A Celebration of Fred David Sack.

In this special Focus Issue on Stomata, it is only fitting to pay tribute to one of the stomatal development field’s founders and greatest champions, Fred David Sack (1947–2015). Fred was born in New York City and raised in a progressive tradition that valued an understanding and appreciation of the human and scientificworlds. Fred attended Stuyvesant High School and Antioch College, where he majored in Sociology and was active in school and antiwar politics. After his graduation, Fred returned to New York City to work as a Research Analyst for the Health Services Mobility Study sponsored by the Research Foundation of the City University of NewYork. In 1973, he became an Assistant Management Analyst for New York City’s Prison Health Service. While living in Brooklyn, Fred discovered the Brooklyn Botanic Garden and quickly cultivated a love of plants and natural habitats that would guide his interests and passions for the rest of his life. Following his new-found love of plants, Fred enrolled as a PhD student in Plant Biology at Cornell University, where he started his work on stomata, writing his dissertation on “The development and ultrastructure of the stomata of Funaria hygrometrica, Hedw.” After earning his PhD in 1982, Fred stayed in upstate New York, becoming a postdoctoral Research Fellow with Professor Carl Leopold, a noted plant physiologist, conservationist, and environmental ethicist, at the Boyce Thomson Institute. Here, Fred began the gravitropism work that defined much of his career. He used video microscopy to study amyloplast movement in living corn columella cells (Sack et al., 1986). These elegant studies showed that amyloplast sedimentation occurs within a timeframe consistent with the starch statolith hypothesis for gravity perception. His newways of addressing classic questions in plant physiology launched Fred’s independent research career. In 1984, Fredwas recruited to an assistant professorship at The Ohio State University. In his 22 years at OSU, Fred rose through the ranks, becoming an associate and full professor, culminating in formal recognition by the university president for his contributions to scholarship and from 2004 to 2006 serving as Chair of the Plant Cell and Molecular Biology program. Although Fred had worked with a number of plant species, notably bryophytes, in his early work, by the late 1980s Arabidopsis thaliana was being recognized as a powerful model organism. The fascinating discovery by Chris Somerville’s group, who isolated a starchless mutant of Arabidopsis in order to study carbohydrate metabolism, gave Fred and Postdoc John Kiss an opportunity to test long-standing hypotheses about gravity perception. Interestingly, it had been reported that when the starchless mutant was reoriented, it still responded to gravity, thus seemingly contradicting the classical starch-statolith hypothesis. More careful work by Fred and John, however, ended up turning the argument around 180 degrees! Fred and John reported that while the starchless mutant did respond to gravity, this mutant also exhibited a much longer presentation and perception time (Kiss et al., 1989). Thus, starch was needed for full gravitropic sensitivity, and data from the studies using the starchless mutant provided strong evidence for a statolith-based model of perception. Numerous other studies using very different approaches also have supported a key role for statoliths in perception (Sack, 1997). Fred made use of another single-celled model system to study gravitropism: protonemata of the moss Ceratodon purpureus. These studies eventually led to the development of a spaceflight experiment that was performed on the ill-fated space shuttle Columbia STS107 mission in 2003. Amazingly after the loss of this spacecraft, the fixed moss samples from Fred’s experiment were recovered on the ground. Fred went on to publish a paper (Kern et al., 2005) that demonstrated the existence of default nonrandom growth patterns that developed in microgravity and suggested that this response was overridden and masked by a constant g-vector on Earth. Throughout his life and career, Fred explored new fields and engaged with different people, but he held a constant love of shape and form in plant biology. In the mid-1990s Fred returned to working on stomata. He combined his expertise in ultrastructuralwork (harkening back to his PhD studies) with his newer appreciation of Arabidopsis genetics to launch an ambitious project to understand how stomata, with their perfectly symmetric two epidermal guard cells flanking a pore, were made, and how their beautiful and orderly patterned distributions on aerial organs (two stomata never touch each other) were organized. This project would intrigue Fred for the rest of his career, as well as inspire a new generation of plant developmental biologists. Before Fred embarked on his genetic studies of stomatal development in Arabidopsis, there was a long history of stomatal physiology and of using stomatal patterns to put living and fossil plants into species groups. Yet, development was rarely considered. Fred was one of the first to consider the dynamics of stomatal development and how cellular choices made early in leaf development enabled or restricted the choices that could be made later. Stomata are visually stunning and could be approached as a simple system upon which layers of complexity could later be added, a system to which Fred seemed intuitively drawn. Whatever his initial motivation for reinvigorating his stomatal research, it would be fair to say that Fred is considered the father of Arabidopsis stomatal development. Fred’s critical publications began in 1995 with the characterization of the first two mutants in the field, too many mouths (tmm) and four lips (flp; Yang and Sack, www.plantphysiol.org/cgi/doi/10.1104/pp.16.01832