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
:: Essay 15.1
Oscillatory Calcium and Proton Gradients in Growing Pollen Tubes
Peter Hepler and Alenka Lovy-Wheeler, University of Massachusetts, Amherst,
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
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
from the tip.
Growing pollen tubes exhibit a pronounced "tip-focused" calcium gradient (
, 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 (
, 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