Tessellations and Pattern Formation in Plant Growth and Development

The shoot apical meristem (SAM) is a dome-shaped collection of cells at the apex of growing plants from which all above-ground tissue ultimately derives. In Arabidopsis thaliana (thale cress), a small flowering weed of the Brassicaceae family (related to mustard and cabbage), the SAM typically contains some three to five hundred cells that range from five to ten microns in diameter. These cells are organized into several distinct zones that maintain their topological and functional relationships throughout the life of the plant. As the plant grows, organs (primordia) form on its surface flanks in a phyllotactic pattern that develop into new shoots, leaves, and flowers. Cross-sections through the meristem reveal a pattern of polygonal tessellation that is suggestive of Voronoi diagrams derived from the centroids of cellular nuclei. In this chapter we explore some of the properties of these patterns within the meristem and explore the applicability of simple, standard mathematical models of their geometry.Comment: Originally presented at: "The World is a Jigsaw: Tessellations in the Sciences," Lorentz Center, Leiden, The Netherlands, March 200

Modelling the Guayule plant growth and development with a Functional Structural Plant Model

The Guayule (Parthenium argentatum, Asteraceae), is a small ramified tree native to the northern Mexico and southwestern United States. The guayule shows a growing interest in research and agriculture (Ray, 1993) due to its hypoallergenic latex properties (Taurines et al., 2019), and seems adapted to South France climate (Sfeir and al., 2014). However, the production itineraries in relation to latex production are still not assessed, and so far little studies were done on the plant structure and functioning. This study aims to propose a first FSPM of the species using the GreenLab model, calibrated from data issued from two varieties in different environmental conditions. The studying methodology is first based on a qualitative architectural analysis (Barthelemy and al., 2007). Second, on the various axis typologies, the development and branching stochastic rules can then be retrieved from field internode distributions collections. Finally, the organ source and sink relations parameters can be fitted from dedicated dry weight measurements (Kang et al., 2018). Experimental plots were hold south of France, close to Montpellier on two varieties CL1 and CLA1, with six environmental conditions related to density (9091 and 62500 plants per hectare) and hydric pressure (no stress, low stress and high stress). 50 plants per environmental conditions were measured. The sampling was optimized to the plant structure and to quantify the polyisoprene and resins contents. The guayule shows a sympodial development is composed of modules with terminal inflorescence. Its architecture corresponds to the Leeuwenberg's model (Hallé et al., 1978). The axes are constituted of successive modules. Over a year, the plant produces eight to nine successive modules. Studying the plant structure, we found out that the number of relay axis per module follows a binomial distribution. The modules are ordered from the plant base to the top. And these modules are composed of internodes whose number also follows a binomial law, which parameters are quite stable from one order to another. In the further modelling process, we thus did consider that the plant elementary unit was the module, called as a meta-phytomer. Under this assumption, we summarized the total dry weight of leaves and internodes per module to build the axis organic series (Buis and Barthou, 1984). Field measurements issued from these two series constituted then a target to be adjusted by the structural functional GreenLab model (Kang et al, 2018) in order to calibrate the organ source parameters. An initial analysis calculated the strength sink of leaves and internodes in a context of free growth and analysed the differences between the two varieties. We are currently applying the methodology to assess the impact on the parameters of development and growth, the effects of planting density and irrigation. This first modelling study hold on two varieties on the Guayule tree shows that the plant structure can be efficiently modelled using a simple module approach. The development parameters, defining the module number of phytomers and branching rules are nearly stable and close for both varieties under the various environmental conditions. First functioning parameters were also retrieved from the measurements. These parameters make it possible to obtain the first stochastic 3D simulations of the Guayule's growth and architecture for both varieties

Simulation of long-term stem diameter variation of Ficus benjamina based on simulated transpiration

Greenhouse microclimate (light, temperature, relative humidity and CO2) and irrigation are important factors for plant growth, development and quality in ornamental horticulture. To optimize plant growth, actual stem diameter growth can be measured and compared with a desired growth pattern. Using the deviation between measured and simulated stem diameter growth, growers can decide whether and in which way the microclimate or irrigation needs to be adjusted. Together with this decision, costs associated with climate control and irrigation must also be taken into account. This will help growers to find a proper balance between cultivation costs and plant growth. In this study, Ficus benjamina was grown from cutting to mature plant in a controlled greenhouse environment. Growing conditions, microclimate as well as plant spacing, closely resembled the ones used in commercial greenhouses. Microclimate, soil water content, leaf temperature, sap flow, stem diameter variation and leaf thickness were continuously measured on three plants. In addition, discrete measurements of leaf area, projected crown surface area, stem water potential, photosynthesis, transpiration and stomatal conductance were performed. These measurements were used to further extend a mechanistic plant model, which allows simulation of long-term stem diameter variation

The Development of Polyamines throughout Brassica rapa over its Lifecycle

Polyamines are naturally produced chemicals in plants involved in growth, development and stress response. The primary objective of my study is to create a profile of changes in the entire life of the plant, in every organ at all stages of development from seed germination to seed formation. We have analyzed polyamines putrescine, spermidine and spermine in all parts of Brassica rapa, a small, rapid growing plant. Parallel to the polyamines, we will also study changes in the activities of the polyamine biosynthetic enzymes and the expression of their genes in different organs at different times. In the next stage of the study, the expression of selected genes will be inhibited by RNAi constructs, allowing further analysis of their role in growth and stress response. Because polyamines play are important in development and lifecycle of plants, altering their presence may be useful in altering plant growth patterns, such as in seasonal crops

Light-regulated plant growth and development.

Plants are sessile and photo-autotrophic; their entire life cycle is thus strongly influenced by the ever-changing light environment. In order to sense and respond to those fluctuating conditions higher plants possess several families of photoreceptors that can monitor light from UV-B to the near infrared (far-red). The molecular nature of UV-B sensors remains unknown, red (R) and far-red (FR) light is sensed by the phytochromes (phyA-phyE in Arabidopsis) while three classes of UV-A/blue photoreceptors have been identified: cryptochromes, phototropins, and members of the Zeitlupe family (cry1, cry2, phot1, phot2, ZTL, FKF1, and LKP2 in Arabidopsis). Functional specialization within photoreceptor families gave rise to members optimized for a wide range of light intensities. Genetic and photobiological studies performed in Arabidopsis have shown that these light sensors mediate numerous adaptive responses (e.g., phototropism and shade avoidance) and developmental transitions (e.g., germination and flowering). Some physiological responses are specifically triggered by a single photoreceptor but in many cases multiple light sensors ensure a coordinated response. Recent studies also provide examples of crosstalk between the responses of Arabidopsis to different external factors, in particular among light, temperature, and pathogens. Although the different photoreceptors are unrelated in structure, in many cases they trigger similar signaling mechanisms including light-regulated protein-protein interactions or light-regulated stability of several transcription factors. The breath and complexity of this topic forced us to concentrate on specific aspects of photomorphogenesis and we point the readers to recent reviews for some aspects of light-mediated signaling (e.g., transition to flowering)

Measurement of plant growth in view of an integrative analysis of regulatory networks

As the regulatory networks of growth at the cellular level are elucidated at a fast pace, their complexity is not reduced; on the contrary, the tissue, organ and even whole-plant level affect cell proliferation and expansion by means of development-induced and environment-induced signaling events in growth regulatory processes. Measurement of growth across different levels aids in gaining a mechanistic understanding of growth, and in defining the spatial and temporal resolution of sampling strategies for molecular analyses in the model Arabidopsis thaliana and increasingly also in crop species. The latter claim their place at the forefront of plant research, since global issues and future needs drive the translation from laboratory model-acquired knowledge of growth processes to improvements in crop productivity in field conditions

When the time is ripe.

The diverse effects of the plant hormone ethylene on development and growth are shaped by the actions of a master regulator of transcription, EIN3

The plant hormone ethylene restricts Arabidopsis growth via the epidermis

The gaseous hormone ethylene plays a key role in plant growth and development, and it is a major regulator of stress responses. It inhibits vegetative growth by restricting cell elongation, mainly through cross-talk with auxins. However, it remains unknown whether ethylene controls growth throughout all plant tissues or whether its signaling is confined to specific cell types. We employed a targeted expression approach to map the tissue site(s) of ethylene growth regulation. The ubiquitin E3 ligase complex containing Skp1, Cullin1, and the F-box protein EBF1 or EBF2 (SCFEBF1/2) target the degradation of EIN3, the master transcription factor in ethylene signaling. We coupled EBF1 and EBF2 to a number of cell type-specific promoters. Using phenotypic assays for ethylene response and mutant complementation, we revealed that the epidermis is the main site of ethylene action controlling plant growth in both roots and shoots. Suppression of ethylene signaling in the epidermis of the constitutive ethylene signaling mutant ctr1-1 was sufficient to rescue the mutant phenotype, pointing to the epidermis as a key cell type required for ethylene-mediated growth inhibition