Endophytic Cryphonectriaceae on native Myrtales: Possible origin of Chrysoporthe canker on plantation-grown Eucalyptus

Botryosphaeriaceae on Syzygium cordatum across a latitudinal gradient in South Africa

The Botryosphaeriaceae is a family of endophytic fungi, many of which are latent pathogens of woody plants. Although extensively sampled in some parts of the world, little is known regarding their occurrence across different environmental conditions. This study considered the presence of the Botryosphaeriaceae on Syzygium cordatum trees across a latitudinal gradient. We examined the relative importance of different environmental factors on the presence of the Botryosphaeriaceae across this latitudinal gradient. Specifically, Botryosphaeriaceae community composition and species richness were analysed. The optimal growth temperature of the most common Botryosphaeriaceae isolates and its relation to isolate origin was also tested in culture. We identified 14 Botryosphaeriaceae species including seven each of Lasiodiplodia and Neofusicoccum species. The maximum historical temperature emerged as the environmental factor that best predicted the presence of Botryosphaeriaceae species in S. cordatum trees, specifically influencing Botryosphaeriaceae community composition. For all the Botryosphaeriaceae species studied in vitro, temperature strongly influenced mycelial growth and they all had an optimal growth temperature of 25 °C. Contrary to our hypothesis, the optimal growth temperature was not related to isolate origin. These results contribute to understanding the presence of the Botryosphaeriaceae in trees and our ability to detect these latent pathogens.

Endophytic Fungi and Ecological Fitness of Chestnuts

Chestnuts (Castanea spp.) are plants of relevant economic interest in the agro-sylvicultural contexts of mountain regions throughout the temperate zone, particularly in the northern hemisphere. In recent years, several biological adversities have repeatedly endangered species belonging to this genus, calling for coordinated actions addressed to contrast their decline. These actions have mainly focused on the control of key pests/pathogens and the improvement of resistance/tolerance by the plant host, while the role of microorganisms as mediators of interactions between plants and the noxious agents has been less considered, essentially by reason of a limited knowledge on their ecological impact. In line with the increasing awareness of the basic importance of microbial symbionts in regulating plant fitness in both natural and crop contexts, this paper offers an overview on the occurrence and effects of endophytic fungi of chestnuts.

Comparative Genomics Analyses of Lifestyle Transitions at the Origin of an Invasive Fungal Pathogen in the Genus Cryphonectria

Forest and agroecosystems, as well as animal and human health, are threatened by emerging pathogens. Following decimation of chestnuts in the United States, the fungal pathogen Cryphonectria parasitica colonized Europe. After establishment, the pathogen population gave rise to a highly successful lineage that spread rapidly across the continent. Core to our understanding of what makes a successful pathogen is the genetic repertoire enabling the colonization and exploitation of host species. Here, we have assembled >100 genomes across two related genera to identify key genomic determinants leading to the emergence of chestnut blight. We found subtle yet highly specific changes in the transition from saprotrophy to latent pathogenicity mostly determined by enzymes involved in carbohydrate metabolism. Large-scale genomic analyses of genes underlying key nutrition modes can facilitate the detection of species with the potential to emerge as pathogens.

Emerging fungal pathogens are a threat to forest and agroecosystems, as well as animal and human health. How pathogens evolve from nonpathogenic ancestors is still poorly understood, making the prediction of future outbreaks challenging. Most pathogens have evolved lifestyle adaptations, which were enabled by specific changes in the gene content of the species. Hence, understanding transitions in the functions encoded by genomes gives valuable insight into the evolution of pathogenicity. Here, we studied lifestyle evolution in the genus Cryphonectria, including the prominent invasive pathogen Cryphonectria parasitica, the causal agent of chestnut blight on Castanea species. We assembled and compared the genomes of pathogenic and putatively nonpathogenic Cryphonectria species, as well as sister group pathogens in the family Cryphonectriaceae (Diaporthales, Ascomycetes), to investigate the evolution of genome size and gene content. We found a striking loss of genes associated with carbohydrate metabolism (CAZymes) in C. parasitica compared to other Cryphonectriaceae. Despite substantial CAZyme gene loss, experimental data suggest that C. parasitica has retained wood colonization abilities shared with other Cryphonectria species. Putative effectors substantially varied in number, cysteine content, and protein length among species. In contrast, secondary metabolite gene clusters show a high degree of conservation within the genus. Overall, our results underpin the recent lifestyle transition of C. parasitica toward a more pathogenic lifestyle. Our findings suggest that a CAZyme loss may have promoted pathogenicity of C. parasitica on Castanea species. Analyzing gene complements underlying key nutrition modes can facilitate the detection of species with the potential to emerge as pathogens.

IMPORTANCE Forest and agroecosystems, as well as animal and human health, are threatened by emerging pathogens. Following decimation of chestnuts in the United States, the fungal pathogen Cryphonectria parasitica colonized Europe. After establishment, the pathogen population gave rise to a highly successful lineage that spread rapidly across the continent. Core to our understanding of what makes a successful pathogen is the genetic repertoire enabling the colonization and exploitation of host species. Here, we have assembled >100 genomes across two related genera to identify key genomic determinants leading to the emergence of chestnut blight. We found subtle yet highly specific changes in the transition from saprotrophy to latent pathogenicity mostly determined by enzymes involved in carbohydrate metabolism. Large-scale genomic analyses of genes underlying key nutrition modes can facilitate the detection of species with the potential to emerge as pathogens.

Evidence for salicylic acid signalling and histological changes in the defence response of Eucalyptus grandis to Chrysoporthe austroafricana

Eucalyptus species are cultivated for forestry and are of economic importance. The fungal stem canker pathogen Chrysoporthe austroafricana causes disease of varying severity on E. grandis. The Eucalyptus grandis-Chrysoporthe austroafricana interaction has been established as a model system for studying Eucalyptus antifungal defence. Previous studies revealed that the phytohormone salicylic acid (SA) affects the levels of resistance in highly susceptible (ZG14) and moderately resistant (TAG5) clones. The aims of this study were to examine histochemical changes in response to wounding and inoculation as well as host responses at the protein level. The anatomy and histochemical changes induced by wounding and inoculation were similar between the clones, suggesting that anatomical differences do not underlie their different levels of resistance. Tyloses and gum-like substances were present after inoculation and wounding, but cell death occurred only after inoculation. Hyphae of C. austroafricana were observed inside dead and living cells, suggesting that the possibility of a hemibiotrophic interaction requires further investigation. Proteomics analysis revealed the possible involvement of proteins associated with cell death, SA signalling and systemic resistance. In combination with previous information, this study forms a basis for future functional characterisation of candidate genes involved in resistance of E. grandis to C. austroafricana.