Peptide growth factor phytosulfokine-alpha contributes to the pollen population effect

Density-dependent pollen germination and tube growth in vitro is a well-doc umented phenomenon, termed the pollen population effect, but far less is kn own about its molecular basis. We present evidence to support phytosulfokin e-alpha [Y(SO3H)lY(SO3H)TQ; PSK-alpha] as a native bioactive factor contrib uting to this effect. Mature pollen grains of Nicotiana tabacum L. var. mac rophylla were incubated in liquid medium for 2 h. Pollen germination freque ncy increased in a density-dependent manner from 625 to 46,000 grains/ml. C onditioned medium, obtained from the medium of pollen cultured at a density of 10,000 pollen grains/ml for 12 h, promoted the germination of pollen cu ltured at a low density (625 grains/ml). A rabbit antiserum against PSK-alp ha specifically inhibited the promotive effect of conditioned medium. Quant ification by enzyme-linked immunosorbent assay showed that the conditioned medium contained 0.4 nM of PSK-alpha. Exogenous PSK-alpha also stimulated p ollen germination in the low-density culture. These results indicate that P SK-alpha is an important regulator involved in the pollen population effect .