Biotic and abiotic controls of nitrogen fixation in cyanobacteria–moss associations

Abstract

Summary Most mosses are colonized by nitrogen (N)‐fixing cyanobacteria. This discovery is relatively recent, which can explain the large knowledge gaps the field is now tackling. For instance, while we have a good understanding of the abiotic controls (e.g. nutrient availability, increased temperature), we still do not know much about the biotic controls of N2 fixation in mosses. I propose here that we should endeavour to position moss–cyanobacteria associations along the mutualism–parasitism continuum under varying abiotic conditions (e.g. nutrient availability). This would finally unravel the nature of the relationship between the partners and will be a big leap in our understanding of the evolution of plant–bacteria interactions using moss–cyanobacteria associations as a model system.

Challenging the term symbiosis in plant–microbe associations to create an understanding across sciences

“…Plant scientists mostly focus on the chemical benefits of the host plant. This explains why physical properties, such as the sheltered habitat for the symbiont in moss– Nostoc associations, are not considered and, thus, the word symbiosis is avoided (Rousk, 2022). Another non‐chemical benefit, the dispersing of sperm by other organisms, is also often not considered to be symbiotic despite contributing to the species’ fitness as shown by the increase in reproductive success of moss when microarthropods disperse their sperm (Shortlidge et al, 2021).…”

“…Another non‐chemical benefit, the dispersing of sperm by other organisms, is also often not considered to be symbiotic despite contributing to the species’ fitness as shown by the increase in reproductive success of moss when microarthropods disperse their sperm (Shortlidge et al, 2021). Controversially, these efforts to classify plant–microbe associations assume that the interaction can be categorized in a binary format (beneficial for one or for both parties), ignoring the reality of plant–microbe interactions that often occur on a continuum depending on the availability of nutrients (Rousk, 2022).…”

What can hornworts teach us?

“…In the majority of plant-cyanobacteria symbioses this is achieved by molecules collectively called hormogonia-inducing factors (HIF) that are produced by the plant host and transform the cyanobacterial cells to hormogonia. Hormogonia are motile cells that can detach from the parent organism and function as dispersal units ( Adams and Duggan, 2008 ; Liu and Rousk, 2022 ; Rousk, 2022 ). The hornwort-cyanobacteria symbiosis is relatively easy to reconstruct under axenic conditions, with A. punctatus being used as the model system to study the morphological, functional, molecular, and chemical processes underlying the symbiosis ( Enderlin and Meeks, 1983 ).…”

“…It is expected that this knowledge can be used in the future to engineer crops capable of initiating the mutualistic interaction with cyanobacteria ( Liu and Rousk, 2022 ; Rousk, 2022 ). This type of interaction has received increasing interest in recent years due to its significant translational potential to boost crop yield without applying additional artificial fertilizer ( Alvarenga and Rousk, 2022 ; Rousk, 2022 ).…”

“…It is expected that this knowledge can be used in the future to engineer crops capable of initiating the mutualistic interaction with cyanobacteria ( Liu and Rousk, 2022 ; Rousk, 2022 ). This type of interaction has received increasing interest in recent years due to its significant translational potential to boost crop yield without applying additional artificial fertilizer ( Alvarenga and Rousk, 2022 ; Rousk, 2022 ). Compared to AM fungi, cyanobacteria, especially the members of the genus Nostoc dominantly present in hornworts, are less dependent on the host, do not necessarily require specialized plant structures like arbuscules, and therefore hold a promising translational potential ( Adams and Duggan, 2008 ).…”