Introduction

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Plant cells commonly expand 100-fold on the way to attaining final form. Such spectacular change is the basis of primary growth, including cell expansion, organ formation or biomass accumulation at a plant community level. Events at a single cell level must be perfectly coordinated for plants to achieve functional and repeatable structures. Up to 18 000 cells are recruited from a root meristem each hour, and most of these cells undergo various degrees of expansion over the hours following division. What events bear on a cell emerging from a meristem or embryo as it undergoes expansive growth?

Cells emerging from a seed embyro, shoot apical meristem, lateral cambium of a tree or a root apex all encounter different fates. Therefore, the dynamics of cell expansion must reflect the characteristics of each tissue that will differentiate from these cell lines. For example, longitudinal versus radial expansion rates of cells giving rise to layers of epidermis and cortex in a coleoptile are sharply contrasting, resulting in long, slender epidermal cells and thicker, shorter cortical cells. In spite of their distinct morphological fates, growth is perfectly coordinated between adjacent cell files to produce a slim, straight coleoptile. On the other hand, differential rates of epidermal and cortical cell expansion in developing leaves result in large intercellular spaces developing in the spongy mesophyll (Section 1.2).

Transition from near-spherical, small cells (about 1 pL) arising from recent cell divisions to enlarged vacuolated and generally elongated cells of a mature plant is therefore subject to genetic and environmental constraints. These constraints manifest themselves in cell expansion through variations in (1) rate of expansion and (2) direction of growth (cell shape and polarity). The genome of each cell sets a programmed pattern of growth which is modulated by environmental factors. For example, formation of an inflorescence proceeds by expansive growth of new floral organs coordinated within a tight develop-mental framework encoded by homeotic genes. In contrast, the exceptional thickening of leaves in response to CO2 enrichment and salinity is achieved by environment signals that modify cellular growth rather than development.

Cell expansion requires maintenance of disequilibria at the cell level: ions must be imported, water must flow along free energy gradients and cell wall bonds must yield to turgor pressure. Cells must therefore use energy-yielding pathways and membrane-bound compartments to acquire ions at the correct osmotic and nutritional levels (Sections 4.1 and 4.2), regulate water inflow and stretch their walls (Section 4.3). Maintenance of electrical and chemical gradients is made possible by active (energised) transport across membranes. Dispersing the free energy stored in electrochemical gradients is controlled largely through channels located in membranes. Transport mech-anisms for ion and water uptake will be discussed in Section 4.1, while Section 4.2 will describe experimental approaches which are leading to a rapid expansion in our knowledge of membrane transport. Finally, the universally important process of cell-wall yielding will be discussed in Section 4.3, showing how resource acquisition and cell enlargement are coordinated.

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