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Cyanobacteria in symbiosis with plants: Protein patterns and regulatory mechanisms
Stockholm University, Faculty of Science, Department of Botany.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cyanobacteria belonging to the heterocystous genus Nostoc, capable of oxygenic photosynthesis and nitrogen fixation via the enzyme nitrogenase, may form symbiotic associations with plants. In these associations most of the N2 fixed by the symbiotic cyanobacteria (cyanobiont) is transferred to the host, which in turn supplies the cyanobiont with fixed carbon. By using a proteomic approach adapted for investigating unsequenced organisms, adaptations of the cyanobiont to symbiotic conditions within the angiosperm Gunnera, for which each generation needs to be newly infected, and the water fern Azolla, which is in permanent association with its cyanobiont, were investigated.

Despite morphological and physiological modifications of the cyanobionts, many basic functions appear to be intact in symbiosis compared to when free-living, as indicated by similar protein levels. Some differences were identified however, and in the view of parallel studies on photoautotrophic and heterotrophic growth of free-living cyanobacteria, these indicated that cellular functions were focused on N2 fixation and the associated heterocyst specific metabolism, and also reflected a mainly heterotrophic growth. Stress responses were induced in both cyanobionts, while surface adaptations mainly in that of Gunnera, possibly a reflection of its intracellular location in combination with the microaerobic and dark conditions inside the Gunnera glands. The heterocyst envelope was reduced, which may be involved in ammonia release. The level of nitrogenase was considerably higher in the Azolla cyanobiont, potentially reflecting a co-evolution with its host plant. The results also indicate that the Azolla cyanobiont may be classified as a new genus. Probably induced by oxygen, some nitrogenase in the Azolla cyanobiont carried a post-translational modification, located within a specific peptide corresponding to the part of nitrogenase that is ADP-ribosylated in certain other N2-fixing bacteria. However, the modification, with a mass of 300-400 Da, was not identified.

The regulation behind heterocyst differentiation, N2-fixation and N-assimilation in symbiosis was also investigated. The mechanisms involving the regulatory proteins NtcA and HetR appear to be intact in symbiosis but distinctly upregulated, generating the higher heterocyst frequencies observed. This upregulation may be induced by a high C:N ratio in symbiosis or a plant effector molecule. These results also indicate that glutamine synthetase levels are reduced in symbiosis by a separate, but unknown mechanism. A sugar uptake regulator located near the hrm hormogonium repressing operon may be involved in carbohydrate uptake in the Gunnera symbiosis. Expression of isoenzymes of glyceraldehyde-3-phosphate dehydrogenase and fructose-1,6-bisphosphatase and a possible redox regulation of certain enzymes may be involved in regulation of metabolic pathways in symbiotic as well as in free-living cyanobacteria. Potential host-induced mechanisms responsible for cyanobiont adaptations, other than the environment offered in symbiosis, remain to be identified.

Place, publisher, year, edition, pages
Stockholm: Botaniska institutionen , 2005. , 50 p.
Keyword [en]
Cyanobacteria, symbiosis, Nostoc, Gunnera, Azolla, nitrogenase, modification, heterotrophic, proteomics, unsequenced
National Category
Botany
Identifiers
URN: urn:nbn:se:su:diva-693ISBN: 91-7155-145-X (print)OAI: oai:DiVA.org:su-693DiVA: diva2:197330
Public defence
2005-11-03, föreläsningssalen, Botanicum, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript. Available from: 2005-10-13 Created: 2005-10-13 Last updated: 2009-12-17Bibliographically approved
List of papers
1. Expression of ntcA, glnB, hetR and nifH and corresponding protein profiles along a developmental sequence in the Nostoc-Gunnera symbiosis
Open this publication in new window or tab >>Expression of ntcA, glnB, hetR and nifH and corresponding protein profiles along a developmental sequence in the Nostoc-Gunnera symbiosis
2004 (English)In: Molecular Plant Microbe Interactions, ISSN 0894-0282, Vol. 17, no 4, 436-443 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24355 (URN)
Available from: 2005-10-13 Created: 2005-10-13 Last updated: 2009-12-17Bibliographically approved
2. Proteomic analyses of the photoauto- and diazotrophically grown cyanobacterium Nostoc sp. PCC 73102
Open this publication in new window or tab >>Proteomic analyses of the photoauto- and diazotrophically grown cyanobacterium Nostoc sp. PCC 73102
Show others...
2007 (English)In: Microbiology, ISSN 1350-0872, Vol. 153, 608-618 p.Article in journal (Refereed) Published
Abstract [en]

The filamentous cyanobacteria of the genus Nostoc are globally distributed, phenotypically complex organisms, capable of cellular differentiation and of forming symbiotic associations with a wide range of plants. To further our understanding of these processes and functions, the proteome of photoautotrophically and diazotrophically grown Nostoc sp. PCC 73102 (N. punctiforme) cells was examined. Extracted proteins were separated into membrane and soluble protein fractions and analysed using two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). The analysis led to the identification of 82 proteins that could be divided into 12 functional categories. Significantly, 65 of these proteins have not been previously documented in the Nostoc proteome. Many of the proteins identified were readily recognized as housekeeping proteins involved in carbon, nitrogen and energy metabolism, but a number of proteins related to stress, motility, secretion and post-translational modifications were also identified. Ten unclassified proteins were also detected, representing potential novel functions. These proteins were highly expressed, suggesting that they play key roles during photoautotrophic and diazotrophic growth. Nineteen of the proteins expressed under the growth conditions examined contained putative thioredoxin (Trx) targets, a motif that functions in redox regulation via redox equivalent mediators and is known to be significant in a wide range of biological processes. These observations contribute to our understanding of the complex Nostoc life cycle.

Identifiers
urn:nbn:se:su:diva-24356 (URN)10.1099/mic.0.29198-0 (DOI)000244422700034 ()
Available from: 2005-10-13 Created: 2005-10-13 Last updated: 2009-12-17Bibliographically approved
3. Protein expression profiles in an endosymbiotic cyanobacterium revealed by a proteomic approach
Open this publication in new window or tab >>Protein expression profiles in an endosymbiotic cyanobacterium revealed by a proteomic approach
2006 (English)In: Molecular Plant-Microbe Interactions, ISSN 0894-0282, Vol. 19, no 11, 1251-1261 p.Article in journal (Refereed) Published
Abstract [en]

Molecular mechanisms behind adaptations in the cyano-bacterium (Nostoc sp.) to a life in endosymbiosis with plants are still not clarified, nor are the interactions between the partners. To get further insights, the proteome of a Nostoc strain, freshly isolated from the symbiotic gland tissue of the angiosperm Gunnera manicata Linden, was analyzed and compared with the proteome of the same strain when free-living. Extracted proteins were separated by two-dimensional gel electrophoresis and were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry combined with tandem mass spectrometry. Even when the higher percentage of differentiated cells (heterocysts) in symbiosis was compensated for, the majority of the proteins detected in the symbiotic cyanobacteria were present in the free-living counterpart, indicating that most cellular processes were common for both stages. However, differential expression profiling revealed a significant number of proteins to be down-regulated or missing in the symbiotic stage, while others were more abundant or only expressed in symbiosis. The differential protein expression was primarily connected to i) cell envelope-associated processes, including proteins involved in exopolysaccharide synthesis and surface and membrane associated proteins, ii) to changes in growth and metabolic activities (C and N), including upregulation of nitrogenase and proteins involved in the oxidative pentose phosphate pathway and downregu-lation of Calvin cycle enzymes, and iii) to the dark, micro-aerobic conditions offered inside the Gunnera gland cells, including changes in relative phycobiliprotein concentrations. This is the first comprehensive analysis of proteins in the symbiotic state.

Identifiers
urn:nbn:se:su:diva-24357 (URN)10.1094/MPMI-19-1251 (DOI)
Available from: 2005-10-13 Created: 2005-10-13 Last updated: 2009-12-17Bibliographically approved
4. Mass spectrometric analysis of the nitrogen-fixing cyanobacterium of the Azolla symbiosis: adaptation and NifH modification
Open this publication in new window or tab >>Mass spectrometric analysis of the nitrogen-fixing cyanobacterium of the Azolla symbiosis: adaptation and NifH modification
(English)Manuscript (Other academic)
Identifiers
urn:nbn:se:su:diva-24358 (URN)
Available from: 2005-10-13 Created: 2005-10-13 Last updated: 2009-12-17Bibliographically approved

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