Hopanoid lipids: from membranes to plant–bacteria interactions

Hopanes were discovered by petroleum geologists as ubiquitous molecular fossils in ancient sedimentary rocks. Later, bacterial hopanoids were identified as their progenitors.Today, phylogenetically diverse bacteria make hopanoids using machinery encoded by a conserved set of genes. The rhizosphere appears to be a niche that is common to many hopanoid-producing bacteria, and some hopanoid producers are known plant symbionts.Bacteria make structurally distinct types of hopanoids, including ones that can covalently bind lipid A. Different hopanoid classes exhibit different properties and likely have specific biological functions.Hopanoids share similar biophysical properties with sterols, such as tuning membrane rigidity and permeability. Though some evidence suggests that hopanoids help order membranes, how such ordering impacts cells and whether hopanoids interact with particular membrane proteins remain to be determined.In vitro, hopanoids contribute to bacterial stress resistance, which may help explain their ability to facilitate beneficial plant–bacteria interactions. However, given that hopanoids can also serve as carriers for plant hormones and that plants themselves make hopanoid-like compounds, it is likely that other mechanisms are additionally at play.