Establishment of a functional symbiosis between the cyanobacterium Nostoc punctiforme and the bryophyte Anthoceros punctatus requires genes involved in nitrogen control and initiation of heterocyst differentiation The GenBank accession numbers for the coding regions of ntcA, hetR and hetF are AY038370, AF318069 and AF288130, respectively.

Abstract

Three mutant strains (ntcA, hetR, hetF) of the cyanobacterium Nostoc punctiforme unable to differentiate heterocysts were characterized and examined for their ability to form a symbiotic association with the bryophyte Anthoceros punctatus. Previously unknown characteristics of the N. punctiforme hetR mutant include differentiation of chilling-resistant akinetes, while vegetative cells of the ntcA mutant randomly lysed, yielding short filaments, following ammonium deprivation. Strains with mutations in hetF and hetR infected A. punctatus with similar frequency to that of wild-type N. punctiforme but did not support growth of the plant partner. These results confirm that the infection of A. punctatus by hormogonia leading to the establishment of an association is physiologically uncoupled from the development of a functional diazotrophic association. They also indicate that heterocyst regulatory elements downstream from HetR and HetF are required in both free-living and symbiotic heterocyst differentiation and nitrogenase expression. A strain with a mutation in the global nitrogen regulator ntcA did not infect A. punctatus despite its ability to differentiate hormogonia at a low frequency. When complemented with one or more copies of ntcA, the mutant strain infected A. punctatus at a similar frequency as the wild-type and supported growth of the plant partner in the absence of combined nitrogen. These results established a connection between the presence of a functional copy of ntcA and the magnitude of hormogonium differentiation, and the behaviour of the formed hormogonia.

Regulation of Cellular Differentiation in Filamentous Cyanobacteria in Free-Living and Plant-Associated Symbiotic Growth States

“…The nature of the plant-derived signal and where it might interact with the developmental cascade of heterocyst differentiation is unknown. Results of analysis of N. punctiforme mutants in the Anthoceros association have established that hetR is essential for symbiotic heterocyst differentiation; therefore, the signal must enter the developmental pathway prior to hetR (217). It is likely that the symbiotic signal enters the pathway prior to activation of NtcA (Fig.…”

“…sigH is not transcribed under various stress or vegetative growth conditions, with or without combined nitrogen, but it is specifically induced in a burst pattern between 1.5 and 6 h following exposure to Anthoceros conditioned medium containing HIF. The genes whose transcription (217). While the ntcA mutant forms hormogonia following shifts in growth conditions, its response to HIF is markedly less strong than that of wild-type cultures and the hormogonia formed by either method fail to infect Anthoceros tissue.…”

“…7), because NtcA appears to be essential for the transcription of genes expressed late in heterocyst maturation and for nitrogenase function (e.g., excision of the nifD element [200]). Interestingly, mutants of N. punctiforme defective in ntcA fail to infect Anthoceros tissue; therefore, their ability to respond to plant signals influencing symbiotic heterocyst differentiation cannot readily be determined (217).…”

A Putative O-Linked β- N -Acetylglucosamine Transferase Is Essential for Hormogonium Development and Motility in the Filamentous Cyanobacterium Nostoc punctiforme

“…In both hormogonium-forming and non-hormogoniumforming strains, motile filaments perform phototaxis to seek out favorable light environments (5) and can establish supracellular structures that arise from the movement of individual filaments within conglomerates (2,6). In certain heterocyst-forming species, such as Nostoc punctiforme, hormogonia serve as the infective unit for establishing symbioses with plants and fungi, after which nitrogen-fixing heterocysts provide the eukaryotic partner with a source of fixed nitrogen (2,7,8). These symbioses are major contributors to global nitrogen fixation (9).…”

Molecular Analysis of Genes in Nostoc punctiforme Involved in Pilus Biogenesis and Plant Infection

“…To investigate the importance of functional pili in the establishment of symbiosis, the frequency of infection was estimated after coculture with the host bryophyte Blasia pusilla (Table 2) (22). Reconstitution of symbiotic associations was performed as described previously (60).…”