signaling pathways for their own benefit. In addition to effectors, microbial pathogens interfere with plant defense responses by producing plant hormones.
29.5 Adaptation Mechanisms in Bryophytes
and Microorganisms
Information regarding the cellular and molecular mechanisms operating in interactions
of bryophytes and their associated microorganisms is quite limited. However, in recent
years, our reference species of bryophytes have allowed us to make important
advances, mainly with understanding interactions between plants and pathogenic
microorganisms. The moss Physcomitrium patens (previously Physcomitrella patens;
P. patens) and the liverwort Marchantia polymorpha (M. polymorpha) are currently
used by several research groups to reveal ancient mechanisms in plant biology. They
have several additional advantages compared to other bryophytes, including available
genomic resources, targeted mutagenesis, and standardized protocols for genetic
transformation. This allows the identification of crucial components in different
types of bryophyte–microorganism interactions, including microbial colonization
strategies and plant defense mechanisms to cope with them.
What we have known until present is that as traqueophytes, P. patens and
M. polymorpha activate a defense response after pathogen invasion, with that
response leading to cell wall associated defenses, increased levels of hormones,
and expression of genes that encode proteins with different roles in plant resistance
(Ponce de León and Montesano 2017; Carella et al. 2019). Some of these key
defense components are conserved between traqueophytes and bryophytes,
suggesting that they were probably present in the common ancestor of land plants.
For example, traqueophytes have developed a system for sensing pathogens by
monitoring the cell wall integrity, upon which they activate reinforcement of the
cell wall, which importantly is required to prevent disease. Receptors at the plasma
membrane can sense oligogalacturonides derived from the degradation of the cell
wall by the action of microbial cell wall degrading enzymes (CWDEs). In P. patens,
treatment with CWDEs and effectors of the bacterial pathogen Pectobacterium
caratovorum activate a signaling cascade, suggesting a functional sensing mechanism. This leads to plant cell wall reinforcement by incorporation of phenolic
compounds and callose to stop further damage. Similarly, Phytopthora palmivora
infection of M. polymorpha involves the induction of a large set of genes encoding
CWDEs, and consequently callose deposition occurs to prevent colonization. Fossil
records have shown the presence of thickened cell walls in tissues of vascular plants
infected with fungal structures 400 Mya, demonstrating that cell wall fortification is
an ancient defense response.
Several hormones involved in traqueophyte defense against microbial pathogens
are also present in bryophytes where they play a role in immunity, including the
hormones salicylic acid (SA) and auxin. Both P. patens and M. polymorpha increase
29 Coevolution of Bryophytes and their Associated Microorganisms
631
29.5 Adaptation Mechanisms in Bryophytes
and Microorganisms
Information regarding the cellular and molecular mechanisms operating in interactions
of bryophytes and their associated microorganisms is quite limited. However, in recent
years, our reference species of bryophytes have allowed us to make important
advances, mainly with understanding interactions between plants and pathogenic
microorganisms. The moss Physcomitrium patens (previously Physcomitrella patens;
P. patens) and the liverwort Marchantia polymorpha (M. polymorpha) are currently
used by several research groups to reveal ancient mechanisms in plant biology. They
have several additional advantages compared to other bryophytes, including available
genomic resources, targeted mutagenesis, and standardized protocols for genetic
transformation. This allows the identification of crucial components in different
types of bryophyte–microorganism interactions, including microbial colonization
strategies and plant defense mechanisms to cope with them.
What we have known until present is that as traqueophytes, P. patens and
M. polymorpha activate a defense response after pathogen invasion, with that
response leading to cell wall associated defenses, increased levels of hormones,
and expression of genes that encode proteins with different roles in plant resistance
(Ponce de León and Montesano 2017; Carella et al. 2019). Some of these key
defense components are conserved between traqueophytes and bryophytes,
suggesting that they were probably present in the common ancestor of land plants.
For example, traqueophytes have developed a system for sensing pathogens by
monitoring the cell wall integrity, upon which they activate reinforcement of the
cell wall, which importantly is required to prevent disease. Receptors at the plasma
membrane can sense oligogalacturonides derived from the degradation of the cell
wall by the action of microbial cell wall degrading enzymes (CWDEs). In P. patens,
treatment with CWDEs and effectors of the bacterial pathogen Pectobacterium
caratovorum activate a signaling cascade, suggesting a functional sensing mechanism. This leads to plant cell wall reinforcement by incorporation of phenolic
compounds and callose to stop further damage. Similarly, Phytopthora palmivora
infection of M. polymorpha involves the induction of a large set of genes encoding
CWDEs, and consequently callose deposition occurs to prevent colonization. Fossil
records have shown the presence of thickened cell walls in tissues of vascular plants
infected with fungal structures 400 Mya, demonstrating that cell wall fortification is
an ancient defense response.
Several hormones involved in traqueophyte defense against microbial pathogens
are also present in bryophytes where they play a role in immunity, including the
hormones salicylic acid (SA) and auxin. Both P. patens and M. polymorpha increase
29 Coevolution of Bryophytes and their Associated Microorganisms
631