It appears there is other microbial community. The metagenome of floating fern Azolla reveals endophytes that do not fix N2 but may denitrfy. The colonization of plants cyanobacteria has been intensively studied in the Gunnera symbiosis. The plant organs colonized in this host are stem glands. These glands secrete viscous mucilage of low pH that attracts Nostoc and induces differentiation on motile hormogonia etc.. For more details consult sciencedirect. com
I agree with Dr. Antai. Additionally, I would like to say more likely you are going to see proteobacteria, firmicutes, and actinobcateria no matter whichever plant type you are investigating. So having this assumption in start would not be much realistic. I think you may consider other things while designing experiment.