Coordination of plant cell growth and division: collective control or mutual agreement?

Plant tissue growth requires the interdependent cellular processes of cytoplasmic growth, cell wall extension and cell division, but the feedbacks that link these processes are poorly understood. Recent papers have revealed developmentally regulated coupling between plant cell growth and progression through both mitotic cycles and endocycles. Modeling has given insight into the effects of cell geometry and tissue mechanics on the orientation of cell divisions. Developmental inputs by auxin have been highlighted in the control of cell turgor, vacuole function and the microtubule dynamics that underlies oriented growth and division. Overall, recent work emphasizes growth and proliferation as processes that are negotiated within and between cells, rather than imposed on cells across tissues.


Introduction
Both in plants and animals, organ growth can tolerate wide variations in cell proliferation through compensatory changes in cell size and shape, supporting the idea that cell growth and division are controlled in parallel by external signals that co-ordinate cell behavior at the tissue and organ level [1,2].At the same time, growth and cell cycle progression appear to be connected by homeostatic feedback loops within each cell [3,4,5 ] and this intracellular coordination would be expected to modify responses to external signals.The relative importance of external and intracellular integration of the processes required for cell and tissue growth is unclear in all multicellular organisms.
In plants, the rate and direction of cell growth depend on the balance between turgor pressure and the resistance of the cell walls to tensile stress [6,7].As the walls yield to turgor pressure, the larger cell volume is occupied through a combination of increased macromolecular synthesis and enlargement of vacuoles (Figure 1) [8].During the proliferative stage, the enlarged cell eventually divides in a particular direction.The coordinated processes of cell growth and division also respond to external signals, such as nutrient availability and mechanical stress [9,10], and to developmental control, typically mediated by hormones and localized expression of transcription factors [11].Here, I review recent insights on the intracellular mechanisms that coordinate plant cell growth and division, how these mechanisms respond to external inputs and how integration within each cell feeds back on the growth of tissues and organs.I focus on meristems and organ primordia, where cell growth and division coexist, discussing initially the coordination of rates, then directions of cell growth and division.

Coordination between rates of growth and cell cycle progression
As mentioned above, turgor pressure is the mechanical driver for plant cell growth.It is often assumed that turgor is constant, but this is not always the case: the emergence of lateral root primordia is facilitated by softening of cell walls in the overlying cortex and epidermis [12] and by localized regulation of turgor mediated by aquaporins [13].It has recently been shown that during the earliest stages of lateral root emergence, enlargement of pericycle cells is accommodated by an auxin-induced reduction in the size of neighboring endodermal cells, presumably requiring turgor changes; without this accommodating response, the lateral root cannot develop [14 ].
The increased cell volume associated with turgor-driven wall extension is occupied by a combination of macromolecular synthesis and vacuolar growth.In the root meristem, auxin has been shown to limit the enlargement of late meristematic cells through rapid post-transcriptional increase in the abundance of vacuolar SNARE proteins, which control vacuolar morphology [15 ].These auxininduced changes were mediated by the actin cytoskeleton and reduced the volume of the vacuole relative to that of the cell [16 ].The auxin-dependent changes in vacuole morphology were proposed to regulate cytosol density during cellular expansion [16 ]; if this is the case, vacuolar function might also be expected to be coordinated with overall macromolecular synthesis (Figure 2).
Ultimately, cell growth depends on macromolecular synthesis, which is coordinated by the conserved Ser/Thr kinases TOR and SnRK1 [10].When sufficient sugar is available, TOR promotes meristem activity and organ growth not only through its conserved role in promoting macromolecular synthesis, but also through direct regulation of the cell cycle regulator E2Fa and potentially through control of cell wall remodeling [17][18][19].At the same time, high sugar levels lead to inhibition of the SnRK1 kinase, which promotes catabolism and inhibits cell cycle progression [20].Recently, a link emerged between SnRK1 and the differential growth that establishes the boundaries between the meristem and emerging organs [21]: overexpression of the catalytic subunit of SnORK1 (AtKIN10) caused organ fusions [22] and AtKIN10 directly interacted with the transcription factor PETAL LOSS, which controls organ boundary development [23].
When nutrients are not limiting, overall meristem activity is high, but the growth rate of neighboring cells within the meristem and developing organs is surprisingly variable [5 ,24 ].Detailed analysis in developing sepals suggested that growth curves are similar in neighboring cells, but shifted and scaled by size [25].This local heterogeneity of growth rates is affected by microtubule dynamics, which probably mediates cellular responses to the mechanical stress that builds up during tissue growth [24 ].Presumably the response of individual cells to local stress leads to variable growth rates through changes in cell wall extensibility [9,26,27].However, it remains unknown whether vacuolar function and turgor pressure might also be locally regulated and whether different rates of cell enlargement are accompanied by variation in biosynthetic rates.
Over time, variable cellular growth rates combined with the imprecision of cell divisions [5 ,28] would be Coordination between cell growth and division Sablowski 55 Overview of the coordinated cellular processes required for meristem and organ primordium growth.

Mechanical stress
Cell cycle [14] Auxin As in animals, it remains unclear how plant cells could assess their size and feed back the information on cell cycle progression (Figure 2).In contrast to the meristem, differentiating organs show a wide range of cell sizes and shapes, suggesting that the mechanisms that link cell growth to cell cycle are developmentally regulated.Accordingly, the coordination between cell size and S-phase entry changes at the transition from meristem to organ identity [33], and cell sizes diverge in developing sepals due to variability in cell cycle length and in the switch to endocycles [34].The shift to endocycles is caused by selective inhibition of mitosis, while allowing repeated re-entry into S-phase; the consequent increase in cell ploidy is believed to increase the physiologically sustainable cell size [35].
Consistent with this permissive role of endoreduplication, the transition to endocycles precedes cell enlargement in the root meristem [36].However, like the coupling between the mitotic cycle and cell size, the relation between endocycles and cell size appears to be developmentally regulated and dependent on cell type [37].

Coordination between oriented growth and division
Morphogenesis depends not only by on the rates, but also the directions of growth [38].Directional cell growth is influenced by the deposition of cellulose microfibrils, which increase tensile strength in the direction along which they are laid down [7].The deposition of cellulose microfibrils is in turn guided by the orientation of cortical microtubules, which serve as tracks for the cellulose synthase complexes that produce the microfibrils [39].
In addition to the established role of cellulose microfibrils, a novel mechanism that controls oriented cell growth has been revealed: targeted vesicle traffic to the edges of cell walls (i.e. to the intersection of wall facets) was required for anisotropic growth in root and leaf primordia, suggesting that edges have a special role in cell wall mechanics [40 ].Like the pattern of microfibril deposition, this targeted secretion depended on microtubule arrays, although in this case the actin cytoskeleton was also involved [40 ] (Figure 3).
The dynamic and self-organizing properties of microtubule arrays make them, and consequently the direction of cell growth, highly sensitive to external inputs.One of these external influences is mechanical stress, which results in part from the growth of connected cells within the tissues [9,  New walls are expected to bear load and alter the distribution of mechanical stress, at least locally.Although it has been considered that the placement of new walls has little effect on overall tissue mechanics, simulations have shown that the rules to orient new cell divisions do affect the local variability of growth and the overall tissue growth [6,47].The placement of cell walls also determines the overall shape of daughter cells, and mechanical models have shown how the shape of individual cells can influence patterns of tissue growth [54].The cumulative effect that a regulated pattern of cell divisions can have on tissue growth remains unclear and is an important topic for future work.

Conclusions and perspectives
The details of how different growth processes interact within and across cells is important are important for our Coordination between cell growth and division Sablowski 57 understanding of how the size and shape of plant organs are genetically determined.An extreme view, articulated by Kaplan in the 1990s, is that subdivision of organs into cells provides physiological support, allows cell specialization and may have mechanical consequences, but rates and orientations of tissue growth are controlled chiefly by supra-cellular cues [55].Many current models of plant morphogenesis embrace a similar view, partly due to the difficulties of implementing spatial models of organ growth with cellular resolution [6].The work reviewed here emphasizes growth as a process of negotiation within and between cells, in which internal coordination of metabolism, cell wall functions and cell cycle progression are integrated with mechanical and chemical signals operating across tissues.The outcome of this intracellular integration, in turn, feeds back on the directions and rates of tissue growth and on patterning.Quantitative imaging of cell behavior combined with computational models that specify the properties and interactions of individual cells [56 ] will be key for future progress in this area.

31.
Schmoller KM, Turner J, Ko ˜ivoma ¨gi M, Skotheim JM: Dilution of the cell cycle inhibitor Whi5 controls budding-yeast cell size.Nature 2015, 526:268-272.A novel molecular mechanism is proposed to links cell cycle progression to cell size in budding yeast.The Cln3 cyclin-Cdk1 kinase controls progression from G1 to S-phase by inactivating the Whi5 transcriptional repressor.The concentration of Whi5 decreases during the cell cycle, until a threshold is reached that allows progression to S-phase.The initial Whi5 concentration in a cell depends on the size of its mother cells: smaller mother cells generate small daughters with higher Whi5 levels, leading to a longer G1 phase and consequently cell size correction.A key aspect of this mechanism is that Whi5 is synthesized at a constant rate, in contrast to the majority of other genes, whose transcription and translation rates scale with the general biosynthetic capacity of the cell, which in turn reflects cell size.

32.
Li Y, Liu D, Lo ´pez-Paz C, Olson BJSC, Umen JG: A new class of cyclin dependent kinase in Chlamydomonas is required for coupling cell size to cell division.eLife 2016, 5:e10767.In Chlamydomonas reinhardtii, cells grow in the light, then stop growing but cycle rapidly through S and M phases in the dark.The number of cycles in the dark reflects the size of the mother cell at the end of the light period and restores cells to their initial size.A mutant screen for perturbed restoration of cell sizes revealed a novel cyclin-dependent kinase (CDKG1), which regulates the number of mitotic cycles.The number of cycles depended on the initial amount of CDKG1, which was proportional to the size of the mother cell.CDKG1 regulates activity of the retinoblastoma (RB) pathway, which had previously been implicated in cell size control in Chlamydomonas.This work reveals that targeted secretion to the edges between cell walls controls oriented cell growth.A fusion between YFP and the Rab GTPase RAB-A5c was transported between the trans-Golgi network and cell wall edges in root and leaf primordia.This localization was lost in mature cells, correlated with the orientation of cell elongation and depended on the integrity of the microtubule and actin cytoskeletons.Live imaging and cell tracking showed that a dominant-negative version of RAB-A5c disrupted oriented cell expansion and sometimes caused rupture of cells, suggesting that their tensile strength had been compromised.A 2D finite-element model suggested that reducing stiffness of cell edges can cause radial cell expansion as seen after expression of the dominant negative version of RAB-A5c.

41.
Sampathkumar A, Krupinski P, Wightman R, Milani P, Berquand A, Boudaoud A, Hamant O, Jo ¨nsson H, Meyerowitz EM: Subcellular and supracellular mechanical stress prescribes cytoskeleton behavior in Arabidopsis cotyledon pavement cells.eLife 2014, 3:e01967.This paper focused on the growth of jigsaw puzzle-shaped epidermal cells to explore the links between mechanical stress, microtubule dynamics and cell wall deposition at the subcellular level.Atomic force microscopy showed that the neck regions of growing epidermal pavement cells were stiffer than lobe regions.This correlated with the microtubule alignment and increased cellulose deposition across the neck region.A finite element model with mechanically isotropic walls predicted anisotropic stress focused on the indenting regions of the pavement cells, suggesting how mechanical stress could mediate the feedback between cell shape and growth.

43.
Sassi M, Ali O, Boudon F, Cloarec G, Abad U, Cellier C, Chen X, Gilles B, Milani P, Friml J: An auxin-mediated shift toward growth isotropy promotes organ formation at the shoot meristem in Arabidopsis.Curr Biol 2014, 24:2335-2342.Auxin was shown to trigger the emergence of floral primordia in the meristem in part by altering microtubule orientation.When primordium initiation was blocked by the auxin transport inhibitor NPA, microtubules were aligned to the meristem circumference, but became disorganized when primordia were induced by auxin treatment.Disruption of microtubule arrays with oryzalin or by a katanin mutation was sufficient to promote primordium outgrowth in apices with defective auxin transport.Finite element models of the growing apex suggested that a shift to anisotropic growth, which is associated with disorganized microtubule arrays, could amplify the effect of local cell wall loosening, which alone would not be sufficient to initiate primordium outgrowth.

48.
Yoshida S, de Reuille PB, Lane B, Bassel GW, Prusinkiewicz P, Smith RS, Weijers D: Genetic control of plant development by overriding a geometric division rule.Dev Cell 2014, 29:75-87.Three-dimensional segmentation and statistical image analysis were used to produce a quantitative description of the patterns of cell growth and division during Arabidopsis embryogenesis.Similar to the early cleavage of animal embryos, cell volumes rapidly decreased with the first embryonic divisions but subsequently remained stable.Cell divisions that conformed to default rules based on cell geometry (see [44]) generated daughter cells with comparable fates; whereas asymmetric divisions correlated with daughter cells of different identities, based on reporter genes.Analysis of auxin response mutants showed that these asymmetric divisions depended on auxin signaling.The asymmetric

56.
Boudon F, Chopard J, Ali O, Gilles B, Hamant O, Boudaoud A, Traas J, Godin C: A computational framework for 3D mechanical modeling of plant morphogenesis with cellular resolution.PLoS Comput Biol 2015, 11:e1003950.In this paper, a mathematical model was developed for the 3D interaction between cell-autonomous mechanical stress (i.e.resulting from a cell's turgor pressure and its cell wall properties) and stress imposed by its connection to surrounding cells.The model was implemented using the finite element method to calculate how tissues deform given a specified cellular structure, local cell wall properties and constant turgor.By comparing simulated shapes with the observed shapes of growing floral buds, the model was used to explore assumptions about differences in cell wall rigidity and anisotropy in organ boundaries and between the abaxial and adaxial regions of sepal primordia.
Figure 1 It will be important to determine what aspect of size (e.g.cell volume, cytoplasmic volume or cell surface area [29]) best correlates with cell cycle progression.A potential molecular mechanism is illustrated by recent work in budding yeast, with dilution of a cell cycle inhibitor whose synthesis rate does not scale with cell volume [31 ].In the unicellular alga Chlamydomonas, cell growth in the light is followed by multiple rounds of rapid division in the light, restoring the initial cell size; in this case, accumulation of a variant cyclin-dependent kinase during light growth determined the subsequent number of divisions and consequently the final cell size [32 ].
cell divisions, which give rise to different cell types.During early embryogenesis, numerous divisions do not follow the default geometric rules described above for proliferative growth.These asymmetric divisions require auxin signaling and correlate with the acquisition of different cell fates[48 ].Presumably auxin re-orients cell division by altering the dynamics of microtubule arrays as discussed above, but the molecular details remain unknown.By changing cell connectivity, intercellular communication and cell fate, asymmetric divisions are expected to have major effects on subsequent development, as shown by recent work on vascular patterning in the growing embryo, in which an auxin-induced source of cytokinin induces periclinal cell divisions in neighboring cells to create vascular progenitors[49 ].This work also suggested that correct patterning depends on the initial cell geometry, which originated from a symmetry-breaking division very early in embryogenesis.What remains unclear, however, is to what extent oriented divisions impact on the mechanics of tissue growth.A causative role for oriented divisions has been suggested based on periclinal divisions seen in subepidermal cells before the outgrowth of leaf primordia[50,51].On the other hand, classic work on the maize tangled-1 mutant, in which the orientation of cell divisions is disrupted, showed relatively modest effects on leaf size and shape[52].More recent work on the development of pitcher leaves in the carnivorous plant Sarracenia purpurea has suggested that changes in the orientation of cell division in subepidermal layers cause differences in primordium growth that initiate the formation of the pitcher [53].It remains difficult, however, to exclude that oriented divisions are a response to mechanical stress within the tissues, which could result from regulation of growth through cell wall mechanics.
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Coordination between cell growth and divisionSablowski 59 www.sciencedirect.comCurrent Opinion in Plant Biology 2016, 34:54-60 divisions generated an organized pattern of cell connectivity (defined as the amount of shared surface between each pair of cells), which could in turn influence intercellular signaling and cell fate.49.De Rybel B, Adibi M, Breda AS, Wendrich JR, Smit ME, Nova ´k O, Yamaguchi N, Yoshida S, Van Isterdael G, Palovaara J: Integration of growth and patterning during vascular tissue formation in Arabidopsis.Science 2014, 345:1255215.This paper showed that vascular patterning in the embryo results from an auxin-induced source of cytokinin that induces periclinal cell divisions in neighboring cells.The dimeric transcription factor TMO/LHW promoted periclinal divisions during vascular development by activating cytokinin synthesis.A 2D simulation of growing and dividing cells, in which high auxin promoted cytokinin synthesis but inhibited cytokinin responses, reproduced the patterning and hormone responses seen in the wild type and in auxin or cytokinin mutants.The model also suggested that correct patterning depends on the initial cell geometry, which originated from a symmetry-breaking division in early embryogenesis.50.Lyndon RF: Tansley Review No. 74 control of organogenesis at the shoot apex.New Phytol 1994, 128:1-18.51.Cunninghame ME, Lyndon RF: The relationship between the distribution of periclinal cell divisions in the shoot apex and leaf initiation.Ann Bot 1986, 57:737-746.52.Smith LG, Hake S, Sylvester AW: The tangled-1 mutation alters cell division orientations throughout maize leaf development without altering leaf shape.Development 1996, 122:481.53.Fukushima K, Fujita H, Yamaguchi T, Kawaguchi M, Tsukaya H, Hasebe M: Oriented cell division shapes carnivorous pitcher leaves of Sarracenia purpurea.Nat Commun 2015, 6:6450.54.Bassel GW, Stamm P, Mosca G, de Reuille PB, Gibbs DJ, Winter R, Janka A, Holdsworth MJ, Smith RS: Mechanical constraints imposed by 3D cellular geometry and arrangement modulate growth patterns in the Arabidopsis embryo.Proc Natl Acad Sci U S A 2014, 111:8685-8690.