Because plants rely on fortuitous processes to
distribute pollen, they must distinguish pathogens and foreign pollen
from gametes of their own species. The first step in a series of cell-cell interactions begins at the stigma, where dehydrated, mature pollen
is hydrated and allowed to grow only if compatible. Identification of
sterile mutations in Arabidopsis thaliana and simultaneous
investigation of the pollen coating composition have advanced the
understanding of pollen-pistil communication at the molecular level. The
extracellular matrix of pollen consists of long-chain lipids and
lipophilic proteins. We have characterized a male-sterile mutation
(cer6-2) that lacks an extracellular pollen coating, affecting
fertilization at the initial step. Instead of allowing pollen hydration
and germination, stigma cells undergo a defense response. Because
cer6-2 sterility can be reversed by hydrating mutant pollen in a
humid chamber, we have been able to obtain suppressing mutations.
Addition of wild-type pollen coat extract to cer6-2 pollen
rescues fertility, implicating the coating as the source of critical
signaling components. We have purified three of the major pollen coat
proteins and are investigating their role in fertilization. The pollen
composition of many races of Arabidopsis manifest striking
differences, hinting at a mechanism for rapid speciation. Differences in
the pollen coat components have allowed genetic mapping of the
variations to a single region. Molecular cloning of purified pollen
proteins and studies to correlate variations of the pollen coat with
fertilization efficiency are currently in progress. These studies
provide insight into pollen-pistil communication and cell-cell
interaction.