Biology Reference
In-Depth Information
tube migration have often made it difficult to assess interactions at downstream
stages, including those near ovules. Here, we examined whether the stigma, style
and ovule-derived signalling interactions are shared among A. thaliana relatives
separated by ~5, 10, or 20 MY [31] (Table 3). The ability of the stigma and style
to promote pollen tube competence was highly conserved (
χ
2; P > 0.05; Table 3,
bottom panel), while the ovule-derived attractant diverged rapidly (
χ
2; P < 0.01;
Table 3, upper panel). For example, A. thaliana pollen tubes inefficiently target
ovules from Arabidopsis arenosa (separated by 5 MY from A. thaliana), rarely
target Olimarabidopsis pumila ovules (10 MY), and fail to target Capsella rubella
or Sysimbrium irio ovules (10 and 20 MY, respectively, Table 3, upper panel).
Because C. rubella and S. irio are challenging to transform, it was not possible to
test whether ovules from these two species are able to guide self-pollen expressing
GFP under our assay conditions. Nonetheless, the ability of A. thaliana pollen
to target ovules correlated with phylogenetic separation [30], suggesting that A.
thaliana pollen tubes are sufficiently diverged that they fail to recognize attrac-
tants from C. rubella and S. irio ovules. Unlike the calcium signals that emanate
from synergids [32,33], the proposed source of micropylar guidance signals, our
results point to a diffusible, heat-labile ovule-derived signal that is sufficiently
complex for rapid divergence - criteria that are most consistent with a protein-
based signal. Pollen tubes perceive this signal at a distance of ~100
µ
m from ovules
after a 5 hour incubation in the assay. To estimate the molecular weight of this
signal, we measured the diffusion rates of fluorescently-labelled dextran molecules
under the same conditions in which the in vitro assay was performed, and calcu-
lated that the ovule-derived signal could measure up to approximately 85 kD (see
methods).
Table 2.
Developmental regulation of short-range guidance signals from ovules.
