Plant growth provides the basis for life on earth and is a process that is intimately linked with human civilizations. Continuous development in agricultural practices and in plant breeding allows us to keep plant production in line with demands. Industrialization, based on an enormous input of energy, mostly fossil fuels derived from biomass produced in the past, made it possible to reach our current standard of living. However, the uncontrolled use of plants and fossil fuels is an important contributor to global warming due to the associated production of CO2. Furthermore, the reserves of this energy source are rapidly being depleted, so alternatives need to be found. Improved plant production is seen at least as a partial solution, as photosynthesis enables the "recycling" of CO2 and fixing of energy. Thus, in recent years, we have seen a rapidly emerging market for bio-energy as well as the production of a myriad of natural products from plants. These bio-energy and bio-product producing crops, however, compete for the available agricultural land used for the production of food and feed, which is already starting to affect market prices of these commodities. Therefore, there is a renewed pressure on plant scientists to find solutions to increase plant productivity in a sustainable way.

Plant growth is intimately connected to the capacity of source organs to produce assimilates. Light is a key energy source and environmental cue controlling development, predominantly via leaves. It is known that growth-promoting signals are perceived in mature leaves and transmitted via unknown signals to developing leaves to regulate their growth. The nature of this transmissible signal is not known, but assimilates, such as sugars are thought to play a key role. Plant hormones also provide long-distance signaling to interface environmental conditions and organ growth. At the cellular level extracellular signals are sensed, transmitted and integrated by intracellular signaling pathways, which on one hand can directly regulate metabolic enzymes and other cellular functions, while on the other hand they feed into the regulation of gene transcription, protein stability protein modifications to quantitatively fine-tune cellular components or behavior. However, little is known about the intracellular signaling pathways in plants that regulate growth or its various components. Genetic approaches are difficult when genes function in an interconnected complex network, and regulate processes that are quantitative, such as growth. Novel methods, together with systems approaches, are needed for multiplex measurements of the outputs of signaling pathways at various complexity levels.

Growth of new organs requires a combination of cell division in or near meristems, cell growth, differentiation and cell expansion. Both developmental and environmental inputs influence organ growth by altering the pool of proliferating cells. These developmental pathways are composed of individual modules consisting of signal(s), transducers, transcriptional regulator(s) and targets. Viewed this way, plant development is a cascade of events that, by continual external and internal input, direct the orderly activation of the hierarchically arranged modules. How these processes are linked and coordinated is not understood.

To gain a systems-wide understanding of any developmental or physiological process, an increasing number of methodologies to obtain "omics" data at various levels and of computational and network-modeling techniques are available. However, a key, sometimes overlooked issue is the precise experimental approach and is the exact source of the "omics" data. To understand a system, one should be able to produce, as far as possible, a list of its parts, to introduce perturbations in the system and to monitor the behavior of the parts following the perturbation. A further source of critical information is time-resolved data, because it can be assumed that changes in concentration/activity of the regulator will inevitably precede the changes in the regulated component.

First and foremost, the sequencing of the genome of Arabidopsis thaliana has launched plant science into the genomics era and provided a gathering platform for plant scientists. This is now rapidly followed by the sequencing of other plant genomes with agricultural importance, including rice, poplar, grapevine, tomato and maize. The impact of having the full list of coding and regulatory sequences for understanding the behavior of plant growth is enormous, as investigators can shift their attention from gene-identification to functional analysis of these genes at the molecular, cellular and whole plant levels. Genomic sequence availability also allowed the development of profiling technologies to monitor gene expression, protein abundance, localization and modifications on a genome-wide scale under a wide range of experimental conditions and in specific cells or tissues. Our ability to simultaneously study the function of virtually all genes encoded by the plant genome, has led to a new more holistic approach to biology named systems biology. Rather than focusing on the function of a few genes in a particular pathway, the emphasis in systems biology is to understand which are the key components regulating specific processes and how such components are connected in "regulatory networks".

As outlined above, plant growth is a particularly intriguing phenomenon as it is under the control of a multitude of interacting regulatory pathways. In this monograph several of the contributing pathways are reviewed, including light signaling (Lopez-Juez and F. Devlin, Chapter 11), the classical hormones auxin (Zago et al., Chapter 8), ethylene (Dugardeyn and Van Der Straeten,

Chapter 10), and brassinosteroids (Clouse, Chapter 9), and signaling pathways including the TOR pathway (Anderson, Chapter 12), Armadillo repeat proteins (Coates, Chapter 15) and the MAPK cascades (Suzuki and Machida, Chapter 13), and protein dephosphorylation mechanisms (Schweigenhofer and Meskiene, Chapter 14). Devoto and Paccanaro (Chapter 17) describe the use of profiling and modeling to analyze signaling pathways on a genome-wide level. Downstream of those signaling pathways,, several key aspects of growth regulation itself are discussed, starting from the unicellular perspective of algae (Bisova, Chapter 18) to the regulation of cell growth, cell division (Doerner, Chapter 1), the switch between division and differentiation (Magyar, Chapter 5), the endoreduplication processes (Yoshizumi et al., Chapter 6) and interactions between cell size and cell numbers (Ferjani et al., Chapter 3) in higher plants. At the whole organ level the role ofthe epidermal layer in growth control is reviewed (Ingram, Chapter 7) and overall organ size control mechanisms are explored (Anatasiou and Lenhard, Chapter 2). Finally, emerging experimental approaches as proteomics (Schulze, Chapter 16) and kinematic analysis of growth (Walter, Chapter 4) are described.

We think it is timely to bring together this overview of the developments in various areas of plant-growth research in this monograph, firstly to give the reader a comprehensive insight into the current state of knowledge in the field. Reading through, it is possible to see common themes emerging from different fields of research and therefore we hope that this book will also stimulate an integrating perspective for future research aimed to better understand the fascinating process that plant growth represents.

March 2008

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