IN THIS ISSUE

Foreword: Plant Cells and Organelles—Beyond the Sum of the Parts

Correspondence to: Harry B. Smith, hsmith{at}aspp.org (E-mail), 301-279-2996 (fax)

In 1839, Schleiden and Schwann enunciated the cell theory, a tenet that would unify the biological sciences as they developed over the subsequent century to spawn the multiple subdisciplines that exist today. Neither Schleiden, a botanist, nor Schwann, a zoologist, was the first to champion the cell as the fundamental entity of biological systems, and the unifying thesis—hardly a theory any longer—is commonly attributed to them not to honor any experimental advance, but as a matter of convenience, or in recognition of their rhetorical deftness, at best. Science, after all, does not occur by edict; rather, it offers a view of the world that is elaborated over time.

Even the view of the cell as the fundamental unit of life continues to change. Indeed, as reductionist approaches to biological inquiry have become ever more refined, to the point that hundreds of biomolecules can now be discussed in terms of atomic coordinates, we are more apt to distinguish species in terms of sequence similarities, amino acid substitutions, and saturation of carbon skeletons than to turn to the cell as a unifying element. This stress on molecular detail should neither be construed as an inability to embrace the big biological picture nor trivialized as a reflection of the technomania that culture critics alternatively praise and berate scientists for perpetrating. The reductionist trend, rather, bespeaks the progress of the scientific method itself, whereby questions are asked and answered to the extent that they can be. In fact, the sophistication that has allowed biology to become subdivided into countless subdisciplines is the very measure of the success of biological science.

From molecular medicine to plant biology, the commitment to reductionism that has propelled scientific inquiry throughout the twentieth century is currently reaping unprecedented rewards under rubrics such as "genomics" and "bioinformatics." And yet, paradoxically, having arrived at a level of technical adeptness such that we are on the verge of writing meaningful chemical formulae for the genomes of diverse organisms, biological science is positioned to embrace the big picture of how life works as never before. Examples abound: gaps in understanding the molecular bases of human learning disabilities and drug addiction can be filled in by turning to cloning studies of Drosophila; yeast mutants that fail to undergo cell division can be used to identify cell cycle regulation in Arabidopsis. The big picture, like a Georges Seurat painting, emerges not in spite of reductionist data, but because of them.

In this special issue, the big-picture integration of molecular genetic data, electron microscopy, comparative sequence analyses, and other reductionist approaches is apparent in 18 review articles that focus our attention on plant biology at the cellular and organellar level. Each of the individual contributions provides the most up-to-date coverage of a selected topic that relates cell structure to plant function. Among the themes presented are: the ultrastructures that support the movement of proteins, membrane constituents, and chromosomes; the compartmentalization of signals that can be mobilized to mediate cell–cell communication; the constitution of membranous barriers that allow for cell energization, integrity, and integration into an organismal context.

The revitalization of the cell theory as a conceptual and not merely a structural unifier is evident throughout this special issue. Insights gained from the molecular regulation of bacterial cell division, for instance, have informed the elucidation of plastid division and development in plants as well as control of the eukaryotic cell cycle. The routing of proteins into chloroplasts has become better understood due to advances in mitochondrial research, which has also offered new paradigms for chromosomal function and a framework for approaching questions of transport across vacuolar and plasma membranes. The means by which plants store proteins as energy and material reserves (to the benefit of the entire food chain) are related to multiple mechanisms of cellular protein and vesicular trafficking, processes fundamental to the secretory system. Even an aspect such as the formation of calcium oxalate crystals within plant cells, a topic that may at first sight seem reductionist—even esoteric—in the extreme, speaks powerfully to fundamental issues of cell sig-naling events, vacuole biogenesis, and membrane compartmentation, as well as wider processes of biomineralization.

All of the themes presented underscore the position of cell biology at the interface between modern reductionist and holistic views of life. Nevertheless, each article is unique in its focus, and is deserving of attention, regardless of the reader's own specialization. It is our hope that this special issue will be helpful not only in cataloguing the current specialized research trends from the literature, but also will convey the excitement of seeing their materialization into the big picture as never before.

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