searching for a space opera novel with a planet of monks who controlled a monopoly on functional immortality via plant-based biotech

A space opera novel with a planet whose inhabitants are a group of ‘monks’ with a quasi-religious monopoly on functional immortality. There’s a parasitic/symbiotic plant organism (I don’t remember if it was microbial algae or a multicellular parasite or if it was sentient or not) native to said planet which can meddle with the biology of its host, fixing any damage, but only the monks have figured out how to communicate with/control it, so if anyone other than them tries to use it, they’ll die horribly of parasitic plant growth body horror. The monks have leveraged said power into various demands, including the establishment of a quasi-serf caste of offworld prospective immortality seekers. A couple decades of servitude in exchange for being allowed to leave and live forever. The book was written in english and it was fairly old.