LIPIDS ARE REQUIRED FOR DIRECTIONAL POLLEN-TUBE GROWTH

Successful pollination and fertilization are absolute requirements for sexual reproduction in higher plants. Pollen hydration, germination a nd penetration of the stigma by pollen tubes are influenced by the exu date on wet stigmas(1) and by the pollen coat in species with dry stig mas(2-5). The exudate allows pollen tubes to grow directly into the st igma, whereas the pollen coat establishes the contact with the stigma. Pollen tubes then grow into the papillae, which are covered by a cuti cle. The components of the exudate or pollen coat that are responsible for pollen tube penetration are not known. To discover the role of th e exudate, we tested selected compounds for their ability to act as fu nctional substitutes for exudate in the initial stages of pollen-tube growth on transgenic stigmaless tobacco plants' that did not produce e xudate. Here we show that lipids are the essential factor needed for p ollen tubes to penetrate the stigma, and that, in the presence of thes e lipids, pollen tubes will also penetrate leaves. We propose that lip ids direct pollen-tube growth by controlling the flow of water to poll en in species with dry and wet stigmas.