Interaction between the moss Physcomitrella patens and Phytophthora: a novel pathosystem for live‐cell imaging of subcellular defence

Abstract

Live-cell imaging of plant-pathogen interactions is often hampered by the tissue complexity and multicell layered nature of the host. Here, we established a novel pathosystem with the moss Physcomitrella patens as host for Phytophthora. The tip-growing protonema cells of this moss are ideal for visualizing interactions with the pathogen over time using high-resolution microscopy. We tested four Phytophthora species for their ability to infect P. patens and showed that P. sojae and P. palmivora were only rarely capable to infect P. patens. In contrast, P. infestans and P. capsici frequently and successfully penetrated moss protonemal cells, showed intracellular hyphal growth and formed sporangia. Next to these successful invasions, many penetration attempts failed. Here the pathogen was blocked by a barrier of cell wall material deposited in papilla-like structures, a defence response that is common in higher plants. Another common response is the upregulation of defence-related genes upon infection and also in moss we observed this upregulation in tissues infected with Phytophthora. For more advanced analyses of the novel pathosystem we developed a special set-up that allowed live-cell imaging of subcellular defence processes by high-resolution microscopy. With this set-up, we revealed that Phytophthora infection of moss induces repositioning of the nucleus, accumulation of cytoplasm and rearrangement of the actin cytoskeleton, but not of microtubules.

On plant defense signaling networks and early land plant evolution

“…did not deposit callose in response to desiccation stress, callose was nonetheless present in these species. In the moss Physcomitrella patens , papillae formation is readily observed close to unsuccessful infection attempts by different Phytophthora pathogens [61]. Oomycete and fungal pathogens also induce ROS [62,63] and inoculations with oomycetes resulted in the accumulation of toxic phenolic compounds in P. patens [61,62].…”

“…In the moss Physcomitrella patens , papillae formation is readily observed close to unsuccessful infection attempts by different Phytophthora pathogens [61]. Oomycete and fungal pathogens also induce ROS [62,63] and inoculations with oomycetes resulted in the accumulation of toxic phenolic compounds in P. patens [61,62]. Similarly, other mosses, including Funaria hygrometrica , also form papillae around fungal penetration sites to prohibit their entry [64,65].…”

“…It shows a strong responsiveness to pathogens or exogenously applied JA in gymnosperms and JA and SA in mosses [62,63,73,143,149,150]. Therefore, it is not surprising that the defense response of P. patens following the inoculation with oomycete and fungal pathogens includes the production of phenolic compounds [61-63]. Cell wall reinforcements in P. patens by lignification after B. cinerea infection was also suggested because of the enhanced expression of the Dirigent-like gene, PpDIR [63]; DIR and DIR-like enzymes function both in lignan and lignin formation [151].…”

“…However, in this study, the increase was genotype dependent, with more susceptible genotypes showing no increase or less of the flavonoid. In P. patens flavonoids seem to also play a role in defense responses, as bacterial elicitors as well as oomycete and fungal pathogens induce CHS [61,62,73]. Furthermore, other genes of the flavonoid biosynthesis pathway are induced by bacterial elicitors [153].…”

An Innate Immunity Pathway in the Moss Physcomitrella patens

“…To assess whether the levels of pathogen defense-related transcripts were altered in Dmpk4a, we measured the response to chitosan of five transcripts involved in defense against pathogens and/or are induced in response to chitosan in both Arabidopsis and P. patens: PAL4, CHS, ERF2, a-DOX, and LOX7 (Oliver et al, 2009;Lehtonen et al, 2012;Machado et al, 2015;Overdijk et al, 2016). Although the response to chitosan (Supplemental Figures 14A and 14B) is slower than to chitin ( Figure 4E), all these transcripts accumulated less in Dmpk4a compared with the wild type (Supplemental Figures 14A to 14E).…”