roots was enhanced by up to threefold and the (65) Zn translocation from roots to shoots increased by up to eightfold, while plant growth was affected to a much smaller degree. Retranslocation of (65) Zn from old into young leaves and from flag leaves to grains also showed marked positive responses to increasing N supply. The results demonstrate that the N-nutritional status of wheat affects major steps in the route of Zn from the growth medium to the grain, including its uptake, xylem transport and remobilization via phloem. Thus, N is a critical player in the uptake and accumulation of Zn in plants, which deserves special attention in biofortification of food crops with Zn.

Leaf traits and decomposition in tropical rainforests: revisiting some commonly held views and towards a new hypothesis

Abstract: Proper estimates of decomposition are essential for tropical forests, given their key role in the global carbon (C) cycle. However, the current paradigm for litter decomposition is insufficient to account for recent observations and may limit model predictions for highly diverse tropical ecosystems. In light of recent findings from a nutrient-poor Amazonian rainforest, we revisit the commonly held views that: litter traits are a mere legacy of live leaf traits; nitrogen (N) and lignin are the key litter traits controlling decomposition; and favourable climatic conditions result in rapid decomposition in tropical forests. Substantial interspecific variation in litter phosphorus (P) was found to be unrelated to variation in green leaves. Litter nutrients explained no variation in decomposition, which instead was controlled primarily by nonlignin litter C compounds at low concentrations with important soil fauna effects. Despite near-optimal climatic conditions, tropical litter decomposition proceeded more slowly than in a climatically less favourable temperate forest. We suggest that slow decomposition in the studied rainforest results from a syndrome of poor litter C quality beyond a simple lignin control, enforcing energy starvation of decomposers. We hypothesize that the litter trait syndrome in nutrient-poor tropical rainforests may have evolved to increase plant access to limiting nutrients via mycorrhizal associations.

Nitric oxide participates in cold-responsive phosphosphingolipid formation and gene expression in Arabidopsis thaliana.

Abstract: Chilling triggers rapid molecular responses that permit the maintenance of plant cell homeostasis and plant adaptation. Recent data showed that nitric oxide (NO) is involved in plant acclimation and tolerance to cold. The participation of NO in the early transduction of the cold signal in Arabidopsis thaliana was investigated. The production of NO after a short exposure to cold was assessed using the NO-sensitive fluorescent probe 4, 5-diamino fluoresceine diacetate and chemiluminescence. Pharmacological and genetic approaches were used to analyze NO sources and NO-mediated changes in cold-regulated gene expression, phosphatidic acid (PtdOH) synthesis and sphingolipid phosphorylation. NO production was detected after 1-4h of chilling. It was impaired in the nia1nia2 nitrate reductase mutant. Moreover, NO accumulation was not observed in H7 plants overexpressing the A. thaliana nonsymbiotic hemoglobin Arabidopsis haemoglobin 1 (AHb1). Cold-regulated gene expression was affected in nia1nia2 and H7 plants. The synthesis of PtdOH upon chilling was not modified by NO depletion. By contrast, the formation of phytosphingosine phosphate and ceramide phosphate, two phosphorylated sphingolipids that are transiently synthesized upon chilling, was negatively regulated by NO. Taken together, these data suggest a new function for NO as an intermediate in gene regulation and lipid-based signaling during cold transduction.

Heat perception and signalling in plants: a tortuous path to thermotolerance.

Abstract: Summary An accurate assessment of the rising ambient temperature by plant cells is crucial for the timely activation of various molecular defences before the appearance of heat damage. Recent findings have allowed a better understanding of the early cellular events that take place at the beginning of mild temperature rise, to timely express heat-shock proteins (HSPs), which will, in turn, confer thermotolerance to the plant. Here, we discuss the key components of the heat signalling pathway and suggest a model in which a primary sensory role is carried out by the plasma membrane and various secondary messengers, such as Ca2+ ions, nitric oxide (NO) and hydrogen peroxide (H2O2). We also describe the role of downstream components, such as calmodulins, mitogen-activated protein kinases and Hsp90, in the activation of heat-shock transcription factors (HSFs). The data gathered for land plants suggest that, following temperature elevation, the heat signal is probably transduced by several pathways that will, however, coalesce into the final activation of HSFs, the expression of HSPs and the onset of cellular thermotolerance.

Carbon and nitrogen dynamics during forest stand development: a global synthesis

Abstract: Our knowledge of carbon (C) and nitrogen (N) dynamics during stand development is not only essential for evaluating the role of secondary forests in the global terrestrial C cycle, but also crucial for understanding long-term C-N interactions in terrestrial ecosystems. However, a comprehensive understanding of forest C and N dynamics over age sequence remains elusive due to the diverse results obtained across individual studies. Here, we synthesized the results of more than 100 studies to examine C and N dynamics during forest stand development. Our results showed that C accumulated in aboveground vegetation, litter and forest floor pools, while the mineral soil C pool did not exhibit significant changes in most studies. The rate of C changes declined with stand age and approached equilibrium during the later stage of stand development. The rate of N changes exhibited linear increases with that of C changes, indicating that N also accrued in various ecosystem components except mineral soil. These results demonstrate that substantial increases in C pools over age sequence are accompanied by N accretion in forest ecosystems. The concurrent C and N dynamics suggest that forest ecosystems may have an intrinsic ability to preclude progressive N limitation during stand development.

Truffle volatiles: from chemical ecology to aroma biosynthesis

Abstract: Truffles (Tuber spp.) are symbiotic fungi that develop underground in association with plant roots. Food connoisseurs describe their scent as sensual, seductive and unique. These mysterious fungi, however, do not produce their aroma for the mere pleasure of humans. Truffle volatiles act as odorant cues for mammals and insects which are thus able to locate the precious fungi underground and spread their spores. They also freely diffuse in the soil and mediate interactions with microorganisms and plant roots, potentially regulating a complex molecular dialogue among soil fauna and flora. The aim of this review is to synthesize 30 yr of research on truffle volatiles, spanning fields of study from chemical ecology to aroma biosynthesis. Specific aspects of truffle volatile ecology and biology will be discussed, including which species have been studied so far and for what purpose, what ecological role has been demonstrated or speculated to exist for specific truffle volatiles, which volatiles are common or unique to certain species and what their biosynthetic route might be. Future challenges in truffle aroma research will also be addressed, focusing on how high-throughput post-genomic technologies may advance our understanding of truffle aroma biosynthesis and chemical ecology.