Abstract: A special issue on plant cell biology is long overdue for JIPB! In the last two decades or so, the plant biology community has been thrilled by explosive discoveries regarding the molecular and genetic basis of plant growth, development, and responses to the environment, largely owing to recent maturation of model systems like Arabidopsis thaliana and the rice Oryza sativa, as well as the rapid development of high throughput technologies associated with genomics and proteomics. However, major gaps remain in our mechanistic understanding of plant processes, especially regarding how a gene or a network of genes spatially and temporally regulates a particular developmental process or a physiological response at the cellular level.Because cells are the most fundamental functional units among living organisms, we won’t understand how and why a plant process is operated until we reveal how cellular activities integrate genes, molecules, and environmental cues for developmental and physiological processes at the tissue, organ and whole plant levels. For example, we now know all the genes in the Arabidopsis genome that are regulated by auxin, and how auxin is perceived by the nuclear signaling pathway leading to the changes in the expression of these genes. For the most part, however, we do not know how these genes modulate cell division, polarized growth, and differentiation. Neither do we know how the perception of auxin results in cytoplasmic events, such as the dynamic remodeling of the cytoskeleton, that clearly contribute to auxin-regulated cellular processes. As the first step in bridging the knowledge gaps in plant biology, it is imperative to elucidate the genetic and molecular mechanisms underlying fundamental cellular processes (e.g. biogenesis and dynamics of subcellular compartments or organelles, cytoskeletal organization and dynamics, and cell wall biogenesis and remodeling) and cellular behaviors (e.g. cell growth, cell division, morphogenesis, polarity establishment, differentiation, and cell-to-cell communication). Studies on these fundamental topics using plant cells as model systems as well as on unique cellular structures (chloroplast, plasmodesmata, cell wall) and processes in plants also contribute to our knowledge of how living cells function in general. Research in cell biology has been greatly aided by rapid advances in technologies like modern laser scanning and spinning disk confocal microscopy and live-cell imaging technologies, as well as improved transmission electron microscopy, which have opened new windows to visualizing and analyzing molecular dynamics in real time and the ultrastructural details of subcellular compartments. When applied to integrate the genetic and genomic in model plant systems such as Arabidopsis, these sophisticated technologies become particularly powerful. Consequently, in recent years we have witnessed exciting and important advances in plant cell biology. In this special issue, we feature a series of review articles from experts, which have highlighted and analyzed recent progress in important topics of plant cell biology, as well as a number of selective original research articles. We have intended to cover broad aspects of plant cell biology with regards to the broad readership of JIPB. The classical topics on the biogenesis,dynamics and function of subcellular compartments (organelles) are experiencing a new life, as relevant new functions are being discovered and the molecular and genetic basis of these processes are being revealed. Faithful sample preservation procedures combined with the high resolution electron tomography technique have opened our eyes to unprecedented details of 3-D subcellular structures (Haas and Otegui). Recent studies have uncovered important roles of chloroplast outer membranes in protein translocation and regulation of metabolic activities and signaling processes (Inoue). Genetic studies have generated new insights into the molecular mechanism underlying the multiplication of the plant peroxisome (Hu). Through serendipitous biochemical and microscopic analysis, molecular identities of provacuolar compartments and endosomal compartments are surfacing (Lam et al.). Members of the conserved RAB guanosine triphosphatase (GTPase) family function as key regulators in vesicular trafficking in the secretory and endocytic pathways. Genomic or bioinformatic analyses not only implicate the functional conservation of RABGTPases, but also pinpoint their specialization in plant processes (Zhang et al.). The cytoskeleton is arguably one of the most actively pursued subjects in plant cell biology, reflecting its paramount importance not only in structural regulation but also in signaling processes of growth, development, and responses to the environment. Consequently, it attracts particular attention in this special issue. Because of the lack of a structurally defined microtubule-organizing center (MTOC), such as the spindle pole body in fungi and the centrosome in animals, unique features of plant microtubule organization demonstrated by various plant cells have intrigued plant cell biologists for many decades. The γ-tubulin complex functions as the microtubule nucleator among eukaryotic organisms. A detailed phylogenetic analysis has indicated that subunits of this complex are well conserved among land plants (Murata). Intriguing MTOC structures are demonstrated by various land plants, especially in bryophytes, which lead to a concept of pleiomorphic and migratory MTOC from an evolutionary perspective (Brown and Lemmon; Brown et al.). The cortical microtubule array directly influences cell morphogenesis and consequently plant development, which has probably received the most scrutiny in the field of the plant cytoskeleton. The dynamic behavior of cortical microtubules is regulated by a large number of diverse microtubule-associated proteins, known as MAPs, including those conserved ones and those unique to plants (Kaloriti et al.; Burk et al.; Zhou et al.). Increasing evidence shows the importance of actin microfilaments in plant growth and development. Isolation and characterization of actin-binding proteins begin to unravel molecular mechanisms that regulate actin organization and dynamics for cellular activities (Su et al.). The cytoskeleton is the muscle for cell polarity establishment, a fundamental process crucial for morphogenesis and cell differentiation. Fucoid algae provide their elegant zygotes for us to visualize the integrated activities of actin microfilaments and microtubules during cell polarization and asymmetric cell division (Bisgrove). Apart from cell morphogenesis, cell polarity, and cell differentiation, cell cycle control is another important aspect of cell behaviors that is intimately related to growth and development. The cell cycle is regulated by developmental signals, hormones and environmental cues, but much is to be learned about the how question. An in silico study identifies novel cell cycle regulators,and may ultimately help us understand complex mechanisms of the cell cycle exhibited by plants (Wang and Yang). Meiosis remains to be a fascinating topic for plant cell biologists. Molecular genetic studies in Arabidopsis have contributed to our basic understanding of the molecular mechanism for meiotic recombination in plants, a fundamental process for inheritance and generation of genetic diversity (Wijeratne and Ma). Besides mysteries about the cell cycle machinery itself, an accompanying question of how organelles are properly partitioned into two daughter cells has always attracted cell biologists’ attentions (Sheahan et al.). The pollen tube communicates intimately with female tissues to render both compatible and self-incompatible interactions.Pollen tubes cultured in vitro provide an exciting system for the investigation of cell-to-cell communication and cellular signaling in plants, especially on signal-mediated cellular activities such as cytoskeletal dynamics, exocytosis, and cell death. Important and exciting progress has been made in self-incompatible reaction-mediated cellular signaling in pollen tubes (Franklin-Tong).Pollen tubes display oscillation of the tip-focused cytosolic calcium gradient that is required for pollen tube growth, which seems to depend on channel-mediated calcium influx. A putative plasma membrane-localized calcium-permeable channel has been identified (Chang et al.). The cell wall provides a framework for cell shape determination, and for the integrity and physical strength of plants. As an integral effort in understanding the evolution of land plants, analysis of cell wall polysaccharides from a moss to advanced gymnosperms reveals both highly conserved and modified features (Nothnagel and Nothnagel). The cell wall’s role in cell-to-cell communication and signaling receives more and more attention. Plasmodesmata, a highly dynamic and regulated channel connecting the symplast and transporting small molecules, proteins and RNAs across plant cells, provide a unique mechanism for cell-to-cell communication in plants. Interestingly, plant viruses have hijacked the plasmodesmata transport machinery to allow them to spread throughout the plant body. Significant progress has been made in this fascinating subject of plant cell biology (Ding and Itaya). In spite of our desire to be as inclusive as possible in this special issue, we have inevitably missed many important subjects that are under intensive scrutiny, as demonstrated by the active research in all cylinders of plant cell biology. It is clear that the field of plant cell biology is breaking the dawn of our mechanistic understanding of cellular behaviors and their underlying linkage to genes and molecules. Ultimately, plants harness splendid cellular behaviors for their fascinating growth and developmental processes,and for their responses to the environment. It is our hope that this special issue shows the inheritance of a traditional emphasis in cell biology by Acta Botanica Sinica, and the commitment to reporting breakthroughs in nderstanding plant cells by the Journal of Integrative Plant Biology. More fascinating tales of plant cells are yet to be told!