Signal-Mediated Depolymerization of Actin in Pollen during the Self-Incompatibility Response

Abstract

Signal perception and the integration of signals into networks that effect cellular changes is essential for all cells. The self-incompatibility (SI) response in field poppy pollen triggers a Ca 2 ؉ -dependent signaling cascade that results in the inhibition of incompatible pollen. SI also stimulates dramatic alterations in the actin cytoskeleton. By measuring the amount of filamentous (F-) actin in pollen before and during the SI response, we demonstrate that SI stimulates a rapid and large reduction in F-actin level that is sustained for at least 1 h. This represents quantitative evidence for s timulus-mediated depolymerization of F-actin in plant cells by a defined biological stimulus. Surprisingly, there are remarkably few examples of sustained reductions in F-actin levels stimulated by a biologically relevant ligand. Actin depolymerization also was achieved in pollen by treatments that increase cytosolic free Ca 2 ؉ artificially, providing evidence that actin is a target for the Ca 2 ؉ signals triggered by the SI response. By determining the cellular concentrations and binding constants for native profilin from poppy pollen, we show that profilin has Ca 2 ؉ -dependent monomeric actinsequestering activity. Although profilin is likely to contribute to stimulus-mediated actin depolymerization, our data suggest a role for additional actin binding proteins. We propose that Ca 2 ؉ -mediated depolymerization of F-actin may be a mechanism whereby SI-induced tip growth inhibition is achieved.

Microtubules Are a Target for Self-Incompatibility Signaling in Papaver Pollen

“…Because SI stimulated rapid actin depolymerization (Snowman et al, 2002), we wondered whether this might be responsible for alterations to the microtubules. We used latrunculin B (LatB) to examine the effect of F-actin depolymerization on the pollen microtubule cytoskeleton.…”

“…We have shown that SI triggers both actin depolymerization (Snowman et al, 2002) and apparent microtubule depolymerization. Moreover, depolymerizing actin with LatB triggers microtubule depolymerization, while depolymerizing microtubules with oryzalin has no effect on actin organization, as also previously shown by Gossot and Geitmann (2007).…”

“…Pollen of Papaver rhoeas was germinated and grown in vitro in liquid germination medium [0.01% H 3 BO 3 , 0.01% KNO 3 , 0.01% Mg(NO 3 ) 2 .6H 2 O, 0.036% CaCl 2 -2H 2 O, and 13.5% Suc] as described previously (Snowman et al, 2002) at 25°C. Pollen was grown for 1 h before any treatments were applied.…”

“…As sPPases are important enzymes for driving biosynthesis, they are crucial for cell growth, so SI, by targeting this enzyme, results in pollen tube inhibition. SI also triggers reorganization and depolymerization of the F-actin cytoskeleton (Geitmann et al, 2000;Snowman et al, 2002). As the actin cytoskeleton is required for pollen tube growth (Gibbon et al, 1999), this represents another mechanism to inhibit incompatible pollen tube growth.…”

Papaver rhoeas S-Determinants and the Signaling Networks They Trigger

“…23.1a,b . Depolymerization of the actin fi lament bundles occurs within a few minutes of SI induction (Snowman et al 2002 ), which provides a highly effective mechanism to rapidly inhibit incompatible pollen tube growth. Two Ca 2+ -dependent actin-binding proteins (ABPs) that could potentially act synergistically in SI-induced depolymerization are profi lin and PrABP80 (a putative gelsolin), which has potent Ca 2+ -dependent severing activity ).…”