Patterns of root growth acclimation: constant processes, changing boundaries

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. (C) 2012 Wiley Periodicals, Inc.