|Ponce De Leon, Ines|
Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2009
Publication Date: 7/24/2009
Citation: Oliver, J.P., Castro, A., Gaggero, C., Cascon, T., Schmelz, E.A., Castresana, C., Leon, I.P. 2009. Pythium infection activates conserved plant defense responses in mosses. New Phytologist. 230: 569-579. Interpretive Summary: In response to attack by fungal pathogens, plants produce similar arrays of defensive biochemical changes to reduce growth and establishment of the offending agent. One way to examine the origin and evolution of plant defense responses to fungal pathogens is to look for commonalities and differences in more primitive plant lineages. Mosses, such as Physcomitrella patens, diverged from vascular plants approximately 700 million years ago and thus represent ideal models to compare against commonly researched plants. To answer these questions, the entire genome of Physcomitrella has now been sequenced yet very little is known about the response of mosses to biotic stresses. Working with the Instituto de Investigaciones Biológicas Clemente Estable (Department of Molecular Biology, Uruguay) scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that the moss Physcomitrella displays numerous induced defense similarities to vascular plants when attacked by the root rotting fungus Pythium. Shared responses include the increased production of reactive oxygen species (ROS), and defense gene transcripts including lipoxygenase (LOX), phenylalanine ammonia lyase (PAL) and chalcone synthase (CHS). Fatty acid derived phytohormone signals such as jasmonic acid and 12-oxo-phytodienoic acid were also significantly elevated within 24 hours of infection. Application of jasmonic acid strongly induced the accumulation of PAL transcripts and inhibited root growth, consistent with a shared and evolutionarily conserved role for these defense signals in both mosses and vascular plants.
Technical Abstract: The moss Physcomitrella patens (P. patens) is a useful model to study abiotic stress responses since it is highly tolerant to drought, salt and osmotic stress. However, little is known about the defense mechanisms activated in this moss after pathogen assault. Here the induction of defense responses in P. patens against the oomycetes Pythium irregulare and Pythium debaryanum was analyzed. P. patens activated multiple and similar defense responses against both Pythium species, including the reinforcement of the cell wall, induction of the defense genes CHS, LOX and PAL, and accumulation of the signaling molecules jasmonic acid (JA) and its precursor 12-oxo-phytodienoic acid (OPDA). Pythium infection induced reactive oxygen species production and caused cell death of moss tissues. Microscopic analysis also revealed intracellular relocation of chloroplasts in Pythium-infected tissues toward the cell wall located close to the infection site. In addition, JA and OPDA induced the expression of PAL and JA inhibited rhizoids growth. Our results show for the first time JA and OPDA accumulation in a moss and suggest that this defense pathway is functional and has been maintained during evolution. Moreover, Pythium infection activates in P. patens common responses to those characterized in vascular plants.