Cell size and growth kinetics are fundamental cellular properties with important physiological implications. Classical studies on yeast, and recently on bacteria, have identified rules for cell size regulation in single cells, but in the more complex environment of multicellular tissues, data have been lacking. In this study, to characterize cell size and growth regulation in a multicellular context, we developed a 4D imaging pipeline and applied it to track and quantify epidermal cells over 3-4 d in Arabidopsis thaliana shoot apical meristems. We found that a cell size checkpoint is not the trigger for G2/M or cytokinesis, refuting the unexamined assumption that meristematic cells trigger cell cycle phases upon reaching a critical size. Our data also rule out models in which cells undergo G2/M at a fixed time after birth, or by adding a critical size increment between G2/M transitions. Rather, cell size regulation was intermediate between the critical size and critical increment paradigms, meaning that cell size fluctuations decay by ∼75% in one generation compared with 100% (critical size) and 50% (critical increment). Notably, this behavior was independent of local cell-cell contact topologies and of position within the tissue. Cells grew exponentially throughout the first >80% of the cell cycle, but following an asymmetrical division, the small daughter grew at a faster exponential rate than the large daughter, an observation that potentially challenges present models of growth regulation. These growth and division behaviors place strong constraints on quantitative mechanistic descriptions of the cell cycle and growth control.cell size | cell growth | cell cycle | homeostasis | plant stem cells H ow cells coordinate growth and division to achieve a particular cell size remains a fundamental question in biology. Our understanding of this basic property of cells is limited, in part, by the lack of quantitative data on cellular growth and size kinetics over multiple generations, especially in higher eukaryotes (1). Classical studies of cell size homeostasis focused on whether division occurred upon reaching a critical size or after a fixed time period has elapsed (2, 3). However, time-lapse studies of single-celled organisms spanning a range of bacteria (4-7) and the yeast Saccharomyces cerevisiae (8) have recently indicated that cell size is regulated by the addition of a fixed volume increment between divisions. Identification of the size regulation behavior constrains the set of feasible molecular scenarios for how growth and division are coordinated with the cell cycle (8-10). In multicellular tissues, the loss of growth and division/cell cycle coordination could have an impact on the organism's development, yet, to the best of our knowledge, cell growth and size kinetics have never before been measured over generations in a tissue context. The experimental challenges are particularly acute because interdivision times are often on the order of tens of hours, cells have a diversity of shapes necessitating di...

It is remarkable how robustly a bacterial species can maintain its preferred size. This capacity is intimately related to control of the cell cycle: cell size and growth rate determine the duration of the cell cycle, which must accommodate the initiation and completion of DNA replication, and the assembly of the division apparatus during steady growth. Although we still lack an integrated view of the interconnections among events in the cell cycle, cell growth and cell size, the development of high-throughput imaging and image-processing protocols has stimulated a renaissance in the field. In this Review, we summarize recent findings, present simple classic models for cell size control, introduce high-throughput data-collection techniques, and explore the mechanisms that coordinate cell size with essential growth and cell cycle processes.