Essay 15.2

Essay 15.2 - Plant Physiology Online: Microtubules,...

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Plant Physiology Online: Microtubules, Microfibrils, and Growth Anisotropy A Companion to Plant Physiology, Fourth Edition by Lincoln Taiz and Eduardo Zeiger Topics Essays Study Questions Readings Help Select Chapter: Search HOME :: CHAPTER 15 :: Essay 15.2 PREVIOUS :: NEXT Essay 15.2 Microtubules, Microfibrils, and Growth Anisotropy Tobias I. Baskin, Biology Department, University of Massachusetts, Amherst, Massachusetts, USA September, 2006 Expansion in Length and Width Growth is a fundamental plant behavior. Where an animal might run, yawn, or bite, a plant may build a sun leaf, a prop root, or a flower. To create these organs, the plant modifies its growth. Because growth is an irreversible increase in volume, these modifications are permanent and can only be reversed by building yet more organs, with continued growth. A few plant organs, such as certain fruits, are spherical. A spherical organ can be built from uniform growth in all directions—that is, isotropic growth. However, most plant organs are asymmetric, either planar, as in leaves and petals, or cylindrical, as in stems and roots. An asymmetric organ is built from spatially nonuniform—that is, anisotropic, growth. To understand how growth anisotropy is controlled, we must combine biochemistry and biomechanics. Early attempts to understand the anisotropy of expansion focused on single cells, in particular the internodal cells of the Nitella species, which grow to be centimeters long (Web Topic 15.4). Their large size facilitates experiments and their habit as single-cell filaments simplifies the biomechanical treatment. In influential work, Paul B. Green found that the growth rate in length exceeded the rate for width by about fivefold throughout development (Figure 1A). That is, growth-rate anisotropy was constant. Constancy was also the case for another alga, Hydrodictyon reticulatum , which also grows as single cell filaments. This was notable not only because growth rates themselves rise and fall as the cells age but also because the shape of the cell changes from short and squat, at birth, to long and thin, at maturity. This suggested that the anisotropy of expansion was built into the growth process at a fundamental level. Therefore, most subsequent work on growth ignored the specific value of growth-rate anisotropy. Figure 1 Measurements of growth anisotropy. A . Plot of cell length versus diameter for two species of filamentous, green algae. From the linear relationship (boxed data points are for each species), expansion rates in length and width are inferred to be proportional, with the slope equal to their ratio. To the right of
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Plant Physiology Online: Microtubules, Microfibrils, and Growth Anisotropy the data for Nitella , the shape of the cells is sketched, not to scale. (Figure modified slightly, from Green 1964; courtesy of Brookhaven National Laboratory.) B
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This note was uploaded on 08/28/2009 for the course BIO 430 taught by Professor Dr.cohen during the Fall '09 term at Kentucky.

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Essay 15.2 - Plant Physiology Online: Microtubules,...

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