Pathogens of green roof mosses and the use of a Physcomitrella mutant collection as a source for elucidating genes involved in the chitosan-induced signaling pathway

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http://urn.fi/URN:ISBN:978-951-51-6468-1
Title: Pathogens of green roof mosses and the use of a Physcomitrella mutant collection as a source for elucidating genes involved in the chitosan-induced signaling pathway
Author: Marttinen, Eeva
Contributor: University of Helsinki, Faculty of Agriculture and Forestry, Department of Agricultural Sciences
Doctoral Programme in Integrative Life Science
Publisher: Helsingin yliopisto
Date: 2020-09-24
Language: en
Belongs to series: URN:ISSN:2342-317X
URI: http://urn.fi/URN:ISBN:978-951-51-6468-1
http://hdl.handle.net/10138/318301
Thesis level: Doctoral dissertation (article-based)
Abstract: Knowledge of the defense responses of mosses against pathogens has gained less attention than has our knowledge of the pathogens of vascular plants. Recently, the use of mosses has gained attention because mosses are used for greenhouse production as well as landscaping. In particular, the landscaping and greening of buildings have become popular because these initiatives offer one solution for mitigating urban problems such as heat islands and flooding. Mosses are an easy and lightweight solution for greening purposes as they can survive the harsh rooftop environment and have great stormwater retention. However, the health of plants is fundamental to achieving the benefits of greening. Like vascular plants, mosses also are susceptible to plant diseases. Many fungi damage mosses by causing brown patches of greenish moss. Brown patches on mosses are a characteristic sign of fungal infection. However, plants have various defense mechanisms, the first of which consists of preexisting structural and chemical defenses. Second, the plant immune system uses specific receptors with which to recognize the molecular structures of microbes that are not present on the surface of the plant's own cells. Receptor-mediated sensing of these structures can trigger early defense responses of plant, which can make the plant resistant to the attacking microbe. The model moss Physcomitrella patens, like the vascular plants, senses the molecular structures of microbes. For example, exposure of P. patens to chitosan—a component of the fungal cell wall—significantly increases peroxidase activity and oxygen radical formation. Oxygen radicals in turn affect many biological events; they can directly damage the pathogen or stimulate the plant's defenses. Currently, little is known about the peroxidase-based defense as well as chitosan-induced signaling pathways of P. patens. The aim of the research presented in this thesis was to study the pathogens of green roof mosses and establishes the host range of the isolated pathogens; the study also utilized a Physcomitrella mutant collection to identify genes involved in chitosan-induced signaling pathway. Fungal species that naturally inhabit mosses at a moss farm in Japan and on green roof environments in southern Finland were isolated and the ability of these fungi to infect and cause disease symptoms on the model moss P. patens was tested. In addition, the pathogenicity of fungus species towards vascular plants was also assessed. To elucidate the genes involved in chitosan-induced peroxidase activity, part of the Physcomitrella mutant collection was screened using the oxidation of 2,2´-azino-bis(3-ethylbenzothiazoine-6-sulfonic acid) as an indicator of peroxidase activity. Genome walking analyses were used to identify which genes were mutated within each moss line of interest. The work described in this thesis demonstrates that mosses used for greening are colonized by many different fungal isolates. These studies reveal that several fungal genera such as Fusarium, Trichoderma, Phoma and Alternaria cause severe symptoms in P. patens. Moreover, our results demonstrate that mosses and vascular plants have common pathogens. The fungal isolates Fusarium avenaceum and Cladosporium oxysporum obtained from moss panels caused disease symptoms on barley and carrots, respectively, and also on two different moss species. The results also demonstrate that the Physcomitrella mutant collection is a valuable source for identifying genes involved in the chitosan-induced signaling pathway. Screening of part of the mutant collection and further analyses revealed that Rossmann fold protein is a significant part of the signaling chain leading to upregulated peroxidase activity induced by chitosan. In addition, this Rossmann fold protein is an important factor for normal lipoxygenase (LOX) expression and might contribute to defense against fungal pathogens. The results from this doctoral thesis provide new insights into the pathogens of green roofs, the host range of the pathogens and the molecular mechanisms involved in disease control and defense responses in moss. The knowledge gained concerning the pathogenicity of Trichoderma isolates and the host range of pathogenic fungi should be considered when planting moss farms and cultivating crops in close proximity to each other or when applying biological control agents containing Trichoderma species to green roofs. Furthermore, these results may encourage the use of the Physcomitrella mutant collection to identify candidate genes for signaling pathways to elucidate the molecular mechanisms underlying the defense responses of mosses.Viherrakentamisen avulla voidaan lievittää hellesaarekeilmiön ja hulevesien aiheuttamia ongelmia kaupunkiympäristössä. Sammalet ovat helppo ja kevyt ratkaisu viherrakentamiseen. Sammalet voivat selviytyä katolla, jossa elinolosuhteet voivat olla ajoittain hyvinkin ankarat ja lisäksi sammalet pidättävät kasvustoonsa merkittäviä määriä sadevesiä. Kuten putkilokasvit, myös sammalet sairastuvat kasvitauteihin. Kasveilla on monenlaisia puolustusmekanismeja. Kasvit tunnistavat spesifisten reseptorien välityksellä mikrobien molekulaarisia rakenteita, joita ei ilmene kasvin omien solujen pinnoilla. Näiden rakenteiden tunnistaminen voi käynnistää puolustusvasteita, jotka saattavat tehdä kasvin taudinkestäväksi. Malliorganismina käytetty nuppusammal aistii myös mikrobien molekulaarisia rakenteita. Nuppusammalen altistaminen kitosaanille, sienen soluseinän komponentille lisää merkittävästi peroksidaasiaktiivisuutta ja happiradikaalien muodostumista. Tutkimuksen tavoitteena oli määrittää sammalviherkattojen taudinaiheuttajia sekä eristettyjen taudinaiheuttajien isäntälajikirjoa. Lisäksi väitöskirjatutkimuksessa käytettiin nuppusammalen mutanttikokoelmaa kitosaanin indusoiman signaalireittiin osallistuvien geenien identifioimiseksi. Väitöskirjatyön tulokset osoittavat, että viheriöittämisessä käytetyt sammallajit asuttavat monia sieni-isolaatteja, joista Fusarium, Trichoderma, Phoma ja Alternaria -sukuihin kuuluvat sienet voivat aiheuttaa vakavia oireita nuppusammalelle. Tutkimuksissa havaittiin myös, että sammalilla ja putkilokasveilla on yhteisiä taudinaiheuttajia. Tulokset osoittavat, että nuppusammalen mutanttikokoelma on arvokas lähde kitosaanin indusoiman signaalireitin osallistuvien geenien tunnistamiseen. Mutanttikokoelman osittaisella seulonnalla osoitettiin, että proteiini, jossa on Rossmann-laskos, on merkittävä osa signaalireittiä, joka johtaa kitosaanin indusoimaan peroksidaasiaktiivisuuteen. Lisäksi tämä proteiini on tärkeä tekijä normaalissa lipoksigenaasin (LOX) ilmentymisessä. Väitöskirjatutkimuksen tulokset antavat uutta tietoa viherkattojen taudinaiheuttajista, niiden isäntälajikirjosta sekä molekyylimekanismeista, jotka liittyvät taudinpuolustukseen. Trichoderma-isolaattien taudinaiheuttamiskyvystä saatua tietoa voidaan hyödyntää käytettäessä Trichoderma-lajeja sisältäviä biologisia kasvinsuojeluaineita viherkatoilla. Lisäksi väitöskirjatyön tulokset voivat rohkaista nuppusammalen mutanttikokoelman käyttöä geenien tunnistamiseksi signalointireiteille sammalten puolustusvasteiden taustalla olevien molekyylimekanismien selvittämiseksi.
Subject: kasvipatologia
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