Plants interact with diverse microbes including those that result in nutrient-acquiring
symbioses. In order to balance the energy cost with the benefit gained, plants employ
a systemic negative feedback loop to control the formation of these symbioses. This
is particularly well-understood in nodulation, the symbiosis between legumes and
nitrogen-fixing rhizobia, and is known as autoregulation of nodulation (AON). However,
much less is understood about the autoregulation of the ancient arbuscular mycorrhizal
symbioses that form between Glomeromycota fungi and the majority of land plants.
Elegant physiological studies in legumes have indicated there is at least some overlap
in the genes and signals that regulate these two symbioses but there are major gaps
in our understanding. In this paper we examine the hypothesis that the autoregulation
of mycorrhizae (AOM) pathway shares some elements with AON but that there are also
some important differences. By reviewing the current knowledge of the AON pathway,
we have identified important directions for future AOM studies. We also provide the
first genetic evidence that CLV2 (an important element of the AON pathway) influences mycorrhizal development in a non-legume, tomato and review the interaction of the autoregulation pathway with plant hormones and nutrient status. Finally, we discuss whether autoregulation may play a role in the relationships plants form with other microbes.