In Arabidopsis, dehiscence of the a

In Arabidopsis, dehiscence of the anthers is usually accompanied by the transfer of pollen to the receptive surface of the
pistil and the initiation of the process of self-pollination.
Following pollen capture, a series of events are set in motion
which culminate in the delivery of sperm cells to the female
gametophyte and double fertilization. The critical events in this
postpollination process relate to the establishment of a
polarized growth pattern within the pollen grain, the elaboration of a pollen tube that extends by tip growth, the invasion
of the tube into tissues of the pistil, and its directed growth
towards the ovary as it homes in on its ovule targets. A major
unresolved question in plant reproductive biology concerns the
molecular basis of pollen tube guidance: how do pollen tubes
navigate through tissues of the pistil and what signals guide
them to the ovules?
The tissues of the pistil are uniquely specialized to promote
these postpollination events (Hill and Lord, 1987; Gasser and
Robinson-Beers, 1993). At the distal end of the Arabidopsis
pistil is the stigma, a globular structure covered with a dense
layer of papillar cells, the specialized epidermal cells that serve
as receptacles for pollen grains. Below the papillar cells, the
subepidermal cells of the stigma converge into a short solid
style made up of a central core of transmitting tissue surrounded by a parenchymatous cortex and the epidermis. The
style connects the stigma to the ovary, which consists of two
locules containing ovules with haploid egg cells. In members
of the Brassicaceae, including Arabidopsis and Brassica,
contact with a papillar cell is a prerequisite for the activation
of pollen and subsequent pollen tube development: pollen
grains do not normally germinate on non-stigmatic surfaces
(Lolle and Cheung, 1993). The cells of the transmitting tissue
that line the path of pollen tubes are metabolically active
secretory cells thought to provide the mechanical and nutritive
environments for promoting normal pollen tube growth.
Further, the high degree of precision in the directionality of
pollen tube growth suggests that signals are continually being
exchanged between pollen tubes and cells of the pistil that line
their path. For example, a well established function of the pistil
is in pollen recognition. In Brassicaceae, this recognition
function is performed by the stigmatic papillar cells, as underscored by the observation that intra- and inter-specific incompatibility barriers operate at the level of the interaction between
these cells and pollen in this family (Hiscock and Dickinson,
1993).
Recently, genetic ablation methodologies (Thorsness et al.,
1993) and classical genetic approaches (Preuss et al., 1993)
have been used to investigate the requirements for successful
pollination in Arabidopsis. However, progress towards the
molecular and/or biochemical identification of molecules that
are essential for pollination as well as the genetic dissection of