Biology Reference
In-Depth Information
Results
The A. thaliana Stigma and Style Confer Pollen Tube
targeting competence
Previous studies indicated that pollen tubes germinated in a simple growth medi-
um cannot be guided to the micropyle [14,22]. Consistent with these observations,
when such assays are performed for A. thaliana, few ovules are targeted (~3%, Table
1). Hence, we instead removed the upper portion of the pistil (the stigma and style
[4,10,14,24]), deposited pollen on the stigma surface and showed that A. thaliana
pollen tubes emerged from the style, travelled across an agarose medium to excised
ovules and successfully entered the micropyle (Fig. 1a, 1b). To facilitate pollen tube
observation, especially after they enter the micropyle and are obscured by the opaque
ovule integument cells, we transformed plants with a GFP reporter under the control
of the pollen-specific LAT52 promoter [25], and identified GFP-expressing lines
with fully functional pollen tubes. Upon reaching the female gametophyte, these
tubes burst and release a large spot of GFP (Fig. 1c, 1d, arrowheads), conveniently
marking targeted ovules. Pollen tubes that grew within ~100
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m of an unfertilized
ovule often made a sharp turn toward an ovule; of the tubes that grew within this
range, ~50% successfully entered the micropyle (Table 1). This targeting efficiency is
significantly higher than that of tubes germinated on agarose (Table 1). Thus, pollen
tubes acquire the ability from pistil tissue to perceive ovule guidance signals, perhaps
by absorbing essential nutrients or undergoing critical developmental transitions; a
similar phenomenon was reported in T. fournieri [22]. In some cases, pollen grains
that germinated on the stigma formed tubes that grew onto the medium, rather than
penetrating the pistil and growing through the style. Nonetheless, these tubes suc-
cessfully targeted excised ovules, suggesting that interaction with the stigma alone is
sufficient to confer pollen tube guidance competence (Table 1); targeting efficiency
was significantly reduced, however, suggesting that direct contact with female cells,
rather than exposure to diffusible factors in the medium, is most important.
Table 1.
In vitro ovule targeting efficiency.
