Showing papers in "Plant Biology in 2019"

Salinity and crop yield

Abstract: Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop-specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na+ , also Cl- , Mg2+ , SO4 2- or HCO3 - contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.

Malate valves: old shuttles with new perspectives.

Abstract: Malate valves act as powerful systems for balancing the ATP/NAD(P)H ratio required in various subcellular compartments in plant cells. As components of malate valves, isoforms of malate dehydrogenases (MDHs) and dicarboxylate translocators catalyse the reversible interconversion of malate and oxaloacetate and their transport. Depending on the co-enzyme specificity of the MDH isoforms, either NADH or NADPH can be transported indirectly. Arabidopsis thaliana possesses nine genes encoding MDH isoenzymes. Activities of NAD-dependent MDHs have been detected in mitochondria, peroxisomes, cytosol and plastids. In addition, chloroplasts possess a NADP-dependent MDH isoform. The NADP-MDH as part of the 'light malate valve' plays an important role as a poising mechanism to adjust the ATP/NADPH ratio in the stroma. Its activity is strictly regulated by post-translational redox-modification mediated via the ferredoxin-thioredoxin system and fine control via the NADP+ /NADP(H) ratio, thereby maintaining redox homeostasis under changing conditions. In contrast, the plastid NAD-MDH ('dark malate valve') is constitutively active and its lack leads to failure in early embryo development. While redox regulation of the main cytosolic MDH isoform has been shown, knowledge about regulation of the other two cytosolic MDHs as well as NAD-MDH isoforms from peroxisomes and mitochondria is still lacking. Knockout mutants lacking the isoforms from chloroplasts, mitochondria and peroxisomes have been characterised, but not much is known about cytosolic NAD-MDH isoforms and their role in planta. This review updates the current knowledge on MDH isoforms and the shuttle systems for intercompartmental dicarboxylate exchange, focusing on the various metabolic functions of these valves.

Species-specific differences in water uptake depth of mature temperate trees vary with water availability in the soil.

Abstract: Temperate tree species differ in their physiological sensitivity to declining soil moisture and drought. Although species-specific responses to drought have often been suggested to be the result of different water uptake depths, empirical evidence for such a mechanism is scarce. Here we test if differences in water uptake depths can explain previously observed species-specific physiological responses of temperate trees to drought and if the water uptake depth of different species varies in response to declining soil moisture. For this purpose, we employed stable oxygen and hydrogen isotopes of soil and xylem water that we collected over the course of three growing seasons in a mature temperate forest in Switzerland. Our data show that all investigated species utilise water from shallow soil layers during times of sufficient soil water supply. However, Fraxinus excelsior, Fagus sylvatica and Acer pseudoplatanus were able to shift their water uptake to deeper soil layers when soil water availability decreased in the topsoil. In contrast, Picea abies, was not able to shift its water uptake to deeper soil layers. We conclude from our data that more drought-resistant tree species are able to shift their water uptake to deeper soil layers when water availability in the topsoil is becoming scarce. In addition, we were able to show that water uptake depth of temperate tree species is a trait with high plasticity that needs to be characterised across a range of environmental conditions.

Community-based native seed production for restoration in Brazil – the role of science and policy

Abstract: Large-scale restoration programmes in the tropics require large volumes of high quality, genetically diverse and locally adapted seeds from a large number of species. However, scarcity of native seeds is a critical restriction to achieve restoration targets. In this paper, we analyse three successful community-based networks that supply native seeds and seedlings for Brazilian Amazon and Cerrado restoration projects. In addition, we propose directions to promote local participation, legal, technical and commercialisation issues for up-scaling the market of native seeds for restoration with high quality and social justice. We argue that effective community-based restoration arrangements should follow some principles: (i) seed production must be based on real market demand; (ii) non-governmental and governmental organisations have a key role in supporting local organisation, legal requirements and selling processes; (iii) local ecological knowledge and labour should be valued, enabling local communities to promote large-scale seed production; (iv) applied research can help develop appropriate techniques and solve technical issues. The case studies from Brazil and principles presented here can be useful for the up-scaling restoration ecology efforts in many other parts of the world and especially in tropical countries where improving rural community income is a strategy for biodiversity conservation and restoration.

Chlorogenic acid‐mediated chemical defence of plants against insect herbivores

Abstract: Chlorogenic acid is one of the most abundant beneficial polyphenols in plants and is well known as a nutritional antioxidant in plant-based foods. Apart from its dietary antioxidant activity, it has been proved to be an efficient defence molecule against a broad range of insect herbivores. In the last two decades, several reports have shown the effectiveness of chlorogenic acid in insect growth deterrence. The pathway for chlorogenic acid biosynthesis in plants was previously elucidated, and metabolic engineering of the principal pathway showed high chlorogenic acid production in tomato plants. Herbivore-mediated induction of chlorogenic acid biosynthesis was also demonstrated both at metabolite and transcript level, although herbivore-mediated molecular regulation of chlorogenic acid biosynthesis is not yet fully elucidated. In this communication, we present our views on the efficacy of chlorogenic acid as an anti-herbivore defence molecule in plants and also discuss its future outlook.

The battle of two ions: Ca2+ signalling against Na+ stress

Abstract: Soil salinity adversely affects plant growth, crop yield and the composition of ecosystems. Salinity stress impacts plants by combined effects of Na+ toxicity and osmotic perturbation. Plants have evolved elaborate mechanisms to counteract the detrimental consequences of salinity. Here we reflect on recent advances in our understanding of plant salt tolerance mechanisms. We discuss the embedding of the salt tolerance-mediating SOS pathway in plant hormonal and developmental adaptation. Moreover, we review newly accumulating evidence indicating a crucial role of a transpiration-dependent salinity tolerance pathway, that is centred around the function of the NADPH oxidase RBOHF and its role in endodermal and Casparian strip differentiation. Together, these data suggest a unifying and coordinating role for Ca2+ signalling in combating salinity stress at the cellular and organismal level.

Regulation of plant peptide hormones and growth factors by post-translational modification

Abstract: The number, diversity and significance of peptides as regulators of cellular differentiation, growth, development and defence of plants has long been underestimated. Peptides have now emerged as an important class of signals for cell-to-cell communication over short distances, and also for long-range signalling. We refer to these signalling molecules as peptide growth factors and peptide hormones, respectively. As compared to remarkable progress with respect to the mechanisms of peptide perception and signal transduction, the biogenesis of signalling peptides is still in its infancy. This review focuses on the biogenesis and activity of small post-translationally modified peptides. These peptides are derived from inactive pre-pro-peptides of approximately 70-120 amino acids. Multiple post-translational modifications (PTMs) may be required for peptide maturation and activation, including proteolytic processing, tyrosine sulfation, proline hydroxylation and hydroxyproline glycosylation. While many of the enzymes responsible for these modifications have been identified, their impact on peptide activity and signalling is not fully understood. These PTMs may or may not be required for bioactivity, they may inactivate the peptide or modify its signalling specificity, they may affect peptide stability or targeting, or its binding affinity with the receptor. In the present review, we will first introduce the peptides that undergo PTMs and for which these PTMs were shown to be functionally relevant. We will then discuss the different types of PTMs and the impact they have on peptide activity and plant growth and development. We conclude with an outlook on the open questions that need to be addressed in future research.

The Myo-inositol pathway does not contribute to ascorbic acid synthesis.

Abstract: Ascorbic acid (AsA) biosynthesis in plants predominantly occurs via a pathway with d-mannose and l-galactose as intermediates. One alternative pathway for AsA synthesis, which is similar to the biosynthesis route in mammals, is controversially discussed for plants. Here, myo-inositol is cleaved to glucuronic acid and then converted via l-gulonate to AsA. In contrast to animals, plants have an effective recycling pathway for glucuronic acid, being a competitor for the metabolic rate. Recycling involves a phosphorylation at C1 by the enzyme glucuronokinase. Two previously described T-DNA insertion lines in the gene coding for glucuronokinase1 show wild type-like expression levels of the mRNA in our experiments and do not accumulate glucuronic acid in labelling experiments disproving that these lines are true knockouts. As suitable T-DNA insertion lines were not available, we generated frameshift mutations in the major expressed isoform glucuronokinase1 (At3g01640) to potentially redirect metabolites to AsA. However, radiotracer experiments with 3 H-myo-inositol revealed that the mutants in glucuronokinase1 accumulate only glucuronic acid and incorporate less metabolite into cell wall polymers. AsA was not labelled, suggesting that Arabidopsis cannot efficiently use glucuronic acid for AsA biosynthesis. All four mutants in glucuronokinase as well as the wild type have the same level of AsA in leaves.

Characterisation of plant growth-promoting rhizobacteria from rhizosphere soil of heat-stressed and unstressed wheat and their use as bio-inoculant.

Abstract: High temperature induces several proteins in plants that enhance tolerance to high temperature shock. The fate of proteins synthesised in microbial cells or secreted into culture media by interacting microbes has not been fully elucidated. The present investigation aimed to characterise plant growth-promoting rhizobacteria (PGPR) isolated from the rhizosphere of wheat genotypes (differing in tolerance to high temperature stress) and evaluate their performance as bioinoculant for use in wheat. Four bacterial strains, viz. Pseudomonas brassicacearum, Bacillus thuringiensis, Bacillus cereus strain W6 and Bacillus subtilis, were isolated from the rhizosphere of heat-stressed and unstressed wheat genotypes. The wheat genotypes were exposed to high temperature stress at 45 °C for 10 days (3 h daily) at pre-anthesis phase. Isolates were identified on the basis of morphology and biochemical characteristics, 16S rRNA gene sequencing and whole cell protein profiles. Results were further complemented by size exclusion chromatography (SEC) with fast protein liquid chromatography (FPLC) and SDS PAGE of 80% ammonium sulphate precipitates of the cell-free supernatants. Isolates were positive for catalase, oxidases and antimicrobial activity. P. brassicacearum from the rhizosphere of the heat-tolerant genotype was more efficient in phosphate solubilisation, bacteriocin production, antifungal and antibacterial activity against Helminthosporium sativum, Fusarium moniliforme and Klebsiella pneumonia, respectively. The inoculated seedlings had significantly higher root and shoot fresh weight, enhanced activity of antioxidant enzymes, proline and protein content. Total profiling of the culture with SDS-PAGE indicated expression of new protein bands in 95 kDa in P. brassicacearum. Temperature-induced changes in PGPR isolates are similar to those in the host plant. P. brassicacearum may be a good candidate for use in biofertiliser production for plants exposed to high temperature stress.

Rapid evolution in native plants cultivated for ecological restoration: not a general pattern.

Abstract: The growing number of restoration projects worldwide increases the demand for seed material of native species. To meet this demand, seeds are often produced through large-scale cultivation on specialised farms, using wild-collected seeds as the original sources. However, during cultivation, plants experience novel environmental conditions compared to those in natural populations, and there is a danger that the plants in cultivation are subject to unintended selection and lose their adaptation to natural habitats. Although the propagation methods are usually designed to maintain as much natural genetic diversity as possible, the effectiveness of these measures have never been tested. We obtained seed of five common grassland species from one of the largest native seed producers in Germany. For each species, the seeds were from multiple generations of seed production. We used AFLP markers and a common garden experiment to test for genetic and phenotypic changes during cultivation of these plants. The molecular markers detected significant evolutionary changes in three out of the five species and we found significant phenotypic changes in two species. The only species that showed substantial genetic and phenotypic changes was the short-lived and predominantly selfing Medicago lupulina, while in the other, mostly perennial and outcrossing species, the observed changes were mostly minor. Agricultural propagation of native seed material for restoration can cause evolutionary changes, at least in some species. We recommend caution, particularly in selfing and short-lived species, where evolution may be more rapid and effects may thus be more severe.

Hypoxia and the group VII ethylene response transcription factor HRE2 promote adventitious root elongation in Arabidopsis

Abstract: Soil water-logging and flooding are common environmental stress conditions that can impair plant fitness. Roots are the first organs to be confronted with reduced oxygen tension as a result of flooding. While anatomical and morphological adaptations of roots are extensively studied, the root system architecture is only now becoming a focus of flooding research. Adventitious root (AR) formation shifts the root system higher up the plant, thereby facilitating supply with oxygen, and thus improving root and plant survival. We used Arabidopsis knockout mutants and overexpressors of ERFVII transcription factors to study their role in AR formation under hypoxic conditions and in response to ethylene. Results show that ethylene inhibits AR formation. Hypoxia mainly promotes AR elongation rather than formation mediated by ERFVII transcription factors, as indicated by reduced AR elongation in erfVII seedlings. Overexpression of HRE2 induces AR elongation to the same degree as hypoxia, while ethylene overrides HRE2-induced AR elongation. The ERFVII transcription factors promote establishment of an AR system that is under negative control by ethylene. Inhibition of growth of the main root system and promotion of AR elongation under hypoxia strengthens the root system in upper soil layers where oxygen shortage may last for shorter time periods.

Potassium-induced decrease in cytosolic Na+ alleviates deleterious effects of salt stress on wheat (Triticum aestivum L.).

Abstract: Accumulation of NaCl in soil causes osmotic stress in plants, and sodium (Na+ ) and chloride (Cl- ) cause ion toxicity, but also reduce the potassium (K+ ) uptake by plant roots and stimulate the K+ efflux through the cell membrane. Thus, decreased K+ /Na+ ratio in plant tissue lead us to hypothesise that elevated levels of K+ in nutrient medium enhance this ratio in plant tissue and cytosol to improve enzyme activation, osmoregulation and charge balance. In this study, wheat was cultivated at different concentrations of K+ (2.2, 4.4 or 8.8 mm) with or without salinity (1, 60 or 120 mm NaCl) and the effects on growth, root and shoot Na+ and K+ distribution and grain yield were determined. Also, the cytosolic Na+ concentration was investigated, as well as photosynthesis rate and water potential. Salinity reduced fresh weight of both shoots and roots and dry weight of roots. The grain yield was significantly reduced under Na+ stress and improved with elevated K+ fertilisation. Elevated K+ level during cultivation prevented the accumulation of Na+ into the cytosol of both shoot and root protoplasts. Wheat growth at vegetative stage was transiently reduced at the highest K+ concentration, perhaps due to plants' efforts to overcome a high solute concentration in the plant tissue, nevertheless grain yield was increased at both K+ levels. In conclusion, a moderately elevated K+ application to wheat seedlings reduces tissue as well as cytosolic Na+ concentration and enhances wheat growth and grain yield by mitigating the deleterious effects of Na+ toxicity.

Germination profiling of lentil genotypes subjected to salinity stress.

Abstract: Salinity is one of the most severe environmental stresses, negatively affecting productivity of salt-sensitive crop species. Given that germination is the most critical phase in the plant life cycle, the present study aimed to determine seed germination potential and associated traits under salt stress conditions as a simple approach to identify salt-tolerant lentil genotypes. The genetic material consisted of six lentil genotypes whose adaptation to various agroclimatic conditions is not well elucidated. Salinity stress was applied by addition of NaCl at three different levels of stress, while non-stressed plants were included as controls. Evaluation of tolerance was performed on the basis of germination percentage, seed water absorbance, root and shoot length, seedling water content, seedling vigour index and number of seedlings with an abnormal phenotype. Overall, our findings revealed that salinity stress substantially affects all traits associated with germination and early seedling growth, with the effect of salinity being dependent on the level of stress applied. It is noteworthy, however, that genotypes responded differently to the varying salinity levels. In this context, Samos proved the most salt-tolerant genotype, indicating its possible use for cultivation under stress conditions. In conclusion, the determination of seed germination and early growth potential may be exploited as an efficient strategy to reveal genetic variation in lentil germplasm of unknown tolerance to salinity stress. This approach allows selection of desirable genotypes at early growth stages, thus enabling more efficient application of various breeding methods to achieve stress-tolerant lentil genotypes.

Arabidopsis thaliana expressing PbBSMT, a gene encoding a SABATH-type methyltransferase from the plant pathogenic protist Plasmodiophora brassicae, show leaf chlorosis and altered host susceptibility.

Abstract: The plant pathogenic protist Plasmodiophora brassicae causes clubroot disease of Brassicaceae. This biotrophic organism can down-regulate plant defence responses. The previously characterised P. brassicae PbBSMT methyltransferase has substrate specificity for salicylic, benzoic and anthranilic acids. We therefore propose a role for the methylation of SA in attenuating plant defence response in infected roots as a novel strategy for intracellular parasitism. We overexpressed PbBSMT under the control of an inducible promoter in Arabidopsis thaliana and performed physiological, molecular and phytopathological analyses with the transgenic plants under control and induced conditions in comparison to the wild type. Upon induction, transcription of PbBSMT was associated with: (1) strong leaf phenotypes from anthocyanin accumulation and chlorosis followed by browning; (2) increased plant susceptibility after infection with P. brassicae that was manifested as more yellow leaves and reduced growth of upper plant parts; and (3) induced transgenic plants were not able to support large galls and had a brownish appearance of some clubs. Microarray data indicated that chlorophyll loss was accompanied by reduced transcription of genes involved in photosynthesis, while genes encoding glucose metabolism, mitochondrial functions and cell wall synthesis were up-regulated. Our results indicate a role for PbBSMT in attenuation of host defence responses in the roots by metabolising a plant defence signal.

Seed germination and dormancy traits of forbs and shrubs important for restoration of North American dryland ecosystems.

Abstract: In degraded dryland systems, native plant community re-establishment following disturbance is almost exclusively carried out using seeds, but these efforts commonly fail. Much of this failure can be attributed to the limited understanding of seed dormancy and germination traits. We undertook a systematic classification of seed dormancy of 26 species of annual and perennial forbs and shrubs that represent key, dominant genera used in restoration of the Great Basin ecosystem in the western United States. We examined germination across a wide thermal profile to depict species-specific characteristics and assessed the potential of gibberellic acid (GA3 ) and karrikinolide (KAR1 ) to expand the thermal germination envelope of fresh seeds. Of the tested species, 81% produce seeds that are dormant at maturity. The largest proportion (62%) exhibited physiological (PD), followed by physical (PY, 8%), combinational (PY + PD, 8%) and morphophysiological (MPD, 4%) dormancy classes. The effects of chemical stimulants were temperature- and species-mediated. In general, mean germination across the thermal profile was improved by GA3 and KAR1 for 11 and five species, respectively. We detected a strong germination response to temperature in freshly collected seeds of 20 species. Temperatures below 10 °C limited the germination of all except Agoseris heterophylla, suggesting that in their dormant state, the majority of these species are thermally restricted. Our findings demonstrate the utility of dormancy classification as a foundation for understanding the critical regenerative traits in these ecologically important species and highlight its importance in restoration planning.

Breeding new varieties for controlled environments

Abstract: Agricultural production in controlled environments is increasingly feasible, and may play an important role in providing nutrition and choice to growing urban centres New technologies in lighting, ventilation, robotics and irrigation are just a few of the innovations that enable production of high-value specialty crops outside of a traditional field setting However, despite all of the advances in the hardware within the plant factory operation, innovation of the most complex machine has been neglected - the plant itself Indoor agricultural operations typically rely on legacy varieties, plants selected and bred for field conditions In the field, phenotypic stability is paramount, as production must be consistent in an unpredictable and changing environment However, the controlled environment affords focus on different breeding priorities as environmental flux, pests, pathogens and post-harvest quality are less formidable barriers to production On the contrary, breeding for controlled environments shifts the focus to a completely different set of plant traits, such as rapid growth, performance in low light environments and active manipulation of plant stature Instead of breeding for phenotypic stability, plants may be bred to maximise genetic plasticity, allowing specific traits to be presented as a function of the quality of the ambient light spectrum In this scenario plant varieties may be grown with optimal size, supporting a focus on consumer traits like flavour or accumulation of health-related compounds Gene editing may be a central technology in the production of designer plants for controlled environments This review considers the opportunity for breeding for controlled environments, with a focus on a revision of priorities for controlled-environment breeders

The Arabidopsis THADA homologue modulates TOR activity and cold acclimation.

Abstract: Low temperature is one of the most important environmental factors that affect global survival of humans and animals and equally importantly the distribution of plants and crop productivity. Survival of metazoan cells under cold stress requires regulation of the sensor-kinase Target Of Rapamycin (TOR). TOR controls growth of eukaryotic cells by adjusting anabolic and catabolic metabolism. Previous studies identified the Thyroid Adenoma Associated (THADA) gene as the major effect locus by positive selection in the evolution of modern human adapted to cold. Here we investigate the role of THADA in TOR signaling and cold acclimation of plants. We applied BLAST searches and homology modeling to identify the AtTHADA (AT3G55160) in Arabidopsis thaliana as the highly probable orthologue protein. Reverse genetics approaches were combined with immunological detection of TOR activity and metabolite profiling to address the role of the TOR and THADA for growth regulation and cold acclimation. Depletion of the AtTHADA gene caused complete or partial loss of full-length mRNA, respectively, and significant retardation of growth under non-stressed conditions. Furthermore, depletion of AtTHADA caused hypersensitivity towards low-temperatures. Atthada displayed a lowered energy charge. This went along with decreased TOR activity, which offers a molecular explanation for the slow growth phenotype of Atthada. Finally, we used TOR RNAi lines to identify the de-regulation of TOR activity as one determinant for sensitivity towards low-temperatures. Taken together our results provide evidence for a conserved function of THADA in cold acclimation of eukaryotes and suggest that cold acclimation in plants requires regulation of TOR.

Optimising seed processing techniques to improve germination and sowability of native grasses for ecological restoration

Abstract: Grasslands across the globe are undergoing expansive degradation due to human impacts and climate change. If restoration of degraded native grassland is to be achieved at the scale now required, cost-effective means for seed-based establishment of grass species is crucial. However, grass seeds present numerous challenges associated with handling and germination performance that must be overcome to improve the efficiency of seeding. Previous research has demonstrated that complete removal of the palea and lemma (husk) maximises germination performance, hence we investigated the effects of complete husk removal on seed handling and germination of four temperate Australian grass species. Three techniques were tested to remove the husk - manual cleaning, flaming or acid digestion (the latter two followed by a manual cleaning step); these techniques were refined and adapted to the selected species, and germination responses were compared. The complete removal of the husk improved seed handling and sowability for all species. Germination was improved in Microlaena stipoides by 19% and in Rytidosperma geniculatum by 11%. Of the husk removal methods tested, flaming was detrimental to seed germination and fatal for one species (R. geniculatum). Compared to manual cleaning, sulphuric acid improved the overall efficacy of the cleaning procedure and increased germination speed (T50) in Austrostipa scabra, Chloris truncata and M. stipoides, and improved final germination in R. geniculatum by 13%. The seed processing methods developed and tested in the present study can be applied to grass species that present similar handling and germination performance impediments. These and other technological developments (seed coating and precision sowing) will facilitate more efficient grassland restoration at large scale.

Nectar yeasts enhance the interaction between Clematis akebioides and its bumblebee pollinator.

Abstract: It has been hypothesised that intense metabolism of nectar-inhabiting yeasts (NIY) may change nectar chemistry, including volatile profile, which may affect pollinator foraging behaviours and consequently plant fitness. However, empirical evidence for the plant-microbe-pollinator interactions remains little known. To test this hypothesis, we use a bumblebee-pollinated vine Clematis akebioides endemic to southwest China as an experimental model plant. To quantify the incidence and density of Metschnikowia reukaufii, a cosmopolitan NIY in floral nectar, a combination of yeast cultivation and microscopic cell-counting method was used. To examine the effects of NIY on plant-pollinator interactions, we used real flowers filled with artificial nectar with or without yeast cells. Then the volatile metabolites produced in the yeast-inoculated nectar were analysed with coupled gas chromatography and mass spectrometry (GC-MS). On average 79.3% of the C. akebioides flowers harboured M. reukaufii, and cell density of NIY was high to 7.4 × 104 cells mm-3 . In the field population, the presence of NIY in flowers of C. akebioides increased bumblebee (Bombus friseanus) pollinator visitation rate and consequently seed set per flower. A variety of fatty acid derivatives produced by M. reukaufii may be responsible for the above beneficial interactions. The volatiles produced by the metabolism of M. reukaufii may serve as an honest signal to attract bumblebee pollinators and indirectly promote the female reproductive fitness of C. akebioides, forming a potentially tripartite plant-microbe-pollinator mutualism.

The role of hybridization and introgression in maintaining species integrity and cohesion in naturally isolated inselberg bromeliad populations.

Abstract: Hybridization is a widespread phenomenon present in numerous lineages across the tree of life. Its evolutionary consequences range from effects on the origin and maintenance, to the loss of biodiversity. We studied genetic diversity and intra- and interspecific gene flow between two sympatric populations of closely-related species, Pitcairnia flammea and P. corcovadensis (Bromeliaceae), which are adapted to naturally fragmented Neotropical inselbergs, based on nuclear and plastidial DNA. Our main results indicate a strong reproductive isolation barrier, although low levels of interspecific gene flow were observed in both sympatric populations. The low rates of intraspecific gene flow observed for both P. corcovadensis and P. flammea populations corroborate the increasing body of evidence that inselberg bromeliad species are maintained as discrete evolutionary units despite the presence of low genetic connectivity. Nuclear patterns of genetic diversity and gene flow revealed that hybridization and introgression might not cause species extinction via genetic assimilation of the rare P. corcovadensis. In the face of reduced intraspecific gene exchange, hybridization and introgression may be important aspects of the Pitcairnia diversification process, with a positive evolutionary impact at the bromeliad community level, and thus contribute to increasing and maintaining genetic diversity in local isolated inselberg populations.

Germination niche breadth varies inconsistently among three Asclepias congeners along a latitudinal gradient

Abstract: Species responses to climate change will be primarily driven by their environmental tolerance range, or niche breadth, with the expectation that broad niches will increase resilience. Niche breadth is expected to be larger in more heterogeneous environments and moderated by life history. Niche breadth also varies across life stages. Therefore, the life stage with the narrowest niche may serve as the best predictor of climatic vulnerability. To investigate the relationship between niche breadth, climate and life stage we identify germination niche breadth for dormant and non-dormant seeds in multiple populations of three milkweed (Asclepias) species. Complementary trials evaluated germination under conditions simulating historic and predicted future climate by varying cold-moist stratification temperature, length and incubation temperature. Germination niche breadth was derived from germination evenness across treatments (Levins Bn ), with stratified seeds considered less dormant than non-stratified seeds. Germination response varies significantly among species, populations and treatments. Cold-moist stratification ≥4 weeks (1-3 °C) followed by incubation at 25/15 °C+ achieves peak germination for most populations. Germination niche breadth significantly expands following stratification and interacts significantly with latitude of origin. Interestingly, two species display a positive relationship between niche breadth and latitude, while the third presents a concave quadratic relationship. Germination niche breadth significantly varies by species, latitude and population, suggesting an interaction between source climate, life history and site-specific factors. Results contribute to our understanding of inter- and intraspecific variation in germination, underscore the role of dormancy in germination niche breadth, and have implications for prioritising and conserving species under climate change.

Suitability of Solanum lycopersicum L. 'Microtom' for growth in Bioregenerative Life Support Systems: exploring the effect of high-LET ionising radiation on photosynthesis, leaf structure and fruit traits.

Abstract: The realisation of manned space exploration requires the development of Bioregenerative Life Support Systems (BLSS). In such self-sufficient closed habitats, higher plants have a fundamental role in air regeneration, water recovery, food production and waste recycling. In the space environment, ionising radiation represents one of the main constraints to plant growth. In this study, we explore whether low doses of heavy ions, namely Ca 25 Gy, delivered at the seed stage, may induce positive outcomes on growth and functional traits in plants of Solanum lycopersicum L. 'Microtom'. After irradiation of seed, plant growth was monitored during the whole plant life cycle, from germination to fruit ripening. Morphological parameters, photosynthetic efficiency, leaf anatomical functional traits and antioxidant production in leaves and fruits were analysed. Our data demonstrate that irradiation of seeds with 25 Gy Ca ions does not prevent achievement of the seed-to-seed cycle in 'Microtom', and induces a more compact plant size compared to the control. Plants germinated from irradiated seeds show better photochemical efficiency than controls, likely due to the higher amount of D1 protein and photosynthetic pigment content. Leaves of these plants also had smaller cells with a lower number of chloroplasts. The dose of 25 Gy Ca ions is also responsible for positive outcomes in fruits: although developing a lower number of berries, plants germinated from irradiated seeds produce larger berries, richer in carotenoids, ascorbic acid and anthocyanins than controls. These specific traits may be useful for 'Microtom' cultivation in BLSS in space, in so far as the crew members could benefit from fresh food richer in functional compounds that can be directly produced on board.

GhSTOP1, a C2H2 type zinc finger transcription factor is essential for aluminum and proton stress tolerance and lateral root initiation in cotton

Abstract: Aluminum (Al) and proton (H+ ) ions are major acid soil stress factors deleteriously affecting plant root growth and crop yield. In our preliminary studies, cotton (Gossypium hirsutum L.) seedlings displayed very sensitive phenotypes to Al and H+ rhizotoxicities. Based on previous Arabidopsis results, we aimed to characterise the Al stress responsive Sensitive to Proton rhizotoxicity 1 (GhSTOP1) transcription system in cotton using RNAi-mediated down-regulation. With the help of seed embryo apex explants, we developed transgenic cotton plants overexpressing a GhSTOP1-RNAi cassette with NPTII selection. Kanamycin-tolerant T1 seedlings were further considered for Al and H+ stress tolerance studies. Down-regulation of the GhSTOP1 increased sensitivity to Al and proton rhizotoxicities, and root growth was significantly reduced in RNAi lines. The expression profile of GhALMT1 (Aluminum-activated Malate Transporter 1), GhMATE (Multidrug and Toxic Compound Extrusion), GhALS3 (Aluminium Sensitive 3) and key genes involved in the GABA shunt were down-regulated in the transgenic RNAi lines. Additionally, the lateral root initiation process was delayed and expression of GhNAC1, which is involved in lateral roots, was also suppressed in transgenic lines. Besides, overexpression of GhSTOP1 in Arabidopsis accelerated root growth and AtMATE and AtALMT1 expression under Al stress conditions. These analyses indicate that GhSTOP1 is essential for the expression of several genes which are necessary for acid soil tolerance mechanisms and lateral root initiation.

Colour evolution within orchids depends on whether the pollinator is a bee or a fly.

Abstract: Orchids are a classic angiosperm model for understanding biotic pollination. We studied orchid species within two species-rich herbaceous communities that are known to have either hymenopteran or dipteran insects as the dominant pollinators, in order to understand how flower colour relates to pollinator visual systems. We analysed features of the floral reflectance spectra that are significant to pollinator visual systems and used models of dipteran and hymenopteran colour vision to characterise the chromatic signals used by fly-pollinated and bee-pollinated orchid species. In contrast to bee-pollinated flowers, fly-pollinated flowers had distinctive points of rapid reflectance change at long wavelengths and a complete absence of such spectral features at short wavelengths. Fly-pollinated flowers also had significantly more restricted loci than bee-pollinated flowers in colour space models of fly and bee vision alike. Globally, bee-pollinated flowers are known to have distinctive, consistent colour signals. Our findings of different signals for fly pollination is consistent with pollinator-mediated selection on orchid species that results from the distinctive features of fly visual systems.

Overexpression of paralogues of the wheat expansin gene TaEXPA8 improves low-temperature tolerance in Arabidopsis

Abstract: Low temperature is one of the important factors limiting wheat yield in cold regions. Expansins are nonenzymatic proteins that loosen cell walls and play important roles in diverse biological processes related to cell wall modification, including development and stress tolerance. Many studies have shown that expansins are involved in resistance to various abiotic stresses, such as heat and drought. However, the role of expansins in response to low-temperature stress remains unclear. Based on our previous transcriptome data of a winter wheat cultivar Dongnongdongmai 2 (DN2), we found that one of the expansin genes, TaEXPA8, was significantly induced by low temperature, indicating a role for TaEXPA8 in cold resistance. In this study, the paralogous TaEXPA8 genes TaEXPA8-A, TaEXPA8-B and TaEXPA8-D were cloned by RT-PCR. These three genes were then transformed into Arabidopsis by the floral dip method. Expression patterns of TaEXPA8 genes in different tissues and in response to several abiotic stresses and hormones were detected by quantitative real-time PCR (qRT-PCR). The results showed that TaEXPA8-A and TaEXPA8-B were expressed mainly in roots, while TaEXPA8-D was expressed predominantly in flowers. TaEXPA8 genes were induced by low-temperature and drought. The overexpression of TaEXPA8-B and TaEXPA8-D enhanced low-temperature resistance and had increased superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activity and soluble protein, MDA and proline content. In summary, our study suggested that the expansins TaEXPA8-B and TaEXPA8-D are involved in the response to low temperature and possibly play a role in cold resistance by activating the protective enzyme system.

How do leaf wetting events affect gas exchange and leaf lifespan of plants from seasonally dry tropical vegetation

Abstract: Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga. We performed scanning electron, bright-field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan. All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species. We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

Stress-driven increase in proline levels, and not proline levels themselves, correlates with the ability to withstand excess salt in a group of 17 Italian rice genotypes

Abstract: In most plant species, a rapid increase in free proline content occurs following exposure to hyperosmotic stress conditions. However, inconsistent results were reported concerning the role of such an increase on the plant response to water shortage or excess salt. Therefore, the possibility that proline accumulation may help the cell to withstand stress conditions, or that it simply represents a stress marker, is still a matter of debate. A possible relationship between proline accumulation and salt tolerance was investigated in a set of 17 Italian rice varieties. Rice seedlings were exposed to increasing salt concentrations during germination and early growth. The resulting levels of free proline were measured separately in shoots and roots and compared to those in untreated controls. Results were related to the corresponding ability of a given genotype to tolerate stress conditions. Neither absolute proline levels in untreated or in salt-stressed seedlings showed a straightforward relationship to the relative tolerance to salt, estimated as conductivity values able to reduce growth by 10 or 50%. Conversely, a highly significant correlation was found between the increase in proline levels in shoots and the ability to withstand stress. The results strengthen a recent hypothesis suggesting than an increase in proline metabolic rates, more than the resulting proline content, may help the cell to counteract the effects of abiotic stress conditions.

Genome-wide identification and expression analysis reveal the potential function of ethylene responsive factor gene family in response to Botrytis cinerea infection and ovule development in grapes (Vitis vinifera L.)

Abstract: The prevention of Botrytis cinerea infection and the study of grape seedlessness are very important for grape industries. Finding correlated regulatory genes is an important approach towards understanding their molecular mechanisms. Ethylene responsive factor (ERF) gene family play critical roles in defence networks and the growth of plants. To date, no large-scale study of the ERF proteins associated with pathogen defence and ovule development has been performed in grape (Vitis vinifera L.). In the present study, we identified 113 ERF genes (VvERF) and named them based on their chromosome locations. The ERF genes could be divided into 11 groups based on a multiple sequence alignment and a phylogenetic comparison with homologues from Arabidopsis thaliana. Synteny analysis and Ka/Ks ratio calculation suggested that segmental and tandem duplications contributed to the expansion of the ERF gene family. The evolutionary relationships between the VvERF genes were investigated by exon-intron structure characterisation, and an analysis of the cis-acting regulatory elements in their promoters suggested potential regulation after stress or hormone treatments. Expression profiling after infection with the fungus, B. cinerea, indicated that ERF genes function in responses to pathogen attack. In addition, the expression levels of most ERF genes were much higher during ovule development in seedless grapes, suggesting a role in ovule abortion related to seedlessness. Taken together, these results indicate that VvERF proteins are involved in responses to Botrytis cinerea infection and in grape ovule development. This information may help guide strategies to improve grape production.

Ten years of native seed certification in Germany - a summary.

Abstract: Many renaturation projects and compensation areas are based on the use of seeds from regional indigenous wild plants; in the following: native or regional seeds. Despite this, such seeds make up only a small proportion of the total number of seeds used for greening projects; in Germany, for example, it is only around 1% (=200 t per year). Although the market for regional seeds is small, it is highly competitive. High-priced native seeds compete with flower mixes of unspecified origin and can only be differentiated from them by reliable quality seals. A quality assurance system based on seed legislation (EU Directive 2010/60, preservation mixtures) has been developed in a few European countries. However, quality assurance ends with the sale of the seeds. Thus, seed use remains unmonitored, and often unsuitable material, or material foreign to the region, is planted in restoration areas. Unfortunately, nature conservation has not made seed-based restoration one of its key issues, neither at the European nor at the national level. Currently there are many different local and regional standards, methods and private certificates that are confusing for users and which provide little continuity and predictability for producers. We recommend the establishment of an EU directive or a broadly agreed recommendation to the EU member states, spearheaded by nature conservation, which would define the standards for producing and using native seeds (e.g. harmonised regions that cross national borders, quality regulations). At the same time, wild plant interest groups should combine existing structures in order to strengthen seed-based restoration through international cooperation.

Maximising the use of native seeds in restoration projects

Abstract: In many nature conservation or mitigation projects the reestablishment of native plant species is hindered by the lack of availability of a suitable soil seed bank or nearby source area for re-colonisation. The increasing number of restoration projects around the world has created a demand for seed material of native species. The Bonn Challenge to bring 350 m ha of the world’s deforested and degraded land into restoration by 2030 (http://www.bonnchallenge.org/content/challenge), could require more than one million tonnes of seed at a sowing density 4 kg ha. Wild harvest of this quantity of material is unsustainable. Consequently, the need to multiply native seed through cultivation from wild-collected seed on specialised farms is increasingly important. However, the adaptation of native seed for these purposes is hindered by uneven germination of seed lots, a dependence of seed viability on the conditions of parent plants, which can vary from year to year, and the uncertainty about the factors that determine the success rate of establishment in a particular restoration site. A second consideration is that the seeds that are being cultivated from wild material are unintentionally submitted to selection and lose their adaptations to specific natural habitats or may become genetically less diverse. A deep understanding of the biology and ecology of seeds, the potential pre-treatments to enhance germination and the factors that are key in establishing and persisting in a community is essential for the successful use of native seed. To increase the collaboration between scientists in plant ecology, genetics, molecular biology, taxonomy, ecology, conservation, seed biology, environmental science, agricultural botany, crop science, breeding and horticulture, an international training network was formed with the specific purpose of improving understanding of seed biology, solving industry-related challenges and increasing public awareness of the role of seeds in restoration projects. This training network, with the acronym NASSTEC (NAtive Seed Science, TEchnology and Conservation), organised an international symposium at the Royal Botanic Gardens, Kew, in September 2017, at which most of the papers published in this special issue were presented. The papers in this Special Issue are broadly clustered around the following topics: technical advances, seed quality for restoration, methods to improve seed germination in dormant seed lots, demands imposed by changing environmental conditions, and the role of national policies in the use of native seed.