Manipulation of Bryophyte Hosts by Pathogenic and Symbiotic Microbes

Abstract

The colonization of plant tissues by pathogenic and symbiotic microbes is associated with a strong and directed effort to reprogram host cells in order to permit, promote and sustain microbial growth. In response to colonization, hosts accommodate or sequester invading microbes by activating a set of complex regulatory programs that initiate symbioses or bolster defenses. Extensive research has elucidated a suite of molecular and physiological responses occurring in plant hosts and their microbial partners; however, this information is mostly limited to model systems representing evolutionarily young plant lineages such as angiosperms. The extent to which these processes are conserved across land plants is therefore poorly understood. In this review, we outline key aspects of host reprogramming that occur during plant–microbe interactions in early diverging land plants belonging to the bryophytes (liverworts, hornworts and mosses). We discuss how further knowledge of bryophyte–microbe interactions will advance our understanding of how plants and microbes co-operated and clashed during the conquest of land.

On plant defense signaling networks and early land plant evolution

“…While the vast body of literature on how land plants deal with phytopathogens is focused primarily on angiosperms, research on gymnosperms and bryophytes is catching up [11-14] – yet, as highlighted by Upson et al [10], ferns and lycophytes have yet to follow suit. Further, at the present time, little is known about the interactions between streptophyte algae and their phytopathogens.…”

Sticking to it: phytopathogen effector molecules may converge on evolutionarily conserved host targets in green plants

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Are effectors deployed during interactions with early diverging land plants?

Our current understanding of plant–microbe interactions is heavily skewed toward angiosperm models, with comparatively less known about how microbes interact with early diverging land plant lineages. This is especially true for plant–pathogen interactions, which are under-represented compared to interactions between early diverging land plants and symbiotic microbes [44,59]. Evidence for the direct action of effector molecules in early diverging lineages is lacking, yet several lines of evidence suggest that phytopathogens use effectors to manipulate these plants.

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Phytophthora palmivora establishes tissue-specific intracellular infection structures in the earliest divergent land plant lineage

“…Whether this dynamic was established early during the coevolution of plants and microbes is unknown. Extensive evidence has demonstrated that early divergent land plants can accommodate arbuscules within their cells ( 7 , 17 ), whereas specialized pathogenic structures like haustoria have not been observed in these plants.…”

“…This suggests that liverworts represent the earliest divergent land plant lineage, although this has yet to yield a consensus among the field ( 20 , 21 ). Many bryophytes are colonized by symbiotic microbes; however, intracellular endosymbiotic structures (arbuscules) have been observed only in liverworts and hornworts ( 7 , 17 , 19 ). Unfortunately, our understanding of plant–pathogen interactions in these plants is extremely limited.…”