Nitrogen starvation response in hornworts and liverworts provides little evidence for complex priming to the cyanobiont

Mutualistic plant-microbe symbiotic interactions are thought to have evolved from a loose association between host plants and microbes when nutrients are limited. Therefore, the molecular network enabling intimate mutualistic plant-microbe symbioses may have evolved from a nutrient starvation response shared by all land plants. While the molecular link between nutrient status and symbiotic interaction is well-established, it remains poorly understood in some systems. This is especially true for the symbiotic associations between plants and cyanobacteria. To test the conservation of the starvation network across land plants as well as to investigate the link between nutrient starvation and symbiosis initiation in the plant-cyanobacteria symbiosis, here we explore the transcriptional responses to nutrient starvation in two non-vascular plant species, a hornwort Anthoceros agrestis and a liverwort Blasia pusilla, forming plant-cyanobacteria endophytic symbioses. We observe a deep conservation of the systemic starvation response across land plants. However, very few if any components of the starvation network appear to be specific to cyanobacteria hosting plants, providing little evidence for extensive and specific priming to the cyanobiont. Moreover, we found that some bioactive molecules known to be important in initiating the plant-mycorrhiza and nodule-forming bacteria symbioses, may also have a similar role in plant-cyanobacteria symbioses. HighlightOur results suggest that the most critical step in establishing plant-cyanobacteria interactions using non-host plants is the attraction of the cyanobiont. This finding has significant impact on crop engineering.