Biology Reference
In-Depth Information
Results
Here we developed an in vitro assay to study pollen tube guidance to excised
A. thaliana ovules. Using this assay we discerned the temporal and spatial
regulation and species-specificity of late stage guidance signals and character-
ized the dynamics of pollen tube responses. We established that unfertilized
A. thaliana ovules emit diffusible, developmentally regulated, species-specific
attractants, and demonstrated that ovules penetrated by pollen tubes rapidly
release diffusible pollen tube repellents.
Conclusion
These results demonstrate that in vitro pollen tube guidance to excised
A. thaliana ovules efficiently recapitulates much of in vivo pollen tube behav-
iour during the final stages of pollen tube growth. This assay will aid in con-
firming the roles of candidate guidance molecules, exploring the phenotypes of
A. thaliana pollen tube guidance mutants and characterizing interspecies
pollination interactions.
Background
After a pollen grain lands on the surface of the pistil, it absorbs water from the
stigma and forms a pollen tube - a long polar process that transports all of the cel-
lular contents, including the sperm [1]. Pollen tubes invade the pistil and migrate
past several different cell types, growing between the walls of the stigma cells,
travelling through the extracellular matrix of the transmitting tissue, and finally
arriving at the ovary, where they migrate up the funiculus (a stalk that supports
the ovule), and enter the micropyle to deliver the two sperm cells-one fertilizes
an egg and other the central cell (Fig. 1a, 1b) [2]. Typically, only one pollen tube
enters the ovule through an opening called the micropyle, terminates its journey
within a synergid cell, and bursts to release sperm cells-a process defined as pollen
tube reception [3].
A combination of genetic and in vitro assays has defined signals that contribute
to the early stages of pollen tube guidance. Chemocyanin, a small basic protein
from lily stigmas, attracts lily pollen tubes in vitro [4], and in A. thaliana, wild
type pollen guidance was abnormal when grown on stigmas over expressing the A.
thaliana chemocyanin homolog [5]. Other signals are active in the nutrient-rich
extracellular matrix secreted by the female transmitting tissue. A pectin that may
promote guidance by mediating adhesion of pollen tubes to this matrix has been
identified in lily [6]. Glycoproteins that likely contribute to guidance have also
been described: in lily, a lipid transfer protein that contributes to adhesion [6],
and in tobacco, two glycoproteins (TTS1 and TTS2) that provide nutritional and
