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WIREs Dev Biol
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Patterns of root growth acclimation: constant processes, changing boundaries

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Abstract Plasticity, the hallmark of plant morphogenesis, extends to kinetics. To enhance acclimation, growing plant organs adeptly adjust their growth rate, up or down. In roots, rates of division and elemental expansion as well as the length of division and elongation zones are readily characterized because of their linear organization, radial symmetry, and indeterminate growth, and can be measured accurately with kinematic methods. Here, for roots, I describe key concepts from kinematics and review patterns of growth and division during acclimation. The growth rate of a root reflects the integral of elemental expansion activity over the span of the growth zone; therefore, an acclimating plant can change the rate of root growth by changing either or both the span of the growth zone or the rate of elemental expansion. The analogous dichotomy exists for cell division where the rate at which cells are produced reflects the integral of cell division rate over the span of the division zone. Surprisingly, expansion responses nearly always involve changes in the length of the growth zone. Similarly, although based on fewer data, changes in cell division rate are rare, whereas changes in meristem length are common. These patterns imply that setting the boundaries for meristem and elongation zone is the key regulatory act for root growth rate acclimation. WIREs Dev Biol 2013, 2:65–73. doi: 10.1002/wdev.94 This article is categorized under: Establishment of Spatial and Temporal Patterns > Cell Sorting and Boundary Formation Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Plant Development > Root Development

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Hypothetical patterns of expansion within a growth zone. The units on the axes are dimensions (T = time; L = length), and a value of 1 is arbitrary. (a) Wholly idealized growth zone in which there is a uniform rate of elemental expansion (blue line). The area under the curve (light blue) equals root growth rate, easily calculated as the product of the length of the growth zone and the rate of elemental expansion (dimension of L/T). Dashed lines show increased expansion rate (red) and increased zone length (green). (b) More realistic root growth zone, with a region corresponding to the meristem (M) and with rates of elemental expansion that vary with position. The area under the elemental expansion rate curve requires integration to calculate but still equals root growth rate. Increases in expansion rates and zone length are indicated, as in (a).

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The commonly observed pattern of changing growth zone length. This example illustrates the response of maize roots experiencing various levels of water‐deficit stress. Data were obtained 48 h after the imposition of stress when the response has essentially reached steady state. The legend refers to the water potential of the vermiculite growth medium.25

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Establishment of Spatial and Temporal Patterns > Cell Sorting and Boundary Formation
Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing
Plant Development > Root Development