EcoMeristem, a model of morphogenesis and competition among sinks in rice. 1. Concept, validation and sensitivity analysis

Abstract

Because of rapid advances in functional genomics there is an increasing demand for models simulating complex traits, such as the physiological and environmental controls of plant morphology. This paper describes, validates and explores the behaviour of the structural–functional model EcoMeristem, developed for cereals in the context of the Generation Challenge Program (GCP; CGIAR). EcoMeristem constructs the plant on the basis of an organogenetic body plan, driven by intrinsic (genetic) behavioural norms of meristems. These norms consist of phenological–topological rules for organ initiation and pre-dimensioning (sink creation) and rules enabling feedbacks of the plant’s resource status on the organogenetic processes. Plant resource status is expressed by a state variable called Internal Competition Index (Ic) calculated daily as the ratio of assimilate source (supply) over the sum of active sinks (demand). Ic constitutes an internal signal analogous to sugar signalling. Ic affects potential phytomer size, tiller initiation, leaf senescence, and carbohydrate storage and mobilisation. The model was calibrated and tested on IR64 rice grown in controlled environments, and validated with field observations for the same cultivar (Philippines). Observed distributions and dynamics of soluble sugars and starch in plant organs supported the model concepts of internal competition and the role of reserves as a buffer for Ic fluctuations. Model sensitivity analyses suggested that plant growth and development depend not only on assimilate supply, but also on organogenesis-based demand. If true, this conclusion has important consequences for crop improvement strategies.

The quest for understanding phenotypic variation via integrated approaches in the field environment

“…In the context of breeding, models have been used for two purposes, first to extend envirotyping beyond the analysis of environment variables per se (i.e. using the model as an integrator that characterizes the growth environment), and second to extend the phenotyping per se at the level of the plot or plant, sometimes described as model-assisted phenotyping (Luquet et al, 2006;Rebolledo et al, 2015). A newly sought outcome from applications of CGMs is to combine their outputs of development and growth variables with knowledge from genomics and understanding of genetic architecture in order to predict the trait combinations, and eventually the allele combinations, that provide improved solutions in breeding (Technow et al, 2015).…”

CN-Wheat, a functional–structural model of carbon and nitrogen metabolism in wheat culms after anthesis. I. Model description

“…As a signal, cytokinins are expressed in arbitrary units (AUs) and are not taken into account in the mass balance. In CN-Wheat, metabolite concentrations are expressed relative to organ structural dry mass, as proposed by several authors (Tabourel-Tayot and Gastal, 1998;Thornley, 1998;Luquet et al, 2006).…”

“…Nevertheless, plants are not fundamentally driven by central regulatory mechanisms and empirical rules may be impaired by environmental variations. In contrast, mechanistic models explicitly account for biological processes of plants and deal with concepts and variables that can be assessed and measured experimentally (TabourelTayot and Gastal, 1998;Luquet et al, 2006;Bertheloot et al, 2011;Grafahrend-Belau et al, 2013).…”

“…Most present FSPMs have mainly addressed the representation of realistic plant architecture to assess interactions with the (a)biotic environment (SaintJean et al, 2004;Cici et al, 2008;Robert et al, 2008;Barillot et al, 2014). Based on calculations of local light interception, various FSPMs account for the assimilation of carbon, but assimilate partitioning is generally solved using a supply-demand approach whereby the supply synthesized by sources is shared among sinks according to their 'demand' (Luquet et al, 2006;Evers et al, 2010;Sarlikioti et al, 2011;Bertheloot et al, 2011). By contrast, an example of mechanistic treatment of sink-source relations for C has been proposed by Allen et al (2005).…”

Oscillations in Functional Structural Plant Growth Models

“…Indeed, this model focuses on processes that are generic enough to be applied to most plant species (biomass prodution and allocation, coupled with the plant topological development); its main principles are shared by several other 'carbon-driven' models (e.g. TOMSIM [10], one extension of the ADELwheat model [5] or EcoMeristem [18]) so parts of this work can be generalized; changes in plant topology are dynamically simulated in interaction with the plant physiological state, which allows modelling phenomena such as fruit abortion or branch appearance. Besides, a strong effort was put on its rigourous mathematical formalization, thus paving the way to the study of its behaviour.…”