Metallothioneins (MTs) are low-molecular-weight, cysteine-rich proteins believed to play a role in cytosolic zinc (Zn) and copper (Cu) homeostasis. However, evidence for the functional properties of ...MTs has been hampered by methodological problems in the isolation and characterization of the proteins. Here, we document that barley (Hordeum vulgare) MT3 and MT4 proteins exist in planta and that they differ in tissue localization as well as in metal coordination chemistry. Combined transcriptional and histological analyses showed temporal and spatial correlations between transcript levels and protein abundance during grain development. MT3 was present in tissues of both maternal and filial origin throughout grain filling. In contrast, MT4 was confined to the embryo and aleurone layer, where it appeared during tissue specialization and remained until maturity. Using state-of-the-art speciation analysis by size-exclusion chromatography inductively coupled plasma mass spectrometry and electrospray ionization time-of-flight mass spectrometry on recombinant MT3 and MT4, their specificity and capacity for metal ion binding were quantified, showing a strong preferential Zn binding relative to Cu and cadmium (Cd) in MT4, which was not the case for MT3. When complementary DNAs from barley MTs were expressed in Cu- or Cd-sensitive yeast mutants, MT3 provided a much stronger complementation than did MT4. We conclude that MT3 may play a housekeeping role in metal homeostasis, while MT4 may function in Zn storage in developing and mature grains. The localization of MT4 and its discrimination against Cd make it an ideal candidate for future biofortification strategies directed toward increasing food and feed Zn concentrations.
The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In ...the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.
Foliar fertilization is a potential tool to increase the use-efficiency of nitrogen (N) fertilizers. However, whilst leaf scorching has frequently been reported, the underlying physiological ...processes are not clear. In the present work, we investigate the intensity of leaf scorching as affected by the balance between ammonium assimilation and accumulation. Leaves were sprayed with urea–ammonium nitrate (UAN) solution without surfactant or applied liquid droplets of urea in different N concentrations with surfactant. UAN solutions without surfactant containing >10% N caused leaf scorching already after 24 h and the severity increased with the N concentration. The same pattern was observed 3 days after the application of urea solutions containing >4% N together with surfactant. The scorching was accompanied by a massive increase in foliar and apoplastic ammonium (NH4+) concentration. Moreover, the activity of glutamine synthetase (GS), most pronouncedly that of the chloroplastic isoform (GS2), decreased a few hours after the application of high N-concentrations. Along with this, the concentration of glutamate—the substrate for GS—decreased. We conclude that leaf scorching is promoted by NH4+ accumulation due to a limitation in N assimilation capacity.
In order to improve our understanding of the regulation of nitrogen assimilation and recycling in wheat (Triticum aestivum L.), we studied the localization of plastidic (GS2) and cytosolic (GS1) ...glutamine synthetase isoenzymes and of glutamate dehydrogenase (GDH) during natural senescence of the flag leaf and in the stem. In mature flag leaves, large amounts of GS1 were detected in the connections between the mestome sheath cells and the vascular cells, suggesting an active transfer of nitrogen organic molecules within the vascular system in the mature flag leaf. Parallel to leaf senescence, an increase of a GS1 polypeptide (GS1b) was detected in the mesophyll cytosol of senescing leaves, while the GS protein content represented by another polypetide (GS1a) in the phloem companion cells remained practically constant in both leaves and stems. Both GDH aminating activity and protein content were strongly induced in senescing flag leaves. The induction occurred both in the mitochondria and in the cytosol of phloem companion cells, suggesting that the shift in GDH cellular compartmentation is important during leaf nitrogen remobilization although the metabolic or sensing role of the enzyme remains to be elucidated. Taken together, our results suggest that in wheat, nitrogen assimilation and recycling are compartmentalized between the mesophyll and the vasculature, and are shifted in different cellular compartments within these two tissues during the transition of sink leaves to source leaves.
•The invasive Rhododendron ponticum is considered an emergent threat in W France.•It thrives in woodlands on acidic soils and appears promoted by climate change.•It locally spreads on moister soils ...via active layering.•Its ability to establish new populations is low due to recruitment limitations.•Management should minimize soil disturbance near established populations.
Understanding the population dynamics of invasive species is essential for identifying the key parameters of their success and guiding management actions. European temperate forests of the Atlantic domain are prone to be invaded by the Pontic Rhododendron (Rhododendron ponticum L. subsp. baeticum (Boiss. & Reut.) Hand.-Mazz.), a shade-tolerate evergreen shrub native to the Iberian peninsula. While the invasion has a long history in Ireland and the British Isles, it is an emerging threat in mainland Europe. It is therefore a timely challenge to better understanding the invasive dynamics of this species in this context. We studied the population dynamics of R. ponticum in 27 invaded forests of north-western France, examining population structure, and individuals’ growth, fertility and fecundity among contrasted light conditions. All invasive populations were found on acidic soils. Irrespective of light conditions, R. ponticum exhibited a regular radial growth, with a clear increase since the 1990s. The majority of stems were younger than 38 years on average, indicating a recent increase in population density. Light availability strongly impacted R. ponticum’s canopy structure, with higher canopy cover and stem numbers under sun conditions. Despite the production of a huge number of viable seeds, the observed number of seedlings was always very low irrespective of light conditions, indicating recruitment limitation and thus a limited ability to establish new, distant populations. In contrast, vegetative propagation via intense layering at the invasion front wave ensured local population spread from initially planted individuals, especially on moist soils. We conclude that where it has been planted, R. ponticum is able to actively layer, thereby forming dense bushes in the forest understories as long as the soil is acidic and relatively moist. The establishment of new, distant populations appears a rare event since recruitment is limited to particular microhabitats related to forest management-associated disturbances. Such disturbance should thus be avoided in sites where the species has already established, unless a post-disturbance eradication of R. ponticum’s seedling can be implemented the following years. The plantation of R. ponticum in forest must be avoided when the soil is acidic and moist. Because R. ponticum’s growth performances are increasing since the 1990s, as a likely consequence of climate change, monitoring established populations is recommended to detect a possible increasing invasiveness.
Questions
Rhododendron ponticum subsp. baeticum is an invasive shrub of growing concern in continental Europe, but little is known about its impact on native plant communities. Here we ask: do ...environmental conditions differ between forest stands invaded by it and uninvaded stands? Do these differences correlate with R. ponticum's cover? Are these differences associated with differences in taxonomic and functional diversity of vascular plant species of the herb layer? Can these vegetation changes be explained by the sorting of certain life‐history traits by R. ponticum‐induced environmental changes?
Location
Several forests invaded by R. ponticum in the French Atlantic domain.
Methods
We recorded vegetation composition and a number of environmental variables in 400‐m2 plots that were established in 64 paired forest stands (32 invaded vs 32 uninvaded). We compiled traits from existing databases. We computed several metrics of taxonomic and functional diversity. We compared environmental variables and diversity metrics between invaded and uninvaded stands. We used correlation and regression analyses to relate them with R. ponticum's cover. We ran RLQ and fourth‐corner analyses to explore the relationships between R. ponticum invasion, environmental variables, species traits, and vegetation composition.
Results
Independent of its abundance, R. ponticum invasion was associated with lower light arrival at the forest floor and increased litter thickness. Concomitantly, species richness and diversity and trait diversity were reduced. The major driver of species assemblages was soil pH, which strongly interacted with the invasion gradient. R. ponticum did not sort species according to traits associated with shade tolerance and thick‐litter tolerance. However, tree and shrub saplings were more abundant in invaded than uninvaded stands, at the expense of graminoid and fern species.
Conclusions
As R. ponticum becomes the dominant shrub, it exerts new selection forces on life‐history traits of extant species, mostly via reduced light availability, increased litter thickness, and physical competition, thereby reducing taxonomic and functional diversity of the herb layer, without impeding tree and shrub self‐regeneration, at least in the short term.
Rhododendron ponticum subsp. baeticum is an invasive shrub of growing concern in temperate Europe. While invading forest understories, it exerts new selection forces on life‐history traits of extant species, mostly via reduced light availability, increased litter thickness, and physical competition, thereby reducing taxonomic and functional diversity of the herb layer, without impeding tree and shrub self‐regeneration in the short term.
This report presents information on all aspects of the biology, ecology and invasion behaviour of Rhododendron ponticum L., particularly R. ponticum subsp. baeticum (Boiss. & Reut.) Hand.-Mazz. The ...main topics presented are: taxonomy, distribution, history of introduction and spread, ecology, biology, impacts, and management. The subspecies baeticum is native to the Iberian Peninsula, where it is close to extinction. This shade-tolerant evergreen shrub has been broadly introduced throughout Europe since the 18
th
century, mainly for ornamental purposes. The invasive taxon likely results from artificial introgressive hybridization with the Appalachian species R. catawbiense Minchx. It is now naturalized in many countries across western Europe and has become a serious invader in the British Isles. In continental Europe, it mostly invades forest ecosystems of the Atlantic domain, especially on acidic, nutrient-poor but moist soils. R. ponticum subsp. baeticum has perfectly adapted to the humid temperate climate of these regions. As a shade-tolerant species, it can spread out its dense canopy below tree canopies, thereby shading out most herb species and tree seedlings and saplings. Its thick litter likely impacts soil chemical and biological features and hence alters ecosystem processes. Though it produces thousands of seeds, the species mostly propagates vegetatively via layering and forms extended clonal thickets. Long-distance dispersal occurs via seeds, but those can only germinate on moss mats, decaying wood and bare soil and seedlings are very vulnerable to drought. The shrub is also known to be a reservoir for phytopathogenic oomycetes of the genus Phytophthora, including P. ramorum and P. kernoviae. R. ponticum subsp. baeticum invasion is an emerging threat to natural habitats and their associated fauna and flora in western continental Europe. Control is still challenging since management operations are not only expensive and time-consuming but also poorly effective due to vigorous resprouting from stumps after cutting and herbicide resistance.
Summary
A novel Arabidopsis thaliana mutant, named hoc, was found to have an high organogenic capacity for shoot regeneration. The HOC locus may be involved in cytokinin metabolism leading to ...cytokinin‐overproduction. In vitro, hoc root explants develop many shoots in the absence of exogenous growth regulators. The mutant displays a bushy phenotype with supernumerary rosettes and with normal phyllotaxy, resulting from precocious axillary meristem development. Genetic and molecular analyses show that the high shoot regeneration and the bushy phenotype are controlled by a recessive single gene, located on chromosome I, next to the GAPB CAPS marker. The mapping data and allelism tests reveal that the hoc mutant is not allelic to other reported Arabidopsis growth‐regulator mutants. In darkness the hoc mutant is de‐etiolated, with a short hypocotyl, opened cotyledons and true leaves. Growth regulator assays reveal that the mutant accumulates cytokinins at about two‐ and sevenfold the cytokinin level of wild‐type plants in its aerial parts and roots, respectively. Consequently, the elevated amounts of endogenous cytokinins in hoc plants are associated with high organogenic capacity and hence bushy phenotype. Thus hoc is the first cytokinin‐overproducing Arabidopsis mutant capable of auto‐regenerating shoots without exogenous growth regulators.
In wheat, nitrogen (N) uptake and remobilisation after flowering contributes largely, in Northern countries, to grain yield and grain protein content. The aim of our study was first to estimate the ...proportion of N taken up and remobilised to the grain as well as their relative efficiency using
15NO
3
−-labelling at flowering. The validity of the technique was assessed in comparison to the N budget calculation method on five winter wheat cultivars grown for 2 years at low and high fertilization input. We estimated that on average 71.2% of grain N originates from remobilisation with significant genotypic differences. Among the five genotypes, significant differences were also found for both N remobilisation efficiency (from 69.8 to 88.8%) and N translocation efficiency (from 89.7 to 93.4%). In parallel, during 1 year, we monitored physiological markers representative of N assimilation and recycling at two sampling dates during the grain filling period. We then examined if there was any relationship between these physiological markers, N absorption and remobilisation estimates and agronomic traits related to yield and grain N content. Nitrate reductase (NR) activity was highly correlated to N absorbed post-flowering and to grain protein content. Glutamine synthetase (GS) activity was even more highly correlated than NR activity to the amount of N remobilised and grain yield. The use of physiological traits such as NR and GS activities as markers of the wheat N status is discussed.
•The leaf economics spectrum is narrower in New Zealand than in European forests.•Sycamore maple increases photosynthetic efficiency of New Zealand evergreen forests.•Sycamore appears a ‘pre-adapted’ ...alien species which fills a formerly empty niche.
Understanding the mechanisms behind the successful invasion of alien plant species remains a significant research challenge in invasion ecology. There are surprisingly few cross-continental comparative studies which investigated interspecific trait differences between native and invaded ranges. Here, we compare leaf functional traits of sycamore (Acer pseudoplatanus L.), a deciduous tree species native to Europe and invasive in New Zealand (NZ), to co-occurring resident woody species in both the native and invaded ranges. We analyze a suite of 14 physiological traits on individuals growing either in shade or in sun light conditions. Despite similar environmental conditions, the leaf economics spectrum of forest communities is narrower in NZ than in France. Sycamore extends the leaf economics spectrum of the recipient community towards greater photosynthetic efficiency in the invaded range at a relatively low construction cost. This feature increases the competitive advantage of sycamore over native species. Biogeographic history likely has resulted in a native insular temperate forest community in NZ with unoccupied trait space along the leaf economics spectrum, making these communities susceptible to invasion by ‘pre-adapted’ alien species, according to the Evolutionary Imbalance Hypothesis. Sycamore may thus successfully invade NZ evergreen forests simply because it fills a functional gap at the fast return end of the leaf economics spectrum, corresponding to a formerly empty niche. As a consequence, forest management can hardly extirpate the invasive alien tree.
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