Cell size and growth regulation in the $\textit{Arabidopsis thaliana}$ apical stem cell niche

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 $\textit{Arabidopsis thaliana}$ 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.This work was supported by the Gatsby Charitable Foundation through Grant GAT3395-PR4 (to H.J.) and Fellowships GAT3272/C and GAT3273-PR1 (to E.M.M.), Swedish Research Council Grant VR2013:4632 and Knut and Alice Wallenberg Foundation Grant KAW2012.0050 (to H.J.), the Howard Hughes Medical Institute and Gordon and Betty Moore Foundation Grant GBMF3406 (to E.M.M.), and National Science Foundation Faculty Early Career Development (CAREER) Program Award MCB-1149328 (to K.C.H.)