Root endosymbioses vitally contribute to plant nutrition and fitness worldwide. Nitrogen-fixing root nodulation, confined to four plant orders, encompasses two distinct types of associations, the interaction of legumes (Fabales) with rhizobia bacteria and actinorhizal symbioses, where the bacterial symbionts are actinomycetes of the genus Frankia. Although several genetic components of the host-symbiont interaction have been identified in legumes, the genetic basis of actinorhiza formation is unknown. Here, we show that the receptor-like kinase gene SymRK, which is required for nodulation in legumes, is also necessary for actinorhiza formation in the tree Casuarina glauca. This indicates that both types of nodulation symbiosis share genetic components. Like several other legume genes involved in the interaction with rhizobia, SymRK is also required for the interaction with arbuscular mycorrhiza (AM) fungi. We show that SymRK is involved in AM formation in C. glauca as well and can restore both nodulation and AM symbioses in a Lotus japonicus symrk mutant. Taken together, our results demonstrate that SymRK functions as a vital component of the genetic basis for both plant-fungal and plant-bacterial endosymbioses and is conserved between legumes and actinorhiza-forming Fagales.actinorhizal symbioses ͉ Casuarina glauca ͉ mycorrhizae ͉ signaling R oot endosymbioses are associations between plants and soil microorganisms involving intracellular accommodation of microbes within host cells. The most widespread of these associations is arbuscular mycorrhiza (AM), which is formed by the majority of land plants with fungi belonging to the phylum Glomeromycota (1). In contrast, nitrogen-fixing nodulation symbioses of plant roots and bacteria are restricted to four orders of eurosid dicots (2). Actinorhiza, formed by members of the Fagales, Rosales, and Cucurbitales with Gram-positive Frankia bacteria, differs from the interaction of legumes with Gramnegative rhizobia in several morphological and cytological aspects (3). Although these differences suggest independent regulatory mechanisms, the close relatedness of nodulating lineages indicates a common evolutionary basis of root nodulation symbioses (2). In the legume-rhizobia interaction, among the key factors mediating recognition between the plant and the bacteria are Nod factors (NFs). NFs are bacterial lipochitooligosaccharides with an N-acetylglucosamine backbone (4). The perception of NFs induces a series of responses in host roots, including ion flux changes and membrane depolarization, rhythmic calcium oscillations in and around the nucleus (calcium spiking), cytoskeletal modifications and root hair curling, and activation of cortical cell divisions (5). Extensive mutant screenings performed in legumes led to the identification of several loci involved in this signaling cascade, and recently most of the corresponding genes were identified by map-based approaches (6). In Lotus japonicus, two genes, NFR1 and NFR5 encoding receptor-like serine/threonine kinases with L...

Nitrogen-fixing root nodule symbioses (RNS) occur in two major forms—Actinorhiza and legume-rhizobium symbiosis—which differ in bacterial partner, intracellular infection pattern, and morphogenesis. The phylogenetic restriction of nodulation to eurosid angiosperms indicates a common and recent evolutionary invention, but the molecular steps involved are still obscure. In legumes, at least seven genes—including the symbiosis receptor-kinase gene SYMRK—are essential for the interaction with rhizobia bacteria and for the Arbuscular Mycorrhiza (AM) symbiosis with phosphate-acquiring fungi, which is widespread in occurrence and believed to date back to the earliest land plants. We show that SYMRK is also required for Actinorhiza symbiosis of the cucurbit Datisca glomerata with actinobacteria of the genus Frankia, revealing a common genetic basis for both forms of RNS. We found that SYMRK exists in at least three different structural versions, of which the shorter forms from rice and tomato are sufficient for AM, but not for functional endosymbiosis with bacteria in the legume Lotus japonicus. Our data support the idea that SYMRK sequence evolution was involved in the recruitment of a pre-existing signalling network from AM, paving the way for the evolution of intracellular root symbioses with nitrogen-fixing bacteria.

Two years of field sampling aimed to establish the predominance and association among the fungal pathogens causing Fusarium ear blight (FEB) in four European countries (Hungary, Ireland, Italy and the UK). A PCR-based method was used to detect four Fusarium species and two varieties of Microdochium nivale present in the samples. The prevalence of FEB pathogens differed significantly between countries. Overall, all pathogens were commonly detected in Ireland and to a lesser extent in the UK. In contrast, only two species, F. graminearum and F. poae, were regularly detected in Italy and Hungary. Fusarium culmorum was rarely detected except in Ireland. Log-linear models were used to determine whether there is the independence of the six FEB pathogens at each sampling site. Significant two-pathogen interactions were frequently observed, particularly in harvest samples; all these significant two-pathogen interactions were of the synergistic type, except between F. poae and F. culmorum, and were generally consistent over the 2 years and four countries. Fusarium graminearum and F. poae were least frequently involved in two pathogen interactions but were involved in most of the nine significant three-pathogen interactions. However, only the interaction between F. graminearum, F. avenaceum and F. poae was significant in both years. Potential implications of the present results in FEB management are discussed.

During sclerotial infection of Sclerotinia sclerotiorum the mycoparasite Coniothyrium minitans penetrates through the host cell wall, which contains ␤-1,3-glucan as its major component. A PCR-based strategy was used to clone a ␤-1,3-glucanase-encoding gene, designated cmg1, from a cDNA library of the fungus. The nucleotide and deduced amino acid sequences of this gene showed high levels of similarity to the sequences of other fungal exo-␤-1,3-glucanase genes. The calculated molecular mass of the deduced protein (without the predicted 24-amino-acid N-terminal secretion signal peptide) was 83,346 Da, and the estimated pI was 4.73. Saccharomyces cerevisiae INVSc1 expressing the cmg1 gene secreted a ϳ100-kDa ␤-1,3-glucanase enzyme (as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) into the culture medium. N-terminal sequence analysis of the purified recombinant enzyme revealed that the secreted enzyme starts at Ala-32, seven amino acids downstream from the predicted signal peptidase cleavage site. The purified recombinant glucanase inhibited in vitro mycelial growth of S. sclerotiorum by 35 and 85% at concentrations of 300 and 600 g ml ؊1 , respectively. A single copy of the cmg1 gene is present in the genome of C. minitans. Northern analyses indicated increases in the transcript levels of cmg1 due to both carbon starvation and the presence of ground sclerotia of S. sclerotiorum; only slight repression was observed in the presence of 2% glucose. Expression of cmg1 increased during parasitic interaction with S. sclerotiorum.