Essay 15.1

Essay 15.1 - Plant Physiology Online Oscillatory Calcium...

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Plant Physiology Online: Oscillatory Calcium and Proton Gradients in Growing Pollen Tubes 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 15.1 PREVIOUS :: NEXT Essay 15.1 Oscillatory Calcium and Proton Gradients in Growing Pollen Tubes Peter Hepler and Alenka Lovy-Wheeler, University of Massachusetts, Amherst, Massachusetts, USA September, 2006 Pollen tube growth delivers the two sperm cells to the embryo sac, and thus is essential for sexual reproduction in higher plants [see Malhó, R. (Ed.) (2006) Plant Cell Monographs (Vol. 3)]. The process has important and unique features: It is extremely fast, reaching rates of up to 1 cm per hour in corn; it is highly polarized, with growth being confined to the tip ("tip-growth"); and it possesses a guidance mechanism that determines the direction of growth. Several aspects of tip-growth have been deciphered; for example, the Golgi apparatus produces vesicles containing cell wall precursor material, and through cytoplasmic streaming these vesicles flow to the apex of the pollen tube, where they fuse with the plasma membrane and secrete their contents into the cell wall. It is unclear how these events are orchestrated to achieve the high degree of polarity evident during pollen tube growth, although recent work indicates that different ions, including notably calcium and protons, play an important role (Hepler et al. 2006). It has been known for many years that both calcium ions and protons are essential for growth (Hepler et al. 2006). Calcium ions must be present in the growth medium at a concentration above 10 μ M, but below 10 mM. A high proton concentration or low pH (e.g., 4.5 to 6.5) also facilitates both pollen germination and tube growth. Considerable work on the intracellular and extracellular expression on these ions has revealed unique features that are crucial for the regulation of pollen tube growth (Holdaway-Clarke and Hepler 2003). For example, ion imaging reveals that the growing pollen tube possesses a "tip-focused" gradient of free calcium at its extreme apex (Figure 1, top images), in which the concentration extends from 3000 nM or higher (e.g., 10,000 nM; Messerli et al. 2000) at the plasma membrane to a basal level of 200 nM or less within 20 μ m from the tip. Figure 1 Growing pollen tubes exhibit a pronounced "tip-focused" calcium gradient ( top , left and right). The gradient oscillates between high (left) and low (right) levels. The pollen tube was injected with the calcium sensitive dye, fura-2-dextran, and photographed using ratio-metric ion imaging. Growing pollen tubes also exhibit a pH gradient in which the tip is slightly acidic ( bottom , left and right). Back from the tip is a prominent alkaline band. This band oscillates between high (left) and low (right) pH. The pollen tube
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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 15.1 - Plant Physiology Online Oscillatory Calcium...

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