Intermittent pollen-tube growth in pistils of alders (Alnus)

Abstract

In alders, where fertilization occurs approximately 8 weeks after pollination, the pollen tube (male gametophyte) grows intermittently in four steps in close association with the development of the ovary and its ovules. Pollen tubes stop growing in the style, at the ovarian locule, and at the chalaza (ovule), before reaching an embryo sac for fertilization. At the stage when the ovary develops an ovule primordium in each of the two locules, many pollen tubes germinate on the stigma, and a few of them reach the style, where they remain for approximately 7 weeks. Thereafter, a single tube resumes growing; with a short stop in the upper space of the ovarian locule, it reaches the older of the two ovules when it has developed a two-nucleate embryo sac. Except in the last step, where the tube grows from the chalaza to an embryo sac (female gametophyte), an eight-nucleate mature embryo sac is not necessary for pollen-tube guidance in the pistil. Although the intermittent pollen-tube growth appears to play an important role in the selection of a single pollen tube from many and one ovule from two, its detection provides insight into the study of the mechanism of pollen-tube guidance.

Fig. 1.

Pollen-tube growth and development of the ovary and ovule at the time of pollination (the end of March) in Alnus.(A) Female inflorescence consisting of ≈300 female flowers. (B) A pistil at the time of pollination. (C) Longitudinal section (LS) of the part of the ovary indicated with a rectangle (asterisk) in B.(D) LS of the part of the style indicated with a rectangle (double asterisks) in B, showing that cells of the style contain many starchy grains. (E) Two stigmas and a style, indicated by a rectangle (triple asterisk) in B, showing pollen tubes (arrows). (F) Part of E magnified to show the pollen tubes (arrows) lying zigzag in the style. (G) A diagram illustrating pollen-tube growth in the pistil soon after pollination. Bold curved lines show pollen tubes, and gray shading shows cells with many starch grains. ova, ovary; pr, ovule primodium; sg, stigma; sy, style. (Scale bar, 5 mm in A, 0.5 mm in B, 200 μmin E, 100 μmin C and F, and 50 μmin D.)

Fig. 2.

Pollen-tube growth and development of ovule and embryo sac in older pistils of Alnus.(A) Female inflorescence ≈7 weeks after pollination. (B) A single pistil detached from A.(C) Longitudinal section (LS) of ovary. (D) Magnified view the top box in C, showing the disappearing starchy grains in the cells of the style. (E) Magnified view of the bottom box in C, showing a tetrad of megaspores in the nucellus. Arrowheads indicate three degenerating megaspores. (F) Magnified view of the middle box in C, showing that the pollen tube lies in a zigzag line and is divided into small branches (arrows) on the surface of the upper region of the ovarian locule. (G) A diagram illustrating pollen-tube growth in an older pistil. Bold curved lines show pollen tubes and the gray shading shows cells which contained many starch grains in the younger pistil. es, one-nucleate embryo sac; ova, ovary; ovu, ovule; vs., vascular tissue; wg, wing. (Scale bar, 10 mm in A, 1 mm in B, 500 μmin C, 100 μmin D and F, and 50 μmin E.)

Fig. 3.

Pollen-tube growth and development of ovule and embryo sac in older pistils of Alnus.(A) One of two ovules, removed from the left side of an ovarian locule (C). (B) Magnified view of the upper box in A, showing an embryo sac with four nuclei (arrowheads). (C) An ovary with two ovules removed, showing that the pollen tube (arrow) grew downward. (D) Magnified view of the rectangle in C, showing that the pollen tube grows to the left ovule. An arrowhead shows the point where the pollen tube penetrated the funiculus. (E) Another ovule, removed from the right side of an ovarian locule (C). (F) Magnified view of the rectangle in E, showing an embryo sac with one nucleus (arrowhead). (G) Magnified view of the lower rectangle in A, showing that the pollen tube (arrow) grew down to the chalaza of the ovule on the left side. (H) A diagram illustrating pollen-tube growth in an older ovary. Bold curved lines show pollen tubes. An arrowhead shows the point where the pollen tube penetrated the funiculus. ch, chalaza; vs., vascular tissue. (Scale bar, 200 μmin A, C, and E, 100 μmin D, and 50 μmin B, F, and G.)

Fig. 4.

Pollen-tube growth and development of an embryo sac at the time of fertilization in Alnus.(A) Longitudinal section of mature ovule. (B) Pollen tube (arrow) in the mature ovule. (C) A diagram illustrating pollen tube growth in the mature ovary. The bold curved line shows the pollen tube. Arrowheads in B and C show the point where the pollen tube penetrated the funiculus. ch, chalaza; es, embryo sac; vs, vascular tissue. (Scale bar, 200 μmin A and 100 μmin B.)