<br/>Plants that can be nodulated by actinomycetes of the genus <em>Frankia</em> are collectively called actinorhizal plants and comprise mostly woody plant species. Compared to <em>Rhizobium-</em> legume interactions, <em></em> actinorhizal symbioses are poorly understood, especially in their molecular aspects. The goal of the research described in this thesis is to study plant gene expression during the development and function of actinorhizal nodules of <em>Alnus glutinosa,</em> by characterizing cDNA clones isolated from a nodule cDNA library, Chapter 1 gives an overview about the development and functioning of actinorhizal nodules, in comparison with legume- <em>Rhizobium</em> interactions.<p>By differential screening, several <em>A. glutinosa</em> cDNA clones were isolated, representing genes expressed at markedly elevated levels in actinorhizal nodules compared to roots. These cDNAs were found to encode products involved in nitrogen metabolism (chapter 2), a hitherto unknown metabolic pathway (chapter 3), and senescence (chapters 4 and 5).<p>Like in legume nodules, ammonium assimilation in actinorhizal nodules is performed by the common glutamine synthetase (GS)/glutamate synthase (GOGAT) pathway. The exported form of fixed nitrogen in <em>Alnus</em> nodules is citrulline. Two cDNA clones isolated were found to encode products related to nitrogen metabolism. pAg 11 encoded a glutamine synthetase (GS), the key enzyme responsible for ammonium assimilation; pAg 118 encoded an acetylornithine transaminase (AOTA) which is involved in the biosynthesis of citrulline (chapter 2). By determining their sites of expression new insight was gained in reassimilation of ammonium in actinorhizal nodules.<p>One nodule-specific clone, pAg 135, was found to encode a polypeptide homologous to a fatty acid reductase, but since fatty alcohols are not found in <em>A. glutinosa</em> nodules it remains to be examined in which metabolic pathway Ag 135 is active (chapter 3).<p>A cDNA (pAg13) encoding a proline-rich polypeptide was also isolated. Apart from proline, the potential mature peptide was also rich in glutamic acid. <em>In situ</em> hybridization showed that this gene was expressed in infected cells during endosymbiont degradation and in the nodule pericycle (chapter 4). Ineffective root nodules that cannot fix nitrogen because the <em>Frankia</em> bacteria do not form vesicles, can be induced by certain <em>Frankia</em> strains on <em>A. glutinosa.</em> They represent compact structures and contain higher amounts of polyphenols than the effective nodules. A comparison of <em>ag13</em> expression between effective and ineffective nodules of <em>A. glutinosa</em> is presented, implying that ag13 expression is indeed correlated with senescence (chapter 5).<p>So far, about 15 nodule-specific/enhanced cDNA clones have been isolated and identified in <em>Alnus glutinosa</em> nodules. In chapter 6, the results currently achieved in plant molecular studies on <em>Alnus glutinosa</em> nodules are summarized and discussed.
|Qualification||Doctor of Philosophy|
|Award date||12 Jun 1996|
|Place of Publication||S.l.|
|Publication status||Published - 1996|
- root nodules
- plant diseases
- alnus glutinosa