Introduction

Lateral plant organs, such as leaves, floral organs and fruits, are produced throughout post-embryonic life. They are ultimately derived from the stem cells of the indeterminate shoot apical meristem (Veit 2006). Yet, while growth of the whole plant does generally not have a fixed endpoint, individual organs only grow to defined sizes and shapes that are determined by the plant's genotype and the identity of the organ; for example, leaves versus floral organs (Ingram and Waites 2006; Mizukami 2001). Our ability to easily distinguish plant species with very similar morphologies but different sizes, for example daisies and marguerites, underlines the strong heritability of organ size, which can only be modified within certain limits by environmental factors. By contrast, evolution has generated an enormous variety of organ sizes, ranging from the giant leaves of some water lilies or bananas, for example, to the more modest dimensions of Arabidopsis thaliana (Fig. 1). Despite recent progress, our understanding of the underlying genetic and molecular mechanisms that control plant organ size is still far from comprehensive. Unraveling these regulatory mechanisms is not only of great scientific interest, but is also of substantial applied relevance, as it

Fig. 1 Two extremes of plant organ sizes observed in nature. In contrast to banana trees with their giant leaves (left), the diminutive weed Arabidopsis thaliana forms only very small lateral organs (right). (Image of the banana tree courtesy of Dr. Philippe Vain, John Innes Centre, Norwich, UK.)

would allow the rational manipulation of organ size and thus biomass production, e.g., for generating biofuels. The following chapter will not only discuss the progress that has been made, focusing mainly on studies in Arabidopsis and other genetic model organisms, but will also attempt to highlight the gaps to be filled in our current knowledge of how plant organs decide for or against further growth.

Plant organs grow via two fundamentally different, yet closely coordinated processes (Fig. 2; Menand and Robaglia 2004). During a first phase of organ development, cells proliferate mitotically and produce new cyto-plasmic mass. After some time, cell proliferation gradually ceases and post-mitotic cells begin to expand by taking up water into their central vacuole. This post-mitotic cell expansion is often accompanied by a ploidy increase due to endoreduplication, for example in leaves and hypocotyls (Sugimoto-Shirasu and Roberts 2003; a process that will be discussed in more detail by Sugimoto-Shirasu in this volume), although in other cases (e.g., Arabidopsis floral organs), expanding cells remain diploid. These two phases will be referred to as growth by proliferation and growth by expansion throughout the text, respectively. It is generally assumed that little to no increase in cytoplas-mic mass occurs as cells grow by expansion (Sugimoto-Shirasu and Roberts

Fig. 2 A schematic overview of the pathways that promote or restrict organ growth. The phases of growth by proliferation and by expansion are shown as temporally separated only for illustrative purposes, although there is a gradual transition between the two phases in planta, with some cells in an organ starting to expand while others still divide. Genes that affect the duration of growth by proliferation are shown in italics, while the underline for EBP1 indicates its effect on the rate of proliferation. For the remaining genes that influence proliferation, the mode of action has not been precisely determined. See text for a detailed discussion of the individual factors

Fig. 2 A schematic overview of the pathways that promote or restrict organ growth. The phases of growth by proliferation and by expansion are shown as temporally separated only for illustrative purposes, although there is a gradual transition between the two phases in planta, with some cells in an organ starting to expand while others still divide. Genes that affect the duration of growth by proliferation are shown in italics, while the underline for EBP1 indicates its effect on the rate of proliferation. For the remaining genes that influence proliferation, the mode of action has not been precisely determined. See text for a detailed discussion of the individual factors

2003). However, it is clear that cells must continue to synthesize plasma and tonoplast membrane and cell wall material to provide for their size increase, implying that growth by expansion is an active process with the potential for regulation.

Thus, several important questions need to be answered with respect to the control of plant organ size. Which factors regulate growth by proliferation and/or expansion, and how do they interact with the basic cell cycle and biosynthetic machinery? How do organs with different identities attain their characteristic size and shape? Is growth of different organs regulated by distinct pathways, or are the same mechanisms active in diverse organ types, with their output being modified by the transcriptional networks that determine organ identity? How has evolution modified size control pathways to generate the amazing variety in organ sizes and shapes that we see in nature? What is actually being measured as an organ grows? Time, size, mass, or something else?

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