4 Discussion
Two successive invasions of Dickeya potato pathogens occurred in
Europe: first D. dianthicola in the middle of the
20th century, this pathogen being now considered as
endemic, and second, D. solani at the beginning of the
21th. Dickeya solani and D. dianthicolacause similar symptoms on potato plants, so that they are expected to
compete for the same resources. Over ten years, our epidemiologic
surveys revealed a successful establishment of D. solani together
with a maintenance of D. dianthicola . Our experiments contributed
to explain such puzzling co-existence by contrasted advantages in
different parts of the plants and therefore partially different
ecological niches.
By comparing ecological traits of D. solani and D.
dianthicola , we indeed found contrasted behaviors in these species with
respect to aerial parts versus tubers of S. tuberosum . The
resident D. dianthicola more efficiently exploited host stems
after an inoculation of the wounded and unwounded roots than did the
invader D. solani in terms of symptom severity, proliferation and
relative fitness in competition assays. In contrast, D. solanimore efficiently initiated the rotting process (especially at a low
bacterial inoculum) and competed in tubers, with potential consequences
on tubers in the fields and during storage. The capacity of D.
solani to cause rooting at a lower bacterial load than D.
dianthicola is likely at least partly due to a higher expression of thepel enzymes, especially PelE that acts as an initiator of the
plant cell wall degradation (Duprey et al., 2016). By comparingpel genes in several Dickeya species, Duprey et al. (2016)
showed that D. solani evolved specific regulatory sequences that
contributes to a different expression levels of the pelD andpelE genes compared to D. dianthicola . The pelAgene is truncated in D. dianthicola (Duprey et al., 2016; Raoul
des Essarts et al., 2019), but full-length and expressed in D.
solani , reinforcing their virulence arsenal. Other metabolic traits
could also contribute to the fitness advantage of D. solani for
exploiting tubers: comparative transcriptomics revealed a higher
expression of the glyoxylate shunt in D. solani than in D.
dianthicola, a pathway that contributes to exploit alternative carbon
sources when sugar availability is low (Raoul des Essarts et al., 2019).
Beside the tripartite interaction between potato plant host, D.
solani and D. dianthicola , additional environmental (soil and
climate) and biological factors (microbiota includingPectobacterium species) may facilitate or limit D. solaniestablishment (Charkowsky 2018; Shyntum et al., 2019; Toth et al.,
2011). Competition and facilitation processes have been well studied in
pathogenic fungi (Al-Naimi, Garrett, & Bockus, 2005; Abdullah et al.,
2017; Gladieux et al., 2015; Zhan & McDonald, 2013). Another factor
that could reduce competition between the two Dickeya species is
their natural low abundance in soils and surface waters, as well as in
tuber seeds that are subjected to prophylactic diagnosis. A small
population size belonging to a single species could proliferate in a
plant individual without any interactions with another Dickeya orPectobacterium pathogen species. These effects linked to size
population and dispersal are expected to delay invasion. In line with
this hypothesis, the observed slow increase of the percentage ofDickeya -positive fields over one decade suggested that theDickeya invasion was still ongoing (2004-2015). Among the
emerging literature on microbial invasion, the D. solani pathogen
appears as a good example illustrating how different ecological
components (here, the plant host and a resident pathogen) should be
considered to understand the biological invasion by a bacterial pathogen
(Cadotte et al., 2018; Germain, Mayfield, & Gilbert, 2018).
The genome sequencing data indicating low diversity in D. solanisuggest a bottleneck during introduction in Europe. The dispersal modes
(either horizontally via soil, surface water, insects and some
agricultural practices or vertically by asymptomatically contaminated
tuber seeds) could contribute to further bottlenecks in D. solani(Charkowsky 2018; Toth et al., 2011). Selection on some genes may also
have further reduced genetic diversity through selective sweeps across
the whole genomes as these bacteria are clonal. Allelic changes related
to quorum-sensing systems were already observed in different plant and
animal pathogens along the host-colonization process (Feltner et al.,
2016; Guidot et al., 2014; Tannières, Lang, Barnier, Shykoff, Faure,
2017), indicating that balancing selection contributed to maintain their
variability. Remarkably, we observed balanced frequencies of two alleles
in the VfmB protein involved in the Vfm-type quorum-sensing inDickeya bacteria. The chemical structure of the Vfm signal is not
known, but it has been involved in tuber rotting and upregulation of
some virulence genes including pectate lyases (pel genes),
proteases (prt genes) and cellulases (cel genes) (Nasser
et al., 2013; Potrykus et al., 2018). Using tuber assays, we observed a
slightly higher aggressiveness in VfmBSer strains
compared to VfmBPro strains. Using transcriptomics ofD. solani pathogens recovered from tuber symptoms, we confirmed
an enrichment of upregulated genes, including pel and prtgenes, in the VfmBSer strain Ds0432.1 compared to
VfmBPro strain IPO2222. Because of the role of the VfmB
protein in Vfm signaling is not completely elucidated, it was premature
to go deeper into the mechanistic characterization of the
vfmBSer and VfmBPro alleles. In this
work, we showed that the VfmBSer strains were more
aggressive than the VfmBPro strains in potato tubers,
but not in potato stem assays, and that the VfmBSerstrains were less competitive than the VfmBPro strains
in tuber and stem symptoms. This environment-dependent advantage of
VfmBser and VfmBPro alleles in plant
infection assays predicts balancing selection in natural populations.
The VfmBser allele could thus provide an advantage when
competition is reduced at the beginning of the establishment in potato
agrosystems and under a high dispersal condition. In contrast,
VfmBser could be outcompeted by VfmBProwhen D. solani is already established. Our epidemiologic data
revealed up and down variation of VfmBser relative
abundance in field populations over the past decade, in agreement with
an increase of competition between D. solani VfmB alleles in the
more recent sampling year.
In conclusion, this study brings novel insights allowing a better
understanding of the pattern and causes of the D. solani invasion
into potato production agrosystems, and the reasons whyD. dianthicola nevertheless persisted. More broadly, this study
contributes to our understanding the ecological determinants of pathogen
invasion and of the conditions for the maintenance of endemic
competitors.