Phenology and function in lycopod-Mucoromycotina symbiosis

Lycopodiella inundata is a lycophyte with a complex life cycle. The gametophytes and the juvenile, mature and retreating sporophytes form associations with Mucoromycotina fine root endophyte (MFRE) fungi, being mycoheterotrophic as gametophytes and mutualistic as mature sporophytes. However, the function of the symbiosis across juvenile and retreating sporophyte life stages remains unknown. We measured carbon-for-nutrient exchanges between L. inundata and MFRE across the transition from newly emerging sporophytes to mature sporophytes and in retreating adult sporophytes. We show MFRE fungi play distinct functional roles at each plant life stage, with evidence of bidirectional exchange of plant C for fungal acquired nutrients (N and P) between mature adult and retreating adult sporophytes and fungi, but no transfer of plant C to fungi and little fungal-acquired nutrient gain in juvenile sporophytes. Furthermore, we show that these functional stages correspond with different cytologies of colonisation. Our results show that MFRE have considerable plasticity in their interactions with the host plant which is related to the developmental stage of the host. This highlights the need for further research into symbiotic fungal function across plant life histories.