Worldwide evidence of a unimodal relationship between productivity and plant species richness

The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

Intransitive competition is widespread in plant communities and maintains their species richness

Intransitive competition networks, those in which there is no single best competitor, may ensure species coexistence. However, their frequency and importance in maintaining diversity in real-world ecosystems remain unclear. We used two large data sets from drylands and agricultural grasslands to assess: (1) the generality of intransitive competition, (2) intransitivity–richness relationships and (3) effects of two major drivers of biodiversity loss (aridity and land-use intensification) on intransitivity and species richness. Intransitive competition occurred in > 65% of sites and was associated with higher species richness. Intransitivity increased with aridity, partly buffering its negative effects on diversity, but was decreased by intensive land use, enhancing its negative effects on diversity. These contrasting responses likely arise because intransitivity is promoted by temporal heterogeneity, which is enhanced by aridity but may decline with land-use intensity. We show that intransitivity is widespread in nature and increases diversity, but it can be lost with environmental homogenisation.

Metabolomics in plant-herbivore interactions: challenges and applications

Metabolomics as the study of the entire set of metabolites of a given organism is an important frontier in life sciences. As a tool that captures the ‘front end’ of cellular machineries, metabolomics is particularly suited to investigate biotic interactions, including for instance the interplay between plants and insects. In this review, we discuss the opportunities and challenges of metabolomics to study plant–herbivore interactions. We first present a brief overview of the typical analytical workflows used in metabolomics and their associated issues, in particular those related to metabolome coverage and compound identification. Second, recent advances in the field of plant–herbivore relationships that are promoted by non-targeted approaches are reviewed, with examples ranging from classical herbivore resistance patterns to plant-mediated interactions across different spatial scales and volatile-mediated tritrophic interactions. Through general considerations and the discussion of a few selected case studies, our review highlights the potential and challenges of metabolomics as a research approach to understand biological interfaces.

Synergies and trade-offs between insect and pathogen resistance in maize leaves and roots

Determining links between plant defence strategies is important to understand plant evolution and to optimize crop breeding strategies. Although several examples of synergies and trade-offs between defence traits are known for plants that are under attack by multiple organisms, few studies have attempted to measure correlations of defensive strategies using specific single attackers. Such links are hard to detect in natural populations because they are inherently confounded by the evolutionary history of different ecotypes. We therefore used a range of 20 maize inbred lines with considerable differences in resistance traits to determine if correlations exist between leaf and root resistance against pathogens and insects. Aboveground resistance against insects was positively correlated with the plant's capacity to produce volatiles in response to insect attack. Resistance to herbivores and resistance to a pathogen, on the other hand, were negatively correlated. Our results also give first insights into the intraspecific variability of root volatiles release in maize and its positive correlation with leaf volatile production. We show that the breeding history of the different genotypes (dent versus flint) has influenced several defensive parameters. Taken together, our study demonstrates the importance of genetically determined synergies and trade-offs for plant resistance against insects and pathogens.

The Effects of Density, Spatial Pattern, and Competitive Symmetry on Size Variation in Simulated Plant Populations

Patterns of size inequality in crowded plant populations are often taken to be indicative of the degree of size asymmetry of competition, but recent research suggests that some of the patterns attributed to size‐asymmetric competition could be due to spatial structure. To investigate the theoretical relationships between plant density, spatial pattern, and competitive size asymmetry in determining size variation in crowded plant populations, we developed a spatially explicit, individual‐based plant competition model based on overlapping zones of influence. The zone of influence of each plant is modeled as a circle, growing in two dimensions, and is allometrically related to plant biomass. The area of the circle represents resources potentially available to the plant, and plants compete for resources in areas in which they overlap. The size asymmetry of competition is reflected in the rules for dividing up the overlapping areas. Theoretical plant populations were grown in random and in perfectly uniform spatial patterns at four densities under size‐asymmetric and size‐symmetric competition. Both spatial pattern and size asymmetry contributed to size variation, but their relative importance varied greatly over density and over time. Early in stand development, spatial pattern was more important than the symmetry of competition in determining the degree of size variation within the population, but after plants grew and competition intensified, the size asymmetry of competition became a much more important source of size variation. Size variability was slightly higher at higher densities when competition was symmetric and plants were distributed nonuniformly in space. In a uniform spatial pattern, size variation increased with density only when competition was size asymmetric. Our results suggest that when competition is size asymmetric and intense, it will be more important in generating size variation than is local variation in density. Our results and the available data are consistent with the hypothesis that high levels of size inequality commonly observed within crowded plant populations are largely due to size‐asymmetric competition, not to variation in local density.

Plant diversity effects on grassland productivity are robust to both nutrient enrichment and drought

Global change drivers are rapidly altering resource availability and biodiversity. While there is consensus that greater biodiversity increases the functioning of ecosystems, the extent to which biodiversity buffers ecosystem productivity in response to changes in resource availability remains unclear. We use data from 16 grassland experiments across North America and Europe that manipulated plant species richness and one of two essential resources—soil nutrients or water—to assess the direction and strength of the interaction between plant diversity and resource alteration on above-ground productivity and net biodiversity, complementarity, and selection effects. Despite strong increases in productivity with nutrient addition and decreases in productivity with drought, we found that resource alterations did not alter biodiversity–ecosystem functioning relationships. Our results suggest that these relationships are largely determined by increases in complementarity effects along plant species richness gradients. Although nutrient addition reduced complementarity effects at high diversity, this appears to be due to high biomass in monocultures under nutrient enrichment. Our results indicate that diversity and the complementarity of species are important regulators of grassland ecosystem productivity, regardless of changes in other drivers of ecosystem function.

Plant communities and soil chemistry of shrub-steppe and pine savannah grasslands in southern British Columbia, Canada

Habitat fragmentation alters the edges of remnant habitat patches. We examined changes in the plant community and soil in relation to distance from edge and edge type for shrub-steppe and pine savannah grasslands in southern British Columbia, Canada. Community composition showed significant nonlinear relationships with distance-to-edge more frequently at paved roads and fruit crops than at dirt roads or control sites (i.e., in the interior of grassland patches), with changes typically extending 25-30 m. More exotic species and fewer native species were found near edges, and edges showed decreased cryptogam cover and increased bare ground, especially near paved roads. The soil factors that best predicted compositional changes were soil pH and Cu/Mn at paved roads, soil pH and nitrogen at fruit crops, and soil resistance at dirt roads. Variation partitioning suggested that both direct (e.g., propagule pressure) and indirect (environmental change) factors mediated edge-related community changes, and provided evidence that nonlinear responses at developed edges were not due to natural gradients. Given the range of grassland patch sizes in this region (many patches 1-100 ha), the edge effects we observed represent a considerable loss of "core" habitat, which must be accounted for in conservation planning and site restoration.

Refining predictions of iberian plant distribution: lessons from p. nigra and p. sylvestris palaeoecological-based habitat suitability models

In recent years, challenged by the climate scenarios put forward by the IPCC and its potential impact on plant distribution, numerous predictive techniques -including the so called habitat suitability models (HSM)- have been developed. Yet, as the output of the different methods produces different distribution areas, developing validation tools are strong needs to reduce uncertainties. Focused in the Iberian Peninsula, we propose a palaeo-based method to increase the robustness of the HSM, by developing an ecological approach to understand the mismatches between the palaeoecological information and the projections of the HSMs. Here, we present the result of (1) investigating causal relationships between environmental variables and presence of Pinus sylvestris L. and P. nigra Arn. available from the 3rd Spanish Forest Inventory, (2) developing present and past presence-predictions through the MaxEnt model for 6 and 21 kyr BP, and (3) assessing these models through comparisons with biomized palaeoecological data available from the European Pollen Database for the Iberian Peninsula.

On the spatial distribution of woody plant species in a tropical montane cloud forest

Abstract The cloud forest is a special type of forest ecosystem that depends on suitable conditions of humidity and temperature to exist; hence, it is a very fragile ecosystem. The cloud forest is also one of the richest ecosystems in terms of species diversity and rate of endemism. However, today, it is one of the most threatened ecosystems in the world. Little is known about tree species distribution and coexistence among cloud forest trees. Trees are essential to understanding ecosystem functioning and maintenance because they support the ecosystem in important ways. For this dissertation, an analysis of woody plant species distribution at a small scale in a north-Peruvian Andean cloud forest was performed, and some of the factors implicated in the observed patterns were identified. Towards that end, different natural factors acting on species distribution within the forest were investigated: (i) intra-specific arrangements, (ii) heterospecific spatial relationships and (iii) relationships with external environmental factors. These analyses were conducted first on standing woody plants and then on seedlings. The woody plants were found to be clumped in the forest, either considering all the species together or each species separately. However, each species presented a specific pattern and specific spatial relationship among different-age individuals. Dispersal mode, growth form and shade tolerance played roles in the final distribution of the species. Furthermore, spatial associations among species, either positive or negative, were observed. These associations were more numerous when considering individuals of the interacting species at different developmental stages, i.e., younger individuals from one species and older individuals from another. Accordingly, competition and facilitation are asymmetric processes and vary throughout the life of an individual. Moreover, some species appear to prefer certain habitat conditions and avoid other habitats. The habitat definition that best explains species distribution is that which includes both environmental and stand characteristics; thus, a combination of these factors is necessary to understanding species' niche preferences. Seedling distribution was also associated with habitat conditions, but these conditions explained less than the 30% of the spatial variation. The position of conspecific adult individuals also affected seedling distribution; although the seedlings of many tree species avoid the vicinity of conspecifics, a few species appeared to prefer the formation of cohorts around their parent trees. The importance of habitat conditions and distance dependence with conspecifics varied among regions within the forest as well as on the developmental stage of the stand. The results from this thesis suggest that different species can coexist within a given space, forming a “puzzle” of species as a result of the intra- and interspecific spatial relationships along with niche preferences and adaptations that operate at different scales. These factors not only affect each species in a different way, but specific preferences also vary throughout species' lifespans. Resumen Resumen El bosque de niebla es uno de los ecosistemas más amenazados del mundo además de ser uno de los más frágiles. Son formaciones azonales que dependen de la existencia de unas condiciones de humedad y temperatura que permitan la formación de nubes que cubran el bosque; lo que dificulta en gran medida su conservación. También es uno de los ecosistemas con mayor riqueza de especies además de tener uno de los mayores porcentajes de endemismos. Uno de los aspectos más importantes para entender el ecosistema, es identificar y entender los elementos que lo componen y los mecanismos que regulan las relaciones entre ellos. Los árboles son el soporte del ecosistema. Sin embargo, apenas hay información sobre la distribución y coexistencia de los árboles en los bosques de niebla. Esta tesis presenta un análisis de la distribución a pequeña escala de las plantas leñosas en un bosque de niebla situado en la cordillera andina del norte de Perú; así como el análisis de algunos de los factores que pueden estar implicados en que se origine la distribución observada. Para este propósito se estudia cómo influyen factores de diferente naturaleza en la distribución de las especies (i) organización intra-específica (ii) relaciones espaciales heterospecíficas y (iii) relación con factores ambientales externos. En estos análisis se estudiaron primero las plantas jóvenes y las adultas, y después las plántulas. Los árboles aparecieron agregados en el bosque, tanto considerando todos a la vez como cuando se estudió cada especie por separado. Sin embargo, cada especie mostró un patrón distinto así como una particular relación espacial entre individuos jóvenes y adultos. El modo de dispersión, la forma de vida y la tolerancia de la especies estuvieron relacionados con el patrón general observado. Se vio también que ciertas especies aparecían relacionadas con otras, tanto de forma positiva (compartiendo zonas) como negativa (apareciendo en áreas distintas). Las asociaciones fueron mucho más numerosas cuando se consideraron los pares de especies en diferente estado de desarrollo, es decir, individuos jóvenes de una especie e individuos mayores de la otra. Eso indicaría que los procesos de competencia y facilitación son asimétricos y además varían durante la vida de la planta. Por otro lado, algunas especies aparecen preferentemente bajo ciertas condiciones de hábitat y evitan otras. La definición de hábitat a la que mejor responden las especies es cuando se incluyen tanto variables ambientales como de masa; así que ambos tipos de variables son necesarias para entender la preferencia de las especies por ciertos nichos. La distribución de las plántulas también estuvo relacionada con condiciones de hábitat, pero eso sólo llegaba a explicar hasta un 30% de la variabilidad espacial. La posición de los adultos de la misma especie también afectó a la distribución de las plántulas. En bastantes especies las plántulas evitan la cercanía de adultos de su misma especie, padres potenciales, aunque algunas especies aisladas mostraron el patrón contrario y aparecieron preferentemente en las mismas áreas que sus padres. La importancia de las condiciones de hábitat y posición de los adultos en la disposición de las plántulas varía de una zona a otra del bosque y además también varía según el estado de desarrollo de la masa.

A bacterial cysteine protease effector protein interferes with photosynthesis to suppress plant innate immune responses

The bacterial pathogen Pseudomonas syringae pv tomato DC3000 suppresses plant innate immunity with effector proteins injected by a type III secretion system (T3SS). The cysteine protease effector HopN1, which reduces the ability of DC3000 to elicit programmed cell death in non-host tobacco, was found to also suppress the production of defence-associated reactive oxygen species (ROS) and callose when delivered by Pseudomonas fluorescens heterologously expressing a P. syringae T3SS. Purified His 6 -tagged HopN1 was used to identify tomato PsbQ, a member of the oxygen evolving complex of photosystem II (PSII), as an interacting protein. HopN1 localized to chloroplasts and both degraded PsbQ and inhibited PSII activity in chloroplast preparations, whereas a HopN1 D299A non-catalytic mutant lost these abilities. Gene silencing of NtPsbQ in tobacco compromised ROS production and programmed cell death.

Sensitization profiles and cross-reactivity among plant allergens. Molecular mechanisms of food allergy development and the role of enhancers

La prevalencia de las alergias está aumentando desde mediados del siglo XX, y se estima que actualmente afectan a alrededor del 2-8 % de la población, pero las causas de este aumento aún no están claras. Encontrar el origen del mecanismo por el cual una proteína inofensiva se convierte en capaz de inducir una respuesta alérgica es de vital importancia para prevenir y tratar estas enfermedades. Aunque la caracterización de alérgenos relevantes ha ayudado a mejorar el manejo clínico y a aclarar los mecanismos básicos de las reacciones alérgicas, todavía queda un largo camino para establecer el origen de la alergenicidad y reactividad cruzada. El objetivo de esta tesis ha sido caracterizar las bases moleculares de la alergenicidad tomando como modelo dos familias de panalergenos (proteínas de transferencia de lípidos –LTPs- y taumatinas –TLPs-) y estudiando los mecanismos que median la sensibilización y la reactividad cruzada para mejorar tanto el diagnóstico como el tratamiento de la alergia. Para ello, se llevaron a cabo dos estrategias: estudiar la reactividad cruzada de miembros de familias de panalérgenos; y estudiar moléculas-co-adyuvantes que pudieran favorecer la capacidad alergénica de dichas proteínas. Para estudiar la reactividad cruzada entre miembros de la misma familia de proteínas, se seleccionaron LTPs y TLPs, descritas como alergenos, tomando como modelo la alergia a frutas. Por otra parte, se estudiaron los perfiles de sensibilización a alérgenos de trigo relacionados con el asma del panadero, la enfermedad ocupacional más relevante de origen alérgico. Estos estudios se llevaron a cabo estandarizando ensayos tipo microarrays con alérgenos y analizando los resultados por la teoría de grafos. En relación al estudiar moléculas-co-adyuvantes que pudieran favorecer la capacidad alergénica de dichas proteínas, se llevaron a cabo estudios sobre la interacción de los alérgenos alimentarios con células del sistema inmune humano y murino y el epitelio de las mucosas, analizando la importancia de moléculas co-transportadas con los alérgenos en el desarrollo de una respuesta Th2. Para ello, Pru p 3(LTP y alérgeno principal del melocotón) se selección como modelo para llevarlo a cabo. Por otra parte, se analizó el papel de moléculas activadoras del sistema inmune producidas por patógenos en la inducción de alergias alimentarias seleccionando el modelo kiwi-alternaria, y el papel de Alt a 1, alérgeno mayor de dicho hongo, en la sensibilización a Act d 2, alérgeno mayor de kiwi. En resumen, el presente trabajo presenta una investigación innovadora aportando resultados de gran utilidad tanto para la mejora del diagnóstico como para nuevas investigaciones sobre la alergia y el esclarecimiento final de los mecanismos que caracterizan esta enfermedad. ABSTRACT Allergies are increasing their prevalence from mid twentieth century, and they are currently estimated to affect around 2-8% of the population but the underlying causes of this increase remain still elusive. The understanding of the mechanism by which a harmless protein becomes capable of inducing an allergic response provides us the basis to prevent and treat these diseases. Although the characterization of relevant allergens has led to improved clinical management and has helped to clarify the basic mechanisms of allergic reactions, it seems justified in aspiring to molecularly dissecting these allergens to establish the structural basis of their allergenicity and cross-reactivity. The aim of this thesis was to characterize the molecular basis of the allergenicity of model proteins belonging to different families (Lipid Transfer Proteins –LTPs-, and Thaumatin-like Proteins –TLPs-) in order to identify mechanisms that mediate sensitization and cross reactivity for developing new strategies in the management of allergy, both diagnosis and treatment, in the near future. With this purpose, two strategies have been conducted: studies of cross-reactivity among panallergen families and molecular studies of the contribution of cofactors in the induction of the allergic response by these panallergens. Following the first strategy, we studied the cross-reactivity among members of two plant panallergens (LTPs , Lipid Transfer Proteins , and TLPs , Thaumatin-like Proteins) using the peach allergy as a model. Similarly, we characterized the sensitization profiles to wheat allergens in baker's asthma development, the most relevant occupational disease. These studies were performed using allergen microarrays and the graph theory for analyzing the results. Regarding the second approach, we analyzed the interaction of plant allergens with immune and epithelial cells. To perform these studies , we examined the importance of ligands and co-transported molecules of plant allergens in the development of Th2 responses. To this end, Pru p 3, nsLTP (non-specific Lipid Transfer Protein) and peach major allergen, was selected as a model to investigate its interaction with cells of the human and murine immune systems as well as with the intestinal epithelium and the contribution of its ligand in inducing an allergic response was studied. Moreover, we analyzed the role of pathogen associated molecules in the induction of food allergy. For that, we selected the kiwi- alternaria system as a model and the role of Alt a 1 , major allergen of the fungus, in the development of Act d 2-sensitization was studied. In summary, this work presents an innovative research providing useful results for improving diagnosis and leading to further research on allergy and the final clarification of the mechanisms that characterize this disease.

Purification and Characterization of AsES Protein A Subtilisin Secreted by Acremonium Strictum is a Novel Plant Defense Elicitor.

In this work, the purification and characterization of an extracellular elicitor protein, designated AsES, produced by an avirulent isolate of the strawberry pathogen Acremonium strictum, are reported. The defense eliciting activity present in culture filtrates was recovered and purified by ultrafiltration (cutoff, 30 kDa), anionic exchange (Q-Sepharose, pH 7.5), and hydrophobic interaction (phenyl-Sepharose) chromatographies. Two-dimensional SDS-PAGE of the purified active fraction revealed a single spot of 34 kDa and pI 8.8. HPLC (C2/C18) and MS/MS analysis confirmed purification to homogeneity. Foliar spray with AsES provided a total systemic protection against anthracnose disease in strawberry, accompanied by the expression of defense-related genes (i.e. PR1 and Chi2-1). Accumulation of reactive oxygen species (e.g. H2O2 and O2̇̄) and callose was also observed in Arabidopsis. By using degenerate primers designed from the partial amino acid sequences and rapid amplification reactions of cDNA ends, the complete AsES-coding cDNA of 1167 nucleotides was obtained. The deduced amino acid sequence showed significant identity with fungal serine proteinases of the subtilisin family, indicating that AsES is synthesized as a larger precursor containing a 15-residue secretory signal peptide and a 90-residue peptidase inhibitor I9 domain in addition to the 283-residue mature protein. AsES exhibited proteolytic activity in vitro, and its resistance eliciting activity was eliminated when inhibited with PMSF, suggesting that its proteolytic activity is required to induce the defense response. This is, to our knowledge, the first report of a fungal subtilisin that shows eliciting activity in plants. This finding could contribute to develop disease biocontrol strategies in plants by activating its innate immunity.

The role of the secondary cell wall in plant resistance to pathogens

Plant resistance to pathogens relies on a complex network of constitutive and inducible defensive barriers. The plant cell wall is one of the barriers that pathogens need to overcome to successfully colonize plant tissues. The traditional view of the plant cell wall as a passive barrier has evolved to a concept that considers the wall as a dynamic structure that regulates both constitutive and inducible defense mechanisms, and as a source of signaling molecules that trigger immune responses. The secondary cell walls of plants also represent a carbon-neutral feedstock (lignocellulosic biomass) for the production of biofuels and biomaterials. Therefore, engineering plants with improved secondary cell wall characteristics is an interesting strategy to ease the processing of lignocellulosic biomass in the biorefinery. However, modification of the integrity of the cell wall by impairment of proteins required for its biosynthesis or remodeling may impact the plants resistance to pathogens. This review summarizes our understanding of the role of the plant cell wall in pathogen resistance with a focus on the contribution of lignin to this biological process.