Diplodia sapinea infection reprograms foliar traits of its pine (Pinus sylvestris L.) host to death.

Infection with the necrotrophic fungus Diplodia sapinea (Fr.) Fuckel is among the economically and ecologically most devastating diseases of conifers in the northern hemisphere and is accelerated by global climate change. This study aims to characterize the changes mediated by D. sapinea infection on its pine host (Pinus sylvestris L.) that lead to the death of its needles. For this purpose, we performed an indoor infection experiment and inoculated shoot tips of pine seedlings with virulent D. sapinea. The consequences for foliar traits, including the phytohormone profile, were characterized at both the metabolite and transcriptome level. Our results showed that D. sapinea infection strongly affected foliar levels of most phytohormones and impaired a multitude of other metabolic and structural foliar traits, such as reactive oxygen species scavenging. Transcriptome analysis revealed that these changes are partially mediated via modified gene expression by fungal exposure. Diplodia sapinea appears to overcome the defense reactions of its pine host by reprogramming gene expression and post-transcriptional controls that determine essential foliar metabolic traits such as the phytohormone profile, cell wall composition and antioxidative system.

Systemic reprogramming of phytohormone profiles and metabolic traits by virulent Diplodia infection in its pine (Pinus sylvestris L.) host.

The widespread ascomycetous fungus Diplodia pinea is a latent, necrotrophic pathogen in Pinus species causing severe damages and world-wide economic losses. However, the interactions between pine hosts and virulent D. pinea are largely not understood. In the present study, systemic defence responses were investigated in non-inoculated, asymptomatic needles and roots of D. pinea infected saplings of two P. sylvestris provenances under controlled greenhouse conditions. Here, we show that D. pinea infection induced a multitude of systemic responses of the phytohormone profiles and metabolic traits. Shared systemic responses of both pine provenances in needles and roots included increased abscisic acid and jasmonic acid levels. Exclusively in the roots of both provenances, enhanced salicylic acid and reduced indole-3-acetic acid levels, structural biomass, and elevated activities of anti-oxidative enzymes were observed. Despite these similarities, the two pine provenances investigated different significantly in the systemic responses of both, phytohormone profiles and metabolic traits in needles and roots. However, the different systemic responses did not prevent subsequent destruction of non-inoculated needles, but rather prevented damage to the roots. Our results provide a detailed view on systemic defence mechanisms of pine hosts that are of particular significance for the selection of provenances with improved defence capacity.

Consequences of Sphaeropsis tip blight disease for the phytohormone profile and antioxidative metabolism of its pine host.

Phytopathogenic fungi infections induce plant defence responses that mediate changes in metabolic and signalling processes with severe consequences for plant growth and development. Sphaeropsis tip blight, induced by the endophytic fungus Sphaeropsis sapinea that spreads from stem tissues to the needles, is the most widespread disease of conifer forests causing dramatic economic losses. However, metabolic consequences of this disease on bark and wood tissues of its host are largely unexplored. Here, we show that diseased host pines experience tissue dehydration in both bark and wood. Increased cytokinin and declined indole-3-acetic acid levels were observed in both tissues and increased jasmonic acid and abscisic acid levels exclusively in the wood. Increased lignin contents at the expense of holo-cellulose with declined structural biomass of the wood reflect cell wall fortification by S. sapinea infection. These changes are consistent with H2 O2 accumulation in the wood, required for lignin polymerization. Accumulation of H2 O2 was associated with more oxidized redox states of glutathione and ascorbate pools. These findings indicate that S. sapinea affects both phytohormone signalling and the antioxidative defence system in stem tissues of its pine host during the infection process.

Mistletoe infestation mediates alteration of the phytohormone profile and anti-oxidative metabolism in bark and wood of its host Pinus sylvestris.

European mistletoe (Viscum album L.) has largely infested Central European forests and causes high mortality probability particularly in dry years. However, little information is available about the consequences of mistletoe infestation for metabolic processes in bark and wood of its host, despite their important roles in infestation defense. We analyzed the tissue hydration, carbohydrate composition, phytohormone profile, reactive oxygen species and anti-oxidant levels in bark and wood of Scots pines (Pinus sylvestris L.), as dependent on mistletoe infestation. As a consequence of mistletoe infestation, host bark and wood showed impaired hydration and reduced total carbon content. In the bark, soluble sugar and lignin contents increased, apparently at the expense of holo-cellulose. Hydrogen peroxide accumulation was accompanied by increased glutathione and decreased reduced ascorbic acid levels. Mistletoe infestation mediated alteration of the phytohormone profile in bark and wood of its host. Cytokinins, jasmonic acid and abscisic acid levels increased in both tissues, whereas salicylic acid and indole-3-acetic acid, which were only detected in the bark, declined. The present results show that mistletoe infestation affects both the host's anti-oxidative defense system and the phytohormone profile after establishment of the xylem tapping haustorium. The significance of these processes for the development of the woody mistletoe stem and the haustorium is discussed.

Genetic diversification of the chestnut blight fungus Cryphonectria parasitica and its associated hypovirus in Germany.

Chestnut blight in south-western Germany was first reported in 1992 and is since expanding in distribution. Here we investigated the invasion history of Cryphonectria parasitica and its associated hypovirus. For this, we characterized 284 isolates collected between 1992 and 2012 for hypovirulence, vegetative compatibility (vc), mating type, and microsatellite haplotype. A total of 27 haplotypes and 15 vc types were observed, although the C. parasitica population analyzed is currently dominated to 50 % by one haplotype and to 64 % by the vc type EU-2. Structure analysis indicated two divergent genetic pools. Over 66 % of the haplotypes belonged to a pool probably originating from northern Italy. Further diversification is expected due to ongoing sexual recombination, but also to new migration and additional introductions. Cryphonectria hypovirus 1 (CHV-1) was found in four of five C. parasitica populations from Baden-Württemberg. Genetic analysis of the 35 CHV-1 isolates obtained revealed that they all belong to the German subtype, although they have clearly diverged from the first German hypovirus isolated in 1992. Our study suggests that C. parasitica has been introduced into Germany several times from two different gene pools, whereas the hypovirus most probably has a single origin.