mechanisms. Eventually, however, it was in vivo studies that led to the breakthrough when Yang’s group supplied apple fruit tissues with 14C-labeled methionine and observed that a labeled compound accumulated under anaerobiosis, a condition that prevents ethylene synthesis. They identified the compound as 1-aminocyclopropane-1-carboxylic acid, or ACC, and showed that it could be readily converted to ethylene by plant tissues under aerobic conditions. This discovery led to the identification of the enzymes responsible for production of ACC from S-adenosylmethionine (SAM) and for conversion of ACC to ethylene, their eventual cloning and characterization, and opportunities for the genetic and chemical manipulation of ethylene biosynthesis and action in plants. Yang also discovered that the methylthioribose group from SAM is recycled back into methionine formation following ACC synthesis to sustain high rates of ethylene production, which has been termed the “Yang Cycle.” In addition, he studied cyanide metabolism (after discovering that cyanide is a by-product of ethylene biosynthesis) and the effects of sulfur dioxide on plant cellular processes.
The elucidation of the ethylene biosynthetic pathway and the enzymes involved stimulated many studies of how this pathway is regulated in plants by both internal and external factors. The specific roles of different gene family members encoding the ethylene biosynthetic enzymes have been elucidated for a number of important plant growth stages. Yang’s work focused attention on ethylene in plant biology, subsequently resulting in the first identification of a plant hormone receptor and a detailed understanding of the molecular signaling pathways by which ethylene is perceived in plant cells. Yang also contributed to the development of ethylene-releasing compounds that allow the convenient application of ethylene to promote fruit ripening or to facilitate harvest. He helped to develop chemical antagonists