Biology Reference
In-Depth Information
Table 3.
Species specificity of short-range guidance signals from ovules.
Targeted A. thaliana Ovules Repel Supernumerary Pollen
Tubes in Vitro
Interestingly, while the ovule-derived attractant in the in vitro assay acted to guide
multiple pollen tubes toward ovules, only one pollen tube gained access to each
micropyle. This is reminiscent of polyspermy blocks in vivo, where only one tube
generally migrates up the funiculus and into the ovule [13]. While the mechanisms
that prevent multiple tubes from even approaching an ovule are highly efficient,
it is nonetheless possible for more than one pollen tube to enter a micropyle. In
wild-type maize, heterofertilization results when the egg and central cell are fertil-
ized by different pollen tubes at a frequency of ~1/50 [34] and in A. thaliana, ~1%
(wild type) and ~10% (feronia) of ovules are penetrated by multiple pollen tubes
[3]. When we performed the in vitro assay with fertilized ovules, many tubes grew
within 100
µ
m, but none entered (Table 1), suggesting that the release of the
ovule attractant terminates after fertilization, or alternatively, that a new signal
repels additional pollen tubes. To distinguish between these possibilities, we used
time-lapse imaging analysis (Fig. 3). While 44% of targeted ovules (n = 143) were
approached by additional pollen tubes, in every instance, these tubes did not enter
the micropyle. Repelled tubes either stalled near the micropyle or turned sharply
away from the targeted ovule (84 ± 42°; n = 61, Fig. 2e-h), a response that was
observed as early as 10 min after a successful targeting event. This effect was fairly
short-range; only tubes that approached within 27 ± 22 (s.d)
µ
m were repelled.
The diffusion rate of a series of dextran molecules through the medium used in
this assay allowed us to estimate that a repellent measuring <10kD could diffuse
