Male cone morphogenesis, pollen development and pollen dispersal mechanism in Ginkgo biloba L.

Abstract

Lu, Y., Wang, L., Wang, D., Wang, Y., Zhang, M., Jin, B. and Chen, P. 2011. Male cone morphogenesis, pollen development and pollen dispersal mechanism in Ginkgo biloba L. Can. J. Plant Sci. 91: 971–981. Ginkgo biloba L. is one of the oldest gymnosperms. Male cone morphogenesis, pollen development and dispersal are important for successful pollination and reproduction. In this study, we investigated the development of male cone, pollen and the sporangial wall in detail. The results indicate that: (1) The primordia of male cones and leaves begin to differentiate in early June and remain open until the following March. The male cones then mature and release pollen in mid-April. The male cones are drooped and approximately perpendicular to the leaves during pollination. (2) The microsporocytes develop from the sporogenous cell and form a tetrahedral tetrad after two simultaneous asymmetrically meioses, then produce a matured four-cell pollen after three polar mitotic divisions. The matured pollen is hemispheric in shape with a large aperture area and three pollen wall layers; once released from the microsporangia, the pollen becomes boat-like in shape. (3) The sporangial walls are eusporangiate and consist of epidermis, endothecium and tapetum. The differentiation of the tapetum occurs separately from that of the epidermis and endothecium, and originates from the outermost layer of sporogenous cells. The sporangial walls exhibit shrinkage of the epidermis, fibrous thickening of the endothecium, and enzymic dissolution of the tapetum during pollen dispersal, which contributes to microsporangia opening. Based on these results, we conclude that there many unique and primitive characteristics of the development of the male cones, pollen and sporangial wall of G. biloba. In addition, we also found that the male cones, pollen and sporangial walls have evolved efficient structural and morphological adaptations to anemophily.

Temporal and spatial characteristics of male cone development inMetasequoia glyptostroboidesHu et Cheng

“…pollen filtration and wastage caused by the leaves. 24,25 In the present study, the male cones on 50-y-old trees of M. glyptostroboides were located mainly in the outside of the crown. This location would obviously expose the male cones to the maximal wind and therefore is advantageous for pollen dispersal by wind.…”

“…After enzymatic lysis of the septum, and the likely induction of programmed cell death, the septum ruptures to form a single locule and is followed by subsequent opening of the anther. 26 Although septum breakdown is absent in Ginkgo, the sporangium opens on the facing sides of two adjacent microsporangia, 25 which is similar to anther opening in angiosperms. However, in M. glyptostroboides, each microsporangium opens directly and independently along the dehiscence line.…”

“…Although microsporangium dehiscence is essential for wind pollination in gymnosperms, few studies have investigated the process of dehiscence. 25 In contrast, anther dehiscence in angiosperms has been studied extensively on a number of species, such as Arabidopsis, Lilium, rice, maize and members of the Solanaceae. 26 The basic processes of anther dehiscence in angiosperms are similar and involve secondary thickening in the anther endothecium, breakdown of the septum, splitting of the stomium and subsequent opening of the anther.…”

“…In angiosperms, 26 all localized secondary thickenings occur in the endothecium and the splitting of the stomium results in anther opening. In Ginkgo, 25 the microsporangial wall is of the eusporangiate form, consisting of 9-11 layers of cells, and the thickening occurs in the endothecium before sporangium dehiscence. In cycads, 27 the thick-walled cells occur in the apex of sporangia.…”

http://www.idpan.poznan.pl/index.php/content/vol-71/1352-71-73-81

“…The shape of sporangia was oval, elongate-elliptical and boat-shaped and in our study some were also almost round (Liu et al 2006). The microsporangia walls exhibit shrinkage of the epidermis, fibrous thickening of the endothecium and enzymatic dissolution of the tapetum during pollen dispersal, which contributes to sporangia opening (Keijzer 1987;Abid and Qaiser 2004;Lu et al 2011). Based on the mode of dehiscence of microsporangia/ anthers there are four types: longitudinal, poricidal, valvular and transverse (Radford et al 1974).…”