Flowers are the most complex structures of plants. Studies of Arabidopsis thaliana, which has typical eudicot flowers, have been fundamental in advancing the structural and molecular understanding of flower development. The main processes and stages of Arabidopsis flower development are summarized to provide a framework in which to interpret the detailed molecular genetic studies of genes assigned functions during flower development and is extended to recent genomics studies uncovering the key regulatory modules involved. Computational models have been used to study the concerted action and dynamics of the gene regulatory module that underlies patterning of the Arabidopsis inflorescence meristem and specification of the primordial cell types during early stages of flower development. This includes the gene combinations that specify sepal, petal, stamen and carpel identity, and genes that interact with them. As a dynamic gene regulatory network this module has been shown to converge to stable multigenic profiles that depend upon the overall network topology and are thus robust, which can explain the canalization of flower organ determination and the overall conservation of the basic flower plan among eudicots. Comparative and evolutionary approaches derived from Arabidopsis studies pave the way to studying the molecular basis of diverse floral morphologies.

The budding of tubular organs from flat epithelial sheets is a vital morphogenetic process. Cell behaviours that drive such processes are only starting to be unraveled. Using live-imaging and novel morphometric methods, we show that in addition to apical constriction, radially oriented directional intercalation of cells plays a major contribution to early stages of invagination of the salivary gland tube in the Drosophila embryo. Extending analyses in 3D, we find that near the pit of invagination, isotropic apical constriction leads to strong cell-wedging. Further from the pit cells interleave circumferentially, suggesting apically driven behaviours. Supporting this, junctional myosin is enriched in, and neighbour exchanges are biased towards the circumferential orientation. In a mutant failing pit specification, neither are biased due to an inactive pit. Thus, tube budding involves radially patterned pools of apical myosin, medial as well as junctional, and radially patterned 3D-cell behaviours, with a close mechanical interplay between invagination and intercalation.

A developing plant organ exhibits complex spatiotemporal patterns of growth, cell division, cell size, cell shape, and organ shape. Explaining these patterns presents a challenge because of their dynamics and cross-correlations, which can make it difficult to disentangle causes from effects. To address these problems, we used live imaging to determine the spatiotemporal patterns of leaf growth and division in different genetic and tissue contexts. In the simplifying background of the speechless (spch) mutant, which lacks stomatal lineages, the epidermal cell layer exhibits defined patterns of division, cell size, cell shape, and growth along the proximodistal and mediolateral axes. The patterns and correlations are distinctive from those observed in the connected subepidermal layer and also different from the epidermal layer of wild type. Through computational modelling we show that the results can be accounted for by a dual control model in which spatiotemporal control operates on both growth and cell division, with cross-connections between them. The interactions between resulting growth and division patterns lead to a dynamic distributions of cell sizes and shapes within a deforming leaf. By modulating parameters of the model, we illustrate how phenotypes with correlated changes in cell size, cell number, and organ size may be generated. The model thus provides an integrated view of growth and division that can act as a framework for further experimental study.

D'Arcy Thompson emphasised the importance of surface tension as a potential driving force in establishing cell shape and topology within tissues. Leaf epidermal pavement cells grow into jigsaw-piece shapes, highly deviating from such classical forms. We investigate the topology of developing Arabidopsis leaves composed solely of pavement cells. Image analysis of around 50,000 cells reveals a clear and unique topological signature, deviating from previously studied epidermal tissues. This topological distribution is established early during leaf development, already before the typical pavement cell shapes emerge, with topological homeostasis maintained throughout growth and unaltered between division and maturation zones. Simulating graph models, we identify a heuristic cellular division rule that reproduces the observed topology. Our parsimonious model predicts how and when cells effectively place their division plane with respect to their neighbours. We verify the predicted dynamics through in vivo tracking of 800 mitotic events, and conclude that the distinct topology is not a direct consequence of the jigsaw piece-like shape of the cells, but rather owes itself to a strongly life history-driven process, with limited impact from cell-surface mechanics.

1The budding of tubular organs from flat epithelial sheets is a vital morphogenetic 2 process. Cell behaviours that drive such processes are only starting to be unraveled. 3Using live imaging and novel morphometric methods we show that in addition to 4 apical constriction, radially oriented directional intercalation of placodal cells plays a 5 major contribution to the early stages of invagination of the salivary gland tube in the 6 Drosophila embryo. Extending analyses in 3D, we find that near the pit of 7 invagination, isotropic apical constriction leads to strong cell wedging, and further 8 from the pit cells interleave circumferentially, suggesting apically driven behaviours. 9Supporting this, junctional myosin is enriched in, and neighbour exchanges biased 10 towards the circumferential orientation. In a mutant failing pit specification, neither 11 are biased due to an inactive pit. Thus, tube budding depends on a radially polarised 12 pattern of apical myosin leading to radially oriented 3D cell behaviours, with a close 13 mechanical interplay between invagination and intercalation. 14 15 6 apical-basal depths in the cells, and uncover cell behaviours in a 3D context: near 1 the pit of invagination, where medial myosin II is strong (Booth et al., 2014), cells are 2 isotropically constricting apically leading to cell wedging, and with distance from the 3 pit cells progressively tilt towards the pit. Cells also interleave apically in a 4 circumferential direction, i.e. they contact different neighbours along their length, a 5 process that can be compared to a T1 transition in depth. This strongly suggests 6 apically driven behaviours, and we show that across the placode junctional myosin II 7 is enriched in circumferential junctions leading to polarised initiation of cell 8 intercalation. This is followed by polarised resolution of exchanges towards the pit, 9 thereby contributing to tissue invagination. forkhead mutants, that fail to form an 10 invagination, only show unproductive intercalations that fail to resolve directionally, 11 likely due to the lack of an active pit. Thus, tube budding depends on a radial pattern 12 of 3D cell behaviours, that are themselves patterned by the radially polarised activity 13 of apical myosin II pools. The continued initiation of cell intercalation but lack of 14 polarised resolution in the fkh mutant, where the invagination is lost, suggest that a 15 tissue-intrinsic mechanical interplay also contributes to successful tube budding. 16 17