Re-engineering a lost trait: , a master regulator of arbuscular mycorrhizal symbiosis, affects genes for immunity and metabolism of non-host Arabidopsis when restored long after its evolutionary loss.

Arbuscular mycorrhizal symbiosis (AM) is a beneficial trait originating with the first land plants, which has subsequently been lost by species scattered throughout the radiation of plant diversity to the present day, including the model . To explore why an apparently beneficial trait would be repeatedly lost, we generated plants expressing a constitutively active form of , a key transcription factor that enables AM within the Common Symbiosis Pathway, which was lost from along with the AM host trait. We characterize the transcriptomic effect of expressing in with and without exposure to the AM fungus (AMF) , and compare these results to the AM model and its knockout mutant . Despite its long history as a non-AM species, restoring in the form of its constitutively active DNA-binding domain to altered expression of specific gene networks. Surprisingly, the effect of expressing in and knocking it out in was strongest in plants not exposed to AMF, which is revealed to be due to changes in genotype causing a transcriptional state which partially mimics AMF exposure in non-inoculated plants. Our results indicate that despite the long interval since loss of AM and in , molecular connections to symbiosis machinery remain in place in this nonAM species, with implications for both basic science and the prospect of engineering this trait for agriculture.