Latrunculin B has different effects on pollen germination and tube growth

The actin cytoskeleton is absolutely required for pollen germination and tube growth, but little is known about the regulation of actin polymer concentrations or dynamics in pollen, Here, we report that latrunculin B (LATB), a potent inhibitor of actin polymerization, had effects on pollen that were distinct from those of cytochalasin D. The equilibrium dissociation constant measured for LATE binding to maize pollen actin was determined to be 74 nM. This high affinity for pollen actin suggested that treatment of pollen with LATE would have marked effects on actin function. Indeed, LATE inhibited maize pollen germination half-maximally at 50 nM, yet it blocked pollen tube growth at one-tenth of that concentration. Low concentrations of LATE also caused partial disruption of the actin cytoskeleton in germinated maize pollen, as visualized by light microscopy and fluorescent-phalloidin staining. The amounts of filamentous actin (F-actin) in pollen were quantified by measuring phalloidin binding sites, a sensitive assay that had not been used previously for plant cells, The amount of F-actin in maize pollen increased slightly upon germination, whereas the total actin protein level did not change, LATE treatment caused a dose-dependent depolymerization of F-actin in populations of maize pollen grains and tubes, Moreover, the same concentrations of LATE caused similar depolymerization in pollen grains before germination and in pollen tubes, These data indicate that the increased sensitivity of pollen tube growth to LATE was not due to general destabilization of the actin cytoskeleton or to decreases in F-actin amounts after germination. We postulate that germination is less sensitive to LATE than tube extension because the presence of a small population of LATE-sensitive actin filaments is critical for maintenance of tip growth but not for germination of pollen, or because germination is less sensitive to partial depolymerization of the actin cytoskeleton.