Are obligatory apomicts invested in the pollen tube transmitting tissue? Comparison of the micropyle ultrastructure between sexual and apomictic dandelions (Asteraceae, Lactuceae)

Abstract

With the exception of the sunflower, little information concerning the micropyle ultrastructure of the family Asteraceae is available. The aim of our study was to compare the micropyle structure in amphimictic and apomictic dandelions. Ultrastructural studies using buds and flowers during anthesis have been done on the micropyle of the sexual and apomictic Taraxacum. In all of the species that were examined, the micropylar canal was completely filled with ovule transmitting tissue and the matrix that was produced by these cells. The ovule transmitting tissue was connected to the ovarian transmitting tissue. The micropyle was asymmetrical because the integument epidermis that forms the transmitting tissue was only on the funicular side. There was a cuticle between the obturator cells and epidermal cells on the other side of integument. The micropylar transmitting tissue cells and theirs matrix reached the synergid apex. The cytoplasm of the transmitting tissue cells was especially rich in rough endoplasmic reticulum (ER), dictyosomes, and mitochondria. No major differences were detected between the micropyle structure of the amphimictic and apomictic species; thus, a structural reduction of obturator does not exist. The ovule transmitting tissue is still active in apomictic dandelions despite the presence of the embryo and endosperm. Differences and similarities between the micropyle structure in the Asteraceae that have been studied to date are discussed.

Fig. 1

Transmitting tissue structure in sexual dandelions. a T. linearisquameum semithin (longitudinal) section through an ovule (Ov) and part of an ovary showing the transmitting tissue: micropylar transmitting tissue (Mtt, black arrow) and ovary transmitting tissue (Ovt), bar = 20 μm. b T. tenuifolium semithin section through the micropylar part of an ovule; integument (Int), micropylar transmitting tissue (Mtt), funicular side (Fs), synergids (ss), and filiform apparatus (white arrow). Bar = 20 μm. c T. linearisquameum Electron micrograph showing micropylar transmitting tissue cells (Mtt); extracellular matrix (exm) and endoplasmic reticulum (Er). Bar = 1.1 μm. d T. tenuifolium Electron micrograph showing micropylar transmitting tissue cells (Mtt); extracellular matrix (exm) and subepidermal cells (sub). Bar = 2.5 μm

Fig. 2

Ultrastructure of transmitting tissue in T. tenuifolium. a Exocytose of small vesicles (white arrows) in the transmitting tissue cells; ER cisternae (Er), dictyosomes (D), and extracellular matrix (exm). Bar = 0.6 μm. b Transverse section through an micropylar transmitting tissue cell; ER cisternae (Er), mitochondrion (m), nucleus (N), bar = 0.6 μm. c A part of a section through a micropyle near synergids; multivesicular bodies (MvB), micropylar transmitting tissue cell (Mtt), micropylar canal cell (mcc), bar = 1 μm. d A part of a section through subepidermal (sub) and micropylar transmitting tissue cells; plasmodesmata (black arrow), dictyosomes (D), ER cisternae (Er), nucleus (N), plastid (P), bar = 0.6 μm

Fig. 3

a, b Ultrastructure of micropylar canal cells in T. tenuifolium. Note the extracellular matrix (Exm) of micropylar transmitting tissue cells, cuticle (star), exocytose of small vesicles (white arrows), nucleus (N), and plastid (P). Bar = 0.75 μm

Fig. 4

Transmitting tissue structure in apomictic dandelion T. officinale s.l. (clone SA-B). a Semithin section through an ovule and a part of an ovary showing the transmitting tissue (Mtt, black arrow), egg cell (eg), synergids (ss), and filiform apparatus (white arrow). Bar = 20 μm. b, c Ultrastructure of micropylar transmitting tissue cells, dictyosomes (D), ER cisternae (RER), plastid (P), and mitochondrion (m). Bar = 0.75 and 0.5 μm

Fig. 5

a−c Semithin section through a young seed of an apomictic dandelion T. officinale s.l. (clone Kraków-Podgórze) showing the micropylar transmitting tissue (Mtt, black arrow), cellular endosperm (En), globular embryo (eb), synergids (s), and filiform apparatus (white arrow). a Bar = 50 μm. b, c Bar = 20 μm. d Ultrastructure of micropylar transmitting tissue cells, note contact of micropylar transmitting tissue cells (Mtt) with synergids (s), filiform apparatus (white arrow), ER cisternae (ER), multivesicular bodies (MvB), extracellular matrix (exm), and mitochondrion (m). Bar = 1.8 and 0.4 μm