Bound by a handshake.

Rubisco—or ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO)—is the basis of Earth's life support system (1). Found in all green plants, algae, and cyanobacteria, this enzyme converts inorganic carbon dioxide (CO2) to organic carbon during photosynthesis. As such, it has shaped past and present climates and provides food and energy for the world's burgeoning populations. In aquatic environments, dissolved CO2 is usually scarce, and to compensate, diverse CO2-concentrating mechanisms have evolved to corral Rubisco into efficient working clusters. In cyanobacteria, Rubisco is encapsulated by proteinaceous shells called carboxysomes (2). In algae, it is packaged into pyrenoids, which are liquid-like organelles (3). Among the earliest of land plants (4), hornworts (Anthocerotophyta) are the only species known to have pyrenoids. On page 1070 of this issue, Robison et al. (5) reveal that the mechanism by which hornworts aggregate Rubisco is unlike anything previously reported. The discovery points to a new potential pathway for engineering crops with improved photosynthetic efficiency. [ABSTRACT FROM AUTHOR]