Plántulas de Populus alba pretratadas con endófitos muestran una mayor tolerancia a Venturia tremulae

Algunos hongos endófitos parecen modular la respuesta de las plantas ante ataques de ciertos patógenos o insectos. En este trabajo se estudió la implicación de hongos endófitos de Populus alba en la tolerancia al patógeno necrotrofo Venturia tremulae, causante de la muerte de ramillos en especies de la sección Populus, siendo P. alba más tolerante que otras especies de la sección. Se evaluó in vitro el efecto de doce cepas de endófitos enfrentándolas al patógeno con el fin de seleccionar aquellas con mayor potencial de biocontrol. Se seleccionaron diez endófitos que fueron inoculados mediante solución esporal de una mezcla de todos ellos, de forma preventiva sobre plántulas de una savia de P. alba. Transcurridos 15 días, se inoculó al patógeno. Aunque en general los síntomas mostrados por las plantas fueron bajos, la mezcla de endófitos redujo la incidencia y severidad de los síntomas causados por el patógeno. Los resultados sugieren que los endófitos de P. alba limitarían los síntomas causados por el patógeno en este hospedante

Causes and consequences of large clonal assemblies in a poplar hybrid zone.

Asexual reproduction is a common and fundamental mode of reproduction in plants. Although persistence in adverse conditions underlies most known cases of clonal dominance, proximal genetic drivers remain unclear, in particular for populations dominated by a few large clones. In this paper, we studied a clonal population of the riparian tree Populus alba in the Douro river basin (northwestern Iberian Peninsula) where it hybridizes with P. tremula, a species that grows in highly contrasted ecological conditions. We used 73 nuclear microsatellites to test whether genomic background (species ancestry) is a relevant cause of clonal success, and to assess the evolutionary consequences of clonal dominance by a few genets. Additional Genotyping-by-Sequencing (GBS) data were produced to estimate the age of the largest clones. We found that a few ancient (over a few thousand years old) and widespread genets dominate the population, both in terms of clone size and number of sexual offspring produced. Interestingly, large clones possessed two genomic regions introgressed from P. tremula, which may have favored their spread under stressful environmental conditions. At the population level, the spread of large genets was accompanied by an overall ancient (>0.1 Myr) but soft decline of effective population size. Despite this decrease, and the high clonality and dominance of sexual reproduction by large clones, the Douro hybrid zone still displays considerable genetic diversity and low inbreeding. This suggests that, even in extreme cases as in the Douro, asexual and sexual dominance of a few large, geographically-extended individuals does not threaten population survival. This article is protected by copyright. All rights reserved

Plántulas de Populus alba pretratadas con endófitos muestran una mayor tolerancia a Venturia tremulae

Algunos hongos endófitos parecen modular la respuesta de las plantas ante ataques de ciertos patógenos o insectos. En este trabajo se estudió la implicación de hongos endófitos de Populus alba en la tolerancia al patógeno necrotrofo Venturia tremulae, causante de la muerte de ramillos en especies de la sección Populus, siendo P. alba más tolerante que otras especies de la sección. Se evaluó in vitro el efecto de doce cepas de endófitos enfrentándolas al patógeno con el fin de seleccionar aquellas con mayor potencial de biocontrol. Se seleccionaron diez endófitos que fueron inoculados mediante solución esporal de una mezcla de todos ellos, de forma preventiva sobre plántulas de una savia de P. alba. Transcurridos 15 días, se inoculó al patógeno. Aunque en general los síntomas mostrados por las plantas fueron bajos, la mezcla de endófitos redujo la incidencia y severidad de los síntomas causados por el patógeno. Los resultados sugieren que los endófitos de P. alba limitarían los síntomas causados por el patógeno en este hospedante

Agronomic performance of Populus deltoides trees engineered for biofuel production

Background: One of the major barriers to the development of lignocellulosic feedstocks is the recalcitrance of plant cell walls to deconstruction and saccharifcation. Recalcitrance can be reduced by targeting genes involved in cell wall biosynthesis, but this can have unintended consequences that compromise the agronomic performance of the trees under feld conditions. Here we report the results of a feld trial of fourteen distinct transgenic Populus deltoides lines that had previously demonstrated reduced recalcitrance without yield penalties under greenhouse conditions. Results: Survival and productivity of the trial were excellent in the frst year, and there was little evidence for reduced performance of the transgenic lines with modifed target gene expression. Surprisingly, the most striking phenotypic efects in this trial were for two empty-vector control lines that had modifed bud set and bud fush. This is most likely due to somaclonal variation or insertional mutagenesis. Traits related to yield, crown architecture, herbivory, patho‑ gen response, and frost damage showed few signifcant diferences between target gene transgenics and empty vector controls. However, there were a few interesting exceptions. Lines overexpressing the DUF231 gene, a putative O-acetyltransferase, showed early bud fush and marginally increased height growth. Lines overexpressing the DUF266 gene, a putative glycosyltransferase, had signifcantly decreased stem internode length and slightly higher volume index. Finally, lines overexpressing the PFD2 gene, a putative member of the prefoldin complex, had a slightly reduced volume index. Conclusions: This feld trial demonstrates that these cell wall modifcations, which decreased cell wall recalcitrance under laboratory conditions, did not seriously compromise frst-year performance in the feld, despite substantial challenges, including an outbreak of a stem boring insect (Gypsonoma haimbachiana), attack by a leaf rust pathogen (Melampsora spp.), and a late frost event. This bodes well for the potential utility of these lines as advanced biofuels feedstocks

Admixture mapping of quantitative traits in Populus hybrid zones: power and limitations

Uncovering the genetic architecture of species differences is of central importance for understanding the origin and maintenance of biological diversity. Admixture mapping can be used to identify the number and effect sizes of genes that contribute to the divergence of ecologically important traits, even in taxa that are not amenable to laboratory crosses because of their long generation time or other limitations. Here, we apply admixture mapping to naturally occurring hybrids between two ecologically divergent Populus species. We map quantitative trait loci for eight leaf morphological traits using 77 mapped microsatellite markers from all 19 chromosomes of Populus. We apply multivariate linear regression analysis allowing the modeling of additive and non-additive gene action and identify several candidate genomic regions associated with leaf morphology using an information-theoretic approach. We perform simulation studies to assess the power and limitations of admixture mapping of quantitative traits in natural hybrid populations for a variety of genetic architectures and modes of gene action. Our results indicate that (1) admixture mapping has considerable power to identify the genetic architecture of species differences if sample sizes and marker densities are sufficiently high, (2) modeling of non-additive gene action can help to elucidate the discrepancy between genotype and phenotype sometimes seen in interspecific hybrids, and (3) the genetic architecture of leaf morphological traits in the studied Populus species involves complementary and overdominant gene action, providing the basis for rapid adaptation of these ecologically important forest trees

Genetic analysis of post-mating reproductive barriers in hybridizing European Populus species

Molecular genetic analyses of experimental crosses provide important information on the strength and nature of post-mating barriers to gene exchange between divergent populations, which are topics of great interest to evolutionary geneticists and breeders. Although not a trivial task in long-lived organisms such as trees, experimental interspecific recombinants can sometimes be created through controlled crosses involving natural F₁'s. Here, we used this approach to understand the genetics of post-mating isolation and barriers to introgression in Populus alba and Populus tremula, two ecologically divergent, hybridizing forest trees. We studied 86 interspecific backcross (BC₁) progeny and >350 individuals from natural populations of these species for up to 98 nuclear genetic markers, including microsatellites, indels and single nucleotide polymorphisms, and inferred the origin of the cytoplasm of the cross with plastid DNA. Genetic analysis of the BC₁ revealed extensive segregation distortions on six chromosomes, and >90% of these (12 out of 13) favored P. tremula donor alleles in the heterospecific genomic background. Since selection was documented during early diploid stages of the progeny, this surprising result was attributed to epistasis, cyto-nuclear coadaptation, heterozygote advantage at nuclear loci experiencing introgression or a combination of these. Our results indicate that gene flow across ‘porous’ species barriers affects these poplars and aspens beyond neutral, Mendelian expectations and suggests the mechanisms responsible. Contrary to expectations, the Populus sex determination region is not protected from introgression. Understanding the population dynamics of the Populus sex determination region will require tests based on natural interspecific hybrid zones

Decline of beech trees in a Mediterranean forest is associated with high rhizosphere oomycete diversity

In the last decades, tree decline in European beech forests has been related to extreme climatic events and with the activity of parasitic oomycetes such as Phytophthora species. In Spain, little is known about the association of beech decline and soil oomycete composition. We hypothesized that the weakening of beech trees is associated with the activity and proliferation of parasitic oomycetes. We studied the rhizosphere oomycete community of large beech trees located in one of the southernmost beech forests in Europe. From soil samples collected in the rhizosphere, we used a metabarcoding approach to explore the oomycete community associated to asymptomatic and declining trees. We also studied fine root parameters in both groups of trees. A total of 99 oomycete amplicon sequence variants (ASVs) were detected. Higher diversity and richness of oomycetes were observed in declining than in asymptomatic trees. The oomycete composition also differed between the two groups of trees. Declining trees showed lower fine root biomass and root density than asymptomatic trees, and root density was negatively correlated with the abundance of Pythium ASV counts. The genus Phytophthora, associated with beech decline in central Europe, was underrepresented in the oomycete community. The results suggest that decline-associated processes in beech trees are related with fine root weakening and loss which probably facilitates the entrance and colonization of opportunistic oomycete microbes. Although members in the genus Pythium do not seem to be a primary factor on beech decline, they probably contribute to the chronic decline of Fagus sylvatica trees