Project: Integrative Systems Analysis of the Shoot Apical Meristem

The aim of the project iSAM is to understand how complex structures and patterns are produced at the growing tip of the plant shoot using combined modelling and experimentation. The shoot tip contains the shoot apical meristem (SAM), a population of dividing, undifferentiated cells that generates leaves and other organs in highly ordered patterns at shoot tips throughout the life of the plant. The SAM therefore has two functions. First it houses at its centre a stem cell population that is stable throughout the life cycle of the plant, which may be hundreds of years in the case of long-lived trees. It also initiates organs, and, since plant growth arises from the repeated production of new organs as the plant grows, the SAM is responsible for specifying all aboveground plant tissues. It therefore determines plant architecture and, indirectly, many aspects of agricultural productivity. The SAM has been extensively studied and we know many of its molecular and cellular components, but we do not understand how these components assemble into the multicellular structure with specific shape and growth dynamics. It is these questions this project will address using an iterative process of analysis, model building, biological testing and refinement. The individual cells within the SAM interact by exchanging signals and the interaction network that feeds back on the 'machineries' of individual cells, controlling local growth through the modulation of division rates. Added up, the local cell proliferation rates, patterned by the signalling networks, lead to specific shape changes. To understand the SAM, we will use a complex systems modelling approach, focusing on the interaction networks provided by the key plant hormones auxin and cytokinin. This proposal links four leading research teams in UK, France and Finland, bringing synergistic expertise and technologies in imaging, modelling, plant hormones and cell cycle to address this important systems problem.

Acronym	iSAM
Project Results (after finalisation)	Morphogenesis requires the coordinated actions of many cells. However, despite extensive focus on the cellular components, we still do not understand how complex structures and patterns are produced. The goal of this project has been to understand the morphogenetic events leading to growth and organ production in the shoot apical meristem (SAM) of higher plants, in an iterative process of analysis, model building, biological testing and refinement. The SAM is a population of dividing, undifferentiated cells that generates leaves and other organs in highly ordered patterns at shoot tips throughout the life of the plant. The SAM is a complex self-maintaining and stable structure, housing at its centre a totipotent stem cell population and initiating organs on its periphery. The SAM produces the cells that comprise all aboveground plant tissues, and defines the number, type and position of lateral organs. SAMs are thus the basis of plant architecture and determinants of major agronomic traits. In view of its importance, the SAM has been extensively studied and a wealth of information is available concerning its molecular and cellular components. Nevertheless, we do not understand how these components assemble into the multicellular structure with specific shape and growth dynamics. It is this question we have addressed. The SAM is a typical example of a complex system, where individual entities, the cells, interact by exchanging signals. It is the overall structure of the interaction network that feeds back on the 'machineries' of individual cells, thereby controlling local growth through the modulation of proliferation rates. Added up, the local cell proliferation rates, patterned by the signalling networks, lead to specific shape changes, which are the emergent properties of the system. To understand the SAM, we have used a complex systems approach. This proposal linked four leading research teams in UK, France and Finland, bringing synergistic expertise and technologies in imaging, modelling, auxin, cytokininin and cell cycle to address this important systems problem. We have focused on the interaction networks, concentrating on two established essential signals, the plant hormones auxin and cytokinin. These integrate stem cell maintenance, organ initiation and meristem organisation through their effects on the component cells. We have provided a detailed spatial description, or map, of these signalling components in the SAM and a quantified link between these signals and cell proliferation rates in different parts of the meristem, based on novel image collection and analysis techniques that allow data to be captured for the whole SAM structure. These have been combined with quantitative analysis of gene expression and with new fluorescent markers developed in this project that provide dynamic reporting of the position of individual cells in the cell cycle. These detailed, multidimensional data have served as direct input into new predictive mathematical models for morphogenesis and gene regulation then further tested through rounds of experimental perturbation and analysis. The outcomes of the project have been a greatly increased understanding of the cellular basis of morphogenesis in the important SAM system. In particular, a new role for the hormone cytokinin in maintaining the phyllotactic pattern of organ emergence has been established during the project, and was published in Nature. A significant number of other publications and reviews have also emerged from the research, and will continue to flow from the work that continues to be ongoing. The project has served to integrate and coordinate meristem research and provide a platform for future work, embed systems modelling approaches in the partner labs and provide important training opportunities.
Network	ERASysBio+
Call	ERASysBio+-2008-01

Project partner

Number	Name	Role	Country
1	Cardiff University	Coordinator	United Kingdom
2	École normale supérieure de Lyon	Partner	France
3	INRIA Montpellier	Partner	France
4	University of Helsinki	Partner	Finland