Biology Reference
In-Depth Information
guidance cues are known [7,8]. Although potential homologs of these proteins
exist in A. thaliana, their role in pollen tube growth is yet to be determined [6].
Figure 1.
Pollen tube targeting in vitro.
(a) Diagram
of a pollinated pistil within an A. thaliana flower. After reaching the stigma (si), pollen (p) extends a
tube through the style (st) to reach the transmitting tract (tt) before entering one of the two ovary (ov) chambers
to target an ovule. (b) Upon reaching the ovule, the pollen tube (pt, green) either grows up the funiculus (f ) or
makes a sharp turn towards the micropyle (m) and enters the ovule. Within the ovule, the pollen tube navigates
towards the female gametophyte (gray) encased by outer (o) and inner (i) integuments, lyses within one of the
two synergid (s) cells that flank an egg cell (e). Upon lysis, one sperm fertilizes the egg cell to form the zygote
and the other fuses with the central cell (c) to form the endosperm. The number of pollen tubes drawn is for
illustration purposes only and does not reflect the quantity typically observed in an assay. (c) Merged fluorescent
and bright field images depicting the final stages of in vitro pollen tube growth. GFP-tagged pollen tubes make
a committed turn (arrows) before entering a virgin ovule and lysing (arrow heads). (d) Diagram and (e) merged
fluorescent and bright field image of in vitro pollen tube guidance assay. Pollen tubes emerge from the cut
portion of the pistil, travel across the agarose medium before entering the excised ovules. Fluorescent green spot
within ovules mark successful pollen tube targeting. Scale bars, 100
ยต
m.
After emerging from the transmitting tract, pollen tubes approach the ovule
micropyle with remarkable precision. Mutants defective in pollen tube guidance
have demonstrated that this process is controlled by a series of molecular signals
that involve pollen tubes, ovule tissues, and female gametophytes [1]. The A.
thaliana mutants, ino [9] and pop2 [10] point to a role for diploid ovule tissue in
pollen tube guidance; these have aberrant interactions between pollen tubes and
diploid ovule cells, yet their female gametophytes appear normal, and in the case
of pop2, can be fertilized with wild-type pollen [11]. Pollen tubes fail to either
reach or enter the micropyle in A. thaliana mutants with nonviable or aberrant
