Abstract
In land plants, two distinct generations, gametophyte and sporophyte, alternate to complete the life cycle. Sporophytes undergo meiosis to produce spores, from which gametophytes develop. Gametophytes produce gametes, which participate in fertilization to produce the zygote, the first cell of the sporophyte generation. In addition to this sexual reproduction pathway, some fern species can undergo apospory or apogamy, processes that bypass meiosis or fertilization, respectively, to alternate between the two generations without changing the chromosome number. Apospory is inducible in the laboratory in various fern species simply by altering the sugar level in the media. In sporophytes induced to undergo apospory, sporophyte regeneration is also observed. The ratio of aposporous gametophytes to regenerated sporophytes varies, in a manner consistent with being dependent on sugar level. Whereas the sugar signaling pathway is yet to be elucidated in lower plants, in angiosperms it has been shown to play a regulatory role in controlling essential processes including flowering and embryo development, which give rise to the gametophyte and the next sporophyte generation, respectively. Here, we present evidence for the role of different sugar levels on the balance of apospory and regeneration in the fern Ceratopteris richardii. The demonstration of crosstalk between sugar signaling and the hormone ethylene signaling in angiosperms prompted us to test the effects of this hormone in combination with sugar on apospory vs. regeneration. These results provide insight into how a group of redifferentiating cells determines which generation to become and lay the groundwork for further analysis of this asexual pathway.
Highlights
In all land plants, the life cycle is characterized by an alternation between two generations; each develops into a multicellular body
Whereas the sugar signaling pathway is yet to be elucidated in lower plants, in angiosperms it has been shown to play a regulatory role in controlling essential processes including flowering and embryo development, which give rise to the gametophyte and the sporophyte generation, respectively
Sugar supplemented at low concentration (0.5% glucose) promoted apospory (Figure 1(a)) and at high concentration (2.5% glucose) promoted regeneration (Figure 1(e))
Summary
The life cycle is characterized by an alternation between two generations; each develops into a multicellular body. The haploid gametophyte and the diploid sporophyte, undergo the core events of sexual reproduction to complete the life cycle. Sporophytes produce haploid spores by specialized cells undergoing meiosis; these spores develop into multicellular gametophytes which produce gametes. In addition to sexual reproduction pathway via meiosis and fertilization, some fern species can reproduce asexually through apogamy or apospory. Diploid gametophytes are produced from somatic cells of sporophytes in the absence of meiosis. Both processes can be induced in the laboratory in the homosporous fern Ceratopteris richardii by altering the level of sugar supplement in the growth media, and in the case of apogamy, by preventing fertilization [5].
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