Role of cytosolic free calcium in the reorientation of pollen tube growth

Abstract

Summary Growing pollen tubes of Agapanthus umbellatus exhibited a tip‐to‐base gradient in cytosolic free calcium ([Ca2+]c). Although this gradient is believed to be involved in pollen tube growth, its role in specifying reorientation is unknown. The direction of pollen tube growth could be modified by iontophoretic micro‐injection, electrical fields (EFs) or photolysis of caged Ca2+. Iontophoretic injection resulted in a temporary cessation of growth, an increase in [Ca2+]c and, upon recovery, reoriented growth. Weak EFs also elevated [Ca2+]c, reduced growth rates and resulted in the reorientation of pollen tubes towards the cathode. Treatment with very low concentrations of the Ca2+‐channel blocker lanthanum chloride, prior to exposure to an EF, inhibited both the increase in [Ca2+]c and reorientation whilst only slightly affecting growth rates. The responses of growth inhibition and reorientation were mimicked when [Ca2+]c was artificially elevated by photoactivating caged Ca2+ (Nitr‐5). Our data suggest that [Ca2+]c is part of a transduction mechanism which enables growing pollen tubes to successfully reorient to directional signals in the style and thus accomplish eventual fertilization of the egg.

Spatial characteristics to calcium signalling; the calcium wave as a basic unit in plant cell calcium signalling

“…Very low concentrations of the channel blockers, La 3+ or Gd 3+ , slightly diminish growth rates but completely block reorientation of the pollen tube in response to an applied electrical ¢eld. Several types of Ca 2+ channel may thus be present (Malho¨et al 1994). This initiation site is an important model for other plant cells.…”

“…Photolysis of loaded caged (1,4,5) InsP 3 in the presence of heparin (to inhibit binding to the (1,4,5) InsP 3 channel) induced only truncated waves (Franklin-Tong et al 1996). A decline in membrane potential induced by brief iontophoresis opens voltage-gated plasma membrane Ca 2+ channels and elevates [Ca 2+ ] c through extensive regions of the pollen tube (Malho¨et al 1994. Pollen tubes also possess a Ca 2+ -activated phospholipase C capable of generating (1,4,5) InsP 3 (Franklin-Tong et al 1996).…”

“…Many £uorescent ratio images of pollen tubes, protoplasts and guard cells have been produced (Gilroy et al 1991;Shacklock et al 1992;Malho¨et al 1994 A total of three main kinetic characteristics of the transient can be recognized; the rise time, the peak and the decay time. The rise time represents a period of increasing recruitment of vesicles into a release mode; in this phase, Ca 2+ release is greater than Ca 2+ uptake.…”

“…Tube growth requires a relatively narrow range of extracellular Ca 2+ concentration. Fluorescence ratio imaging studies in a variety of di¡erent pollen tubes have shown steep (3 mM3200 nM) tip-high gradients of [Ca 2+ ] c con¢ned to the apical region 15^20 mm (Malho¨et al 1994;Pierson et al 1996). A spatial separation of Ca 2+ channels from Ca 2+ ATPases in the plasma membrane is thought to underpin the gradient (Feijo¨et al 1995).…”

Exocytosis Precedes and Predicts the Increase in Growth in Oscillating Pollen Tubes

“…This essential ion (Brewbaker and Kwack, 1963) forms a steep tipfocused gradient at the extreme apex of the tube (Rathore et al, 1991;Miller et al, 1992;Malhó et al, 1994;Pierson et al, 1994;Roy et al, 1999) where it may stimulate exocytosis (Battey et al, 1999;Roy et al, 1999). Further studies have revealed that the Ca 2+ gradient oscillates in magnitude with the same period as growth rate; however, it follows the increase in growth rate by 10 to 408 (Messerli et al, 2000;Cá rdenas et al, 2008).…”