Why do arbuscular mycorrhizal fungi retain active circadian clock genes?

Question

Symbiotic arbuscular mycorrhizal fungi colonize the root systems of perhaps 80% of modern-day plant families, and are responsible for assisting with intake of nutrients required for plant growth and other important functions (e.g., protection from parasites, see https://academic.oup.com/pcp/article/47/1/154/1867594 ). In short, they make the abundance of terrestrial plant life possible! Yet, we do not know important things about them, such as, why do they retain circadian clock functionality, when they are 1) subterranean, 2) obligate symbiotes, and 3) it has been ~460 million years since they associated with plants, quite long enough to delete or inactivate through mutations any genes with no useful function. So what advantages might retention of this ability confer?

Answer 1

Actually, there isn't research out there that would answer this question, that I've found.

Some of the things one might consider (when submitting a research proposal, planning a research program, etc.! Hint, hint!) might be:

1) Do arbuscular mycorrhizal fungi (AM for short) have life stages where they might profit by utilizing small amounts of light, or, conversely, survive by hunkering down when daylight (with drought, heat associated) is present?

2) Do AM need to synchronize growth periods to that of their symbiotic host plant, in order to colonize or function well once emplaced? And if so, how would that work?

3) If there is no evolutionary advantage per se in their survival or symbiotic functioning to circadian rhythm, for what reason might the circadian clock have been retained? For example, it may regulate other biochemical cycles within them, and simply a biochemically "convenient" way of doing that (as opposed to other systems of regulation) -- such other feedback regulation systems lacking, it is retained by default)? You could obtain evidence for that by considering other biochemical cycles that MAY have been regulated by circadian clock at some point (and may still be in some plant families), but are now regulated by other means -- what degree of complexity of control was involved in the shift, vs. that of how the crucial system(s) in AM function?

General note: the study of AM and their functions is highly pertinent to considering the effects of climate change, since if climate change makes it impossible for AM to survive in soils, then agriculture there becomes impractical. No agriculture = no food for us mammals!