the synthesis of SA after pathogen infection, leading to the activation of defense
genes. The SA pathways could have been present in the common ancestor of land
plants and played a role in defense since all genes of this pathway are present in
bryophytes but not in algae. A remarkable discovery was the difference of the
jasmonic acid (JA) pathway between bryophytes and traqueophytes, starting from
the fact that the active hormone (JA conjugated to isoleucine; JA-Ile) is not produced
in bryophytes. In traqueophytes, JA-Ile binds to the receptor COI1 (coronatineinsensitive 1) and activates a signaling pathway that reprogrammed the expression
of hundreds of genes, while in M. polymorpha COI1 recognizes to its precursor,
dinor oxophytodienoic acid (dinor-OPDA) (Monte et al. 2018). The reason for the
evolutionary change in ligand specificity is a single amino acid substitution in COI1.
Interestingly, dinor-OPDA play a role in bryophyte defense like JA-Ile in
traqueophytes, and both hormones have an antagonistic effect with the SA pathway.
Thus, ancient features of the jasmonate pathway were already present in early land
plants.
Several evidences demonstrate that pathogen effectors influence bryophyte
defense responses. For example, P. palmivora forms digit-like protruding structures
within M. polymorpha living cells which invaginate, thereby forming haustoria-like
structures that as in traqueophytes release effectors, mainly RXLRs (the family of
phytopathogen effector proteins termed RXLR is defined by a secretion signal
peptide followed by a conserved N-terminal domain which contains the sequence
Arg-Xaa-Leu-Arg) (Carella et al. 2018). The expression of RXLR is associated with
successful colonization events in the liverwort, demonstrating the importance of
these molecules for pathogenic lifestyle. Pseudomonas syringae effectors target
M. polymorpha defenses to provoke disease by interacting with receptors that
sense its presence, by interfering with the SA response and by repressing host
defense gene expression (Gimenez-Ibanez et al. 2019).
The recently gained knowledge on bryophyte defense mechanisms and pathogen
infection strategies is just starting to reveal for us a very small piece of the complete
picture of these interactions from the perspective of both organisms. Further research
on these descendants of the first land plants and their associated microorganisms will
certainly advance and improve our understanding on how bryophytes–microorganism interactions coevolved.
Guillermo Reboledo
632
G. Reboledo and I. Ponce de León
genes. The SA pathways could have been present in the common ancestor of land
plants and played a role in defense since all genes of this pathway are present in
bryophytes but not in algae. A remarkable discovery was the difference of the
jasmonic acid (JA) pathway between bryophytes and traqueophytes, starting from
the fact that the active hormone (JA conjugated to isoleucine; JA-Ile) is not produced
in bryophytes. In traqueophytes, JA-Ile binds to the receptor COI1 (coronatineinsensitive 1) and activates a signaling pathway that reprogrammed the expression
of hundreds of genes, while in M. polymorpha COI1 recognizes to its precursor,
dinor oxophytodienoic acid (dinor-OPDA) (Monte et al. 2018). The reason for the
evolutionary change in ligand specificity is a single amino acid substitution in COI1.
Interestingly, dinor-OPDA play a role in bryophyte defense like JA-Ile in
traqueophytes, and both hormones have an antagonistic effect with the SA pathway.
Thus, ancient features of the jasmonate pathway were already present in early land
plants.
Several evidences demonstrate that pathogen effectors influence bryophyte
defense responses. For example, P. palmivora forms digit-like protruding structures
within M. polymorpha living cells which invaginate, thereby forming haustoria-like
structures that as in traqueophytes release effectors, mainly RXLRs (the family of
phytopathogen effector proteins termed RXLR is defined by a secretion signal
peptide followed by a conserved N-terminal domain which contains the sequence
Arg-Xaa-Leu-Arg) (Carella et al. 2018). The expression of RXLR is associated with
successful colonization events in the liverwort, demonstrating the importance of
these molecules for pathogenic lifestyle. Pseudomonas syringae effectors target
M. polymorpha defenses to provoke disease by interacting with receptors that
sense its presence, by interfering with the SA response and by repressing host
defense gene expression (Gimenez-Ibanez et al. 2019).
The recently gained knowledge on bryophyte defense mechanisms and pathogen
infection strategies is just starting to reveal for us a very small piece of the complete
picture of these interactions from the perspective of both organisms. Further research
on these descendants of the first land plants and their associated microorganisms will
certainly advance and improve our understanding on how bryophytes–microorganism interactions coevolved.
Guillermo Reboledo
632
G. Reboledo and I. Ponce de León