3.2 Dickeya dianthicola exhibited a fitness advantage
over D. solani in potato plants.
Plant assays were performed in the greenhouse to compare the incidence
of blackleg symptoms after inoculation by D. dianthicola orD. solani on non-wounded potato plants. FiveD. dianthicola strains (RNS11-47-1-1A, CFBP1888, CFBP2982,
CFBP2015, MIE34) and five D. solani strains (3337 =
RNS08-23-3-1A, IPO2222, RNS05-1-2A, Ds0432.1, PPO9019) were used in
these assays.
In a first assay, each strain was inoculated individually on 15 plants
and the number of asymptomatic and symptomatic plants were counted at 61
days post inoculation (dpi). A Kruskal-Wallis test revealed differences
between D. solani and D. dianthicola in terms of symptom
incidence (k=7; DF=1; p=8 x 10-3): D.
dianthicola was found more virulent than D. solani (percentages
of symptomatic plants of 61% ± 10 versus 15% ± 11; Figure 3a).
In a second plant assay, the five bacterial cultures (i.e., the five
strains) of each species were assembled to constitute two experimental
populations and each was inoculated on 15 plants. A mix of the two
species (10 strains) was also inoculated on 15 plants. The assay was
duplicated (2 x 15 plants per treatment). We compared symptom incidence
at 61 dpi. Kruskal-Wallis test revealed differences between the three
treatments, i.e., D. solani strain mixture, D. dianthicolastrain mixture and species mixture (k=6.1; DF=2; p=0.04). Pairwise
comparison (Post-hoc Tukey test) showed that D. solani was less
virulent than D. dianthicola (F=-2.5; p= 0.03) and the species
mixture (F=1.9; p= 0.10). However, symptom incidence caused by the
species mixture did not differ from those of the D. dianthicolamixture (F=-0.5; p= 0.86). Post-hoc Tukey p-values and mean value (± SE)
of the percentage of symptomatic plants inoculated by the D.
dianthicola (53% ± 7), D. solani (23% ± 3) and mixed (47% ±
0) populations are presented in the figure 3b . To assess the
dynamics of the appearance of symptomatic plants, we also drew a disease
progress curve for each replicate (Figure S3a ) and compared the
areas under disease progress curve (Figure S3b ). Pairwise
comparisons (Post-hoc Tukey tests) confirmed that D. solani was
less virulent than D. dianthicola (F=-336;
p<10-11) and the species mixture (F=386;
p<10-11).
To evaluate which pathogen had emerged at the beginning of the infection
process, hence to identify the most probable causative agent of the
observed symptoms, we used qPCR to quantify pathogen loads. When the two
species mixtures were inoculated separately, D. dianthicola andD. solani reached the mean values ± SE of 2 x
1011 ± 1 x 104 cells and 3 x
109 ± 1 x 102 cells per gram of
symptomatic stem tissues, respectively. In eight co-infected symptomatic
plants, quantifications of the pathogen abundance revealed an excess ofD. dianthicola with a CI median value of 10-5(Figure 3c ). The eight CI values differed from one
(Kruskal-Wallis test, k=9.5; DF=1; p= 2 x 10-3),
meaning that D. dianthicola outcompeted D. solani in
blackleg tissues. Altogether, these data revealed a fitness advantage ofD. dianthicola in terms of multiplication within lesions in the
aerial parts of the host in either the absence or presence of theD. solani invader.