Benoît Marçais

Impact of Erysiphe alphitoides on transpiration and photosynthesis in Quercus robur leaves

Oak powdery mildew, (Erysiphe alphitoides) causes one of the most common diseases of oaks. We assessed the impact of this pathogen on photosynthesis and water relations of infected leaves using greenhouse-grown oak seedlings. Transpiration of seedlings infected by oak powdery mildew was also investigated. Altogether, E. alphitoides had a low impact on host gas exchange whether at the leaf or whole plant scale. Maximal stomatal conductance of infected leaves was reduced by 20–30% compared to healthy controls. Severely infected seedlings did not experience any detectable change of whole plant transpiration. The reduction in net CO2 assimilation, An, was less than proportional to the fraction of leaf area infected. Powdery mildew reduced both the maximal light-driven electron flux (Jmax) and the apparent maximal carboxylation velocity (Vcmax) although Vcmax was slightly more impacted than Jmax. No compensation for the infection occurred in healthy leaves of partly infected seedlings as the reduced photosynthesis in the infected leaves was not paralleled by increased An levels in the healthy leaves of the seedlings. However, E. alphitoides had a strong impact on the leaf life-span of infected leaves. It is concluded that the moderate effect of E. alphitoides on oak might be related to the small impact on net CO2 assimilation rates and on tree transpiration; nevertheless, the severe reduction in leaf life-span of heavily infected leaves may lead to decreased carbon uptake over the growth season.

Identification of quantitative trait loci affecting ectomycorrhizal symbiosis in an interspecific F1 poplar cross and differential expression of genes in ectomycorrhizas of the two parents: Populus deltoides and Populus trichocarpa

A Populus deltoides × Populus trichocarpa F1 pedigree was analyzed for quantitative trait loci (QTLs) affecting ectomycorrhizal development and for microarray characterization of gene networks involved in this symbiosis. A 300 genotype progeny set was evaluated for its ability to form ectomycorrhiza with the basidiomycete Laccaria bicolor. The percentage of mycorrhizal root tips was determined on the root systems of all 300 progeny and their two parents. QTL analysis identified four significant QTLs, one on the P. deltoides and three on the P. trichocarpa genetic maps. These QTLs were aligned to the P. trichocarpa genome and each contained several megabases and encompass numerous genes. NimbleGen whole-genome microarray, using cDNA from RNA extracts of ectomycorrhizal root tips from the parental genotypes P. trichocarpa and P. deltoides, was used to narrow the candidate gene list. Among the 1,543 differentially expressed genes (p value ≤ 0.05; ≥5.0-fold change in transcript level) having different transcript levels in mycorrhiza of the two parents, 41 transcripts were located in the QTL intervals: 20 in Myc_d1, 14 in Myc_t1, and seven in Myc_t2, while no significant differences among transcripts were found in Myc_t3. Among these 41 transcripts, 25 were overrepresented in P. deltoides relative to P. trichocarpa; 16 were overrepresented in P. trichocarpa. The transcript showing the highest overrepresentation in P. trichocarpa mycorrhiza libraries compared to P. deltoides mycorrhiza codes for an ethylene-sensitive EREBP-4 protein which may repress defense mechanisms in P. trichocarpa while the highest overrepresented transcripts in P. deltoides code for proteins/genes typically associated with pathogen resistance.

Structure of Collybia fusipes populations in two infected oak stands

Collybia fusipes is the cause of a root rot of oak trees. We studied the structure of C. fusipes populations in two infected oak stands by using somatic incompatibility and DNA amplification. Isolates were obtained from different oak root systems or from within the same root system and somatic incompatibility groups (SIG) were identified. Many small SIGs that seldom encompassed more than one root system were present in both stands. More than one SIG was usually present on an individual root system: there were 3·1 ± 1·3 SIGs on the pedunculate oaks and 2·2 ± 0·6 on the red oaks. The largest SIG contained more than 70% of the isolates obtained from the root system of 14 of the 20 trees studied. Isolates that belonged to the same SIG usually had the same ribosomal intergenic spacer. It is concluded that C. fusipes spreads poorly from tree to tree by vegetative means.

Genetics of somatic incompatibility in Collybia fusipes

The genetics of somatic incompatibility in tetrapolar Collybia fusipes was studied using eight dikaryotic isolates collected from the wild and their experimentally derived progeny. Monokaryons from each isolate were all paired with the same unrelated monokaryon and also paired together in all combinations. The somatic compatibility of the two resulting sets of dikaryons was studied. Two different types of somatic incompatible interaction were observed, lightly or heavily pigmented lines developing between the two isolates. The dikaryons that had one nuclear type in common and one coming from sibling monokaryons were compatible in 7–27% of the cases, incompatible with a lightly pigmented interaction in 30–93% and incompatible with a heavily pigmented interaction in 0–53%. The results suggest that at least three to four loci control the somatic incompatibility in C. fusipes, one of them alone controlling the heavily pigmented interaction.

Responses of the root rot fungus Collybia fusipes to soil waterlogging and oxygen availability.

Collybia fusipes is a common root rot fungus in mature pedunculate oak forest, that causes drastic destruction of the tree root systems, especially in dry or mildly waterlogged soils. We wanted to check, under controlled conditions or in forest ecosystems, whether reduced O2 during saturation of the soil by water could interact with disease evolution. Susceptibility of waterlogged oak seedlings to C. fusipes was tested in a greenhouse and the survival of the pathogen in woody substrates was assessed in hydromorphic soils in a forest. A direct and detrimental effect of soil waterlogging on C. fusipes survival was evidenced both under controlled conditions and in forest stands. Growth of C. fusipes mycelium on agar media was monitored under low O2 mole fraction and compared to that of Armillaria mellea and Heterobasidion annosum. A drastic reduction in mycelial growth was evidenced in C. fusipes and H. annosum but not in A. mellea.

Inoculation of Mature Pedunculate Oaks (Quercus robur) with the Root Rot Fungus Collybia fusipes: Relationships with Tree Vigour and Soil Factors

The severity of natural infections induced by the root rot fungus Collybia fusipes depends on soil factors. Severely infected trees usually show reduced vigour, as evidenced by poor growth. However, it is not known whether reduced vigour could be a cause of the severe infection. The objective was to clarify the relationships between soil factors, tree vigour and susceptibility of Quercus robur to C. fusipes by artificially inoculating mature trees. Two experiments compared oak trees of different ages and dominance class and oak trees growing on different type of soils. The inoculum survival and the infection success were poor in both experiments when waterlogging was severe. Inoculum survival was better in soil with increasing sand/clay ratio, carbon/nitrogen ratio and decreasing pH and calcium, magnesium and phosphorus availability. The relationships between oak vigour and success of infection or surface area of lesions were inconsistent, with a slightly higher infection success on co-dominant/suppressed oaks in one experiment and a higher surface area of successful infection on dominant oaks in the second.

Comparison of the Susceptibility of Quercus petraea, Q. robur and Q. rubra to Collybia fusipes

Collybia fusipes is the cause of a root rot of Quercus petraea (sessile oak), Q. robur (pedunculate oak) and Q. rubra (red oak). This parasite is often reported to cause problems in stands of red oaks and field evidence suggests that this North American oak species is more susceptible than the two European oak species. Young saplings of the three oak species and of chestnut, Castanea sativa, in the glasshouse, and also mature sessile and red oaks growing in the same stand were inoculated with C. fusipes to compare their susceptibility. Red oak, both as young seedlings and mature trees, was more susceptible to C. fusipes than sessile oak. Chestnut seedlings were as susceptible as sessile oak. Susceptibility of pedunculate oak seedlings was intermediate between red oak and sessile oak. In one experiment this species was significantly less susceptible than red oak, and in the other it was as susceptible.