The cyanobacterium, <i>Nostoc punctiforme</i> can protect against programmed cell death and induce defence genes in <i>Arabidopsis thaliana</i>

Cyanobacteria can form nitrogen-fixing symbioses with a broad range of plant species. Unlike other plant-bacteria symbioses, little is understood about the immunological responses induced by plant cyanobionts (symbiotic cyanobacteria). Here, we demonstrated that the model plant-symbiotic cyanobacteria, <i>Nostoc punctiforme</i> is capable of protecting against programmed cell death (PCD) when induced in <i>Arabidopsis thaliana</i> suspension cell cultures. We also profiled the early transcriptomic changes that were induced in response to conditioned medium (CM) from <i>N. punctiforme</i> cell cultures. Interestingly, the reduction in PCD was preceded by the induction of genes associated with defence and immunity, the most striking of which were a number of WRKY-family transcription factors. Down-regulated genes included those involved in the regulation of cell growth and differentiation. This work is the first to show that a cyanobiont can affect plant PCD and provides a useful transcriptome resource for studying early plant cell responses to symbiotic cyanobacteria.