The Feather Moss Hylocomium splendens Affects the Transcriptional Profile of a Symbiotic Cyanobacterium in Relation to Acquisition and Turnover of Key Nutrients

Abstract

Moss-cyanobacteria symbioses were proposed to be based on nutrient exchange, with hosts providing C and S while bacteria provide N, but we still lack understanding of the underlying molecular mechanisms of their interactions. We investigated how contact between the ubiquitous moss Hylocomium splendens and its cyanobiont affects nutrient-related gene expression of both partners. We isolated a cyanobacterium from H. splendens and co-incubated it with washed H. splendens shoots. Cyanobacterium and moss were also incubated separately. After 1 week, we performed acetylene reduction assays to estimate N₂ fixation and RNAseq to evaluate metatranscriptomes. Genes related to N₂ fixation and the biosynthesis of several amino acids were up-regulated in the cyanobiont when hosted by the moss. However, S-uptake and the biosynthesis of the S-containing amino acids methionine and cysteine were down-regulated in the cyanobiont while the degradation of selenocysteine was up-regulated. In contrast, the number of differentially expressed genes in the moss was much lower, and almost no transcripts related to nutrient metabolism were affected. It is possible that, at least during the early stage of this symbiosis, the cyanobiont receives few if any nutrients from the host in return for N, suggesting that moss-cyanobacteria symbioses encompass relationships that are more plastic than a constant mutualist flow of nutrients.

Keywords: Boreal forests; Metatranscriptomics; Nitrogen fixation; Plant–microbe interactions; Sulfur; Symbiosis.

Fig. 1

Heatmap representations of distance matrices of variance-stabilizing transformed transcriptomic data from the cyanobacterial strain UCTE CB012 (a) and the feather moss H. splendens (b) in the co-incubated and solitary states. Cyanobacterium: strain UCTE CB012 incubated by itself. Moss: H. splendens incubated by itself. Moss + cyanobacterium: co-incubation of H. splendens and UCTE CB012. n = 4

Fig. 2

Principal component analyses comparing variance-stabilizing transformed transcriptomic datasets of the cyanobacterial strain UCTE CB012 (a) and the feather moss H. splendens (b) in the solitary and co-incubated states. Cyanobacterium: strain UCTE CB012 incubated by itself. Moss: H. splendens incubated by itself. Moss + cyanobacterium: co-incubation of H. splendens and UCTE CB012. n = 4

Fig. 3

MA plots comparing the log2 fold changes in differential gene expression between the co-incubated and solitary states of the cyanobacterium UCTE CB012 (a) and H. splendens (b) after 1 week. Genes that were significantly up-regulated (positive values) or down-regulated (negative values) in the co-incubated states (p < 0.05) are highlighted in green. Triangular spots represent data that extrapolates the limits of the y axis. n = 4

Fig. 4

Changes in gene expression and N₂ fixation activity in the symbiosis between the cyanobacterium UCTE CB012 and the feather moss H. splendens. a Number of genes in functional categories from the Cluster of Orthologous Genes database that presented significant positive or negative differential expression under co-culture between the cyanobiont UCTE CB012 and H. splendens. b Increased N₂ fixation activity in colonies of the strain UCTE CB012 in co-incubation with shoots of H. splendens as revealed by acetylene reduction assays. Bars represent average values and lines indicate standard errors, while letters above bars in b represent significant differences observed with the Tukey test (p < 0.05). Cyano: strain UCTE CB012 incubated by itself. Moss + cyano: co-incubation of H. splendens and UCTE CB012. n = 4