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The life cycle of land plants is characterized by alternating generations of sexual gametophytes and asexual sporophytes. Unlike seed plants, seed-free vascular plants, including ferns, initiate and maintain pluripotent meristems during their gametophyte phase to sustain body expansion and drive the formation of sexual organs for reproduction. This review summarizes meristem development among various fern species during the gametophyte phase, focusing on the cellular basis of meristem initiation, proliferation, and termination. We review the different types of gametophytic meristems in ferns, including apical cell (AC)-based meristems, multicellular apical meristems, and multicellular marginal meristems. We highlight both conserved and lineage-specific patterns of cell division, which are closely associated with these meristem identities and play crucial roles in shaping gametophytic morphology. Additionally, we highlight recent progress in understanding the dynamics of cell division and growth that drive meristem development, through studies that integrate confocal live imaging and computational quantitative analysis. Furthermore, we discuss the influence of environmental and genetic factors on cell division activity in fern gametophytes, including conserved transcriptional regulators that sustain meristem indeterminacy and proliferation in the model fern Ceratopteris richardii.
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LEAFY demonstrates functions in reproductive development of the gametophyte but not the sporophyte of the fern Ceratopteris richardii
Flowers are a key reproductive innovation of the angiosperms. Seed plant reproductive axes (including flowers) evolved as reproductively specialized shoots of the land plant diploid sporophyte, with the gamete-producing haploid gametophyte becoming reduced and enclosed within ovules and microsporangia. The transcription factor LEAFY (LFY) initiates floral development, yet it predates flowers and is found across all land plants. LFY function outside angiosperms is known from the moss Physcomitrium patens, where it controls the first cell division of the sporophyte, and from the model fern Ceratopteris richardii, a seedless vascular plant where CrLFY1 and CrLFY2 maintain vegetative meristem activity. However, how LFY’s floral role evolved remains unclear. Using over-expression, we uncover new roles for CrLFY1/2 in fern gametophyte reproduction, in sperm cells and in the gametophyte's multicellular notch meristem. While no sporophytic reproductive function was detected in terms of time to sporing, over-expression supports a role in frond compounding and in the zygote's first cell division. Our findings suggest a potentially ancestral LFY function in fern haploid-stage reproduction, which might have been co-opted into the sporophyte during the origin of the flower.
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- Award ID(s):
- 1920408
- PAR ID:
- 10649329
- Publisher / Repository:
- The Company of Biologists
- Date Published:
- Journal Name:
- Development
- ISSN:
- 0950-1991
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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