Hopanoid lipids: from membranes to plant–bacteria interactions

Key Points 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. Hopanoid lipids are structurally and functionally related to sterols, which are important building blocks of membranes. In this Review, Newman, Silipo and colleagues explore the diversity of bacterial hopanoids and their roles in stress adaptation and plant symbiosis. Lipid research represents a frontier for microbiology, as showcased by hopanoid lipids. Hopanoids, which resemble sterols and are found in the membranes of diverse bacteria, have left an extensive molecular fossil record. They were first discovered by petroleum geologists. Today, hopanoid-producing bacteria remain abundant in various ecosystems, such as the rhizosphere. Recently, great progress has been made in our understanding of hopanoid biosynthesis, facilitated in part by technical advances in lipid identification and quantification. A variety of genetically tractable, hopanoid-producing bacteria have been cultured, and tools to manipulate hopanoid biosynthesis and detect hopanoids are improving. However, we still have much to learn regarding how hopanoid production is regulated, how hopanoids act biophysically and biochemically, and how their production affects bacterial interactions with other organisms, such as p