Russ Monson and the evolution of C₄ photosynthesis

Early in his career, Russ Monson produced a series of influential eco-physiological papers that helped lay the foundation for the study of C₄ plant evolution. Among the most important was a 1984 paper with Maurice Ku and Gerry Edwards that outlined the pathway for the evolutionary bridge from C₃ to C₄ photosynthesis. This model proposed C₄ photosynthesis arose out of a shuttle that imported photorespiratory metabolites into bundle sheath (BS) cells, where glycine decarboxylase cleaved off CO₂, allowing it to accumulate and be efficiently refixed by BS Rubisco. By the mid-1990's, Monson's research focus had shifted away from C₄ plants, save for one 2003 paper on C₃ versus C₄ stomatal control with Travis Huxman, and a series of critical reviews on C₄ evolution. These reviews heavily influenced the modern synthesis of C₄ evolutionary studies, which incorporates phylogenomic understanding with physiological, molecular, and structural characterizations of trait shifts in multiple evolutionary lineages. Subsequent research supported the Monson et al. model from 1984, by showing a glycine shuttle occurs in nearly all C₃-C₄ intermediate species identified. Monson also examined the physiological controls over the ecological distribution of C₃, C₃-C₄ intermediate, and C₄ photosynthesis, building our understanding of the fitness value of the intermediate and C₄ pathway in relevant microenvironments. By establishing the foundation for discoveries that followed, Russ Monson can rightly be considered a leading pioneer contributing to the evolutionary biology of C₄ photosynthesis.