The endophytic fungi of certain grasses and other herbaceous plants have long been known to provide plants with anti-herbivore defense compounds, but there is little information about whether the endophytes of trees also engage in such mutualisms. We investigated the influence of the endophytic fungus Cladosporium cladosporioides on the chemical defenses of black poplar (Populus nigra) trees and the consequences for feeding preference and fitness of herbivorous insects and insect community assembly. Endophyte colonization increased both constitutive- and induced poplar defenses. Generalist Lymantria dispar larvae preferred and performed better on uninfected over endophyte-infected poplar leaves, most likely due to higher concentrations of salicinoids in endophytic leaves and the endophyte-produced alkaloid stachydrine. Under field conditions, the endophytic fungus also shapes insect community assembly in young black poplar trees. Our results show that endophytic fungi can play a major role both in defending trees against herbivorous insects and in structuring insect communities.

Diplodia sapinea causes Diplodia tip blight (DTB) and is recognised as an opportunistic necrotrophic pathogen affecting conifers. While DTB is associated with abiotic stress, the impact of biotic stress in the host on D. sapinea’s lifestyle shift is unknown. Observed co-occurrences of D. sapinea and Melampsora pinitorqua, causing pine twisting rust on Scots pine ( Pinus sylvestris), instigated an investigation into their interaction with and influence on the defence mechanisms of the host. We hypothesised that M. pinitorqua infections predispose the trees to D. sapinea by stressing the host and altering the shoot metabolites. Trees in a pine plantation in central Sweden were sampled over time to study pathogen biomass and host metabolites. The symptoms of both pathogens were consistent over years, and the preceding season’s symptoms affected the metabolic starting points pre-infection and M. pinitorqua’s proliferation. Symptoms of M. pinitorqua altered shoot metabolites more than fungal biomass, with co-symptomatic trees exhibiting elevated M. pinitorqua biomass. D. sapinea’s biomass pre-symptoms was independent of previous disease symptoms and infection by M. pinitorqua. Some trees showed tolerance to M. pinitorqua, with delayed rust infections and minimal DTB symptoms. This trait may improve the resilience of pine plantations, but more work is needed.

Microbial volatiles are important factors in symbiotic interactions with plants. Mortierella hyalina is a beneficial root-colonizing fungus with a garlic-like smell, and promotes growth of Arabidopsis seedlings. GC-MS analysis of the M. hyalina headspace and NMR analysis of the extracted essential oil identified the sulfur-containing volatile tris(methylthio)methane (TMTM) as the major compound. Its incorporation in seedlings was shown by 34S labeling experiment. Under sulfur deficiency, TMTM downregulated sulfur deficiency-responsive genes, prevented glucosinolate (GSL) and glutathione (GSH) diminishment, and sustained plant growth. However, excess TMTM led to accumulation of GSH and GSL and reduced plant growth. Since TMTM is not directly incorporated into cysteine, we propose that the volatile from M. hyalina influences the plant sulfur metabolism by interfering with the GSH metabolism, and alleviates sulfur imbalances under sulfur stress.

The host-pathogen combinations - Malus domestica (apple)/ ‘Candidatus Phytoplasma mali´, Prunus persica (peach)/‘Ca. P. prunorum´ and Pyrus communis (pear)/‘Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within some weeks or years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular morphology, affecting phloem mass flow. In contrast, in pear mass flow and physicochemical characteristics of phloem sap increased. No changes in phytohormone levels were detected in pear but in apple and peach trees, where defence- and stress-related phytohormones increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

The host-pathogen combinations - Malus domestica (apple)/´Candidatus Phytoplasma mali´, Prunus persica (peach)/‘Ca. P. prunorum´ and Pyrus communis (pear)/‘Ca. P. pyri´ show different course of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within some weeks to few years period. So far, in these phytoplasma-plant interactions neither morphological nor physiological differences have been studied in the hosts. Phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular morphology, causing negative effects on phloem mass flow. In contrast, in pear mass flow and physicochemical characteristics of phloem sap increased. No changes in phytohormone levels were detected in pear but in apple and peach trees, where defence- and stress-related phytohormones increased. If compared with peach and pear trees, the data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.