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REGULATION AND FUNCTION OF HAM GENES AND MERISTEM DEVELOPMENT IN CERATOPTERIS RICHARDII
The growth of land plants depends on a group of pluripotent stem cells in a tissue called the meristem. Seed plants initiate and maintain different types of meristems at the asexual sporophyte stage, and they generate sexual gametophytes, which are dependent on their sporophytes and are devoid of a meristem. In contrast, aside from forming indeterminate meristems at the sporophyte stage, seedless vascular plants, including ferns, also develop meristems in their gametophytes to drive gametophyte development and formation of sexual organs. To date, compared to the well-characterized cell behaviors and regulatory pathways in the meristems of seed plants, the molecular and cellular basis of meristem development in seedless ferns is still poorly understood.
In several seed plants, the HAIRY MERISTEM (HAM) family transcription factors play important roles in maintaining the indeterminacy of shoot apical meristems and promoting the de novo formation of axillary meristems. In the first part of this dissertation, through constructing a comprehensive phylogeny, I found that HAM family members are widely present in land plants and duplicated in a common ancestor of flowering plants, leading to the formation of two distinct groups: type I and type II. In addition, HAM members from different seed plants and seedless plants are able to replace the roles of the Arabidopsis type-II HAM genes, maintaining established shoot apical meristems and promoting the initiation of new stem cell niches in Arabidopsis. Furthermore, preliminary functional studies of the HAM homolog (CrHAM) in the model fern Ceratopteris richardii suggest that CrHAM is required for maintaining the indeterminacy of multicellular meristems in Ceratopteris gametophytes. Collectively, these results indicate that HAM family members may serve as common regulators in control of meristem development in both seed plants and seedless vascular plants.
In the remaining chapter of this dissertation, long-term time-lapse confocal imaging was performed using Ceratopteris stable transgenic plants, in which each individual cell (nucleus) was labelled with a fluorescent marker. Real-time lineage, identity, and division activity of each single cell from meristem initiation to establishment in Ceratopteris gametophytes were then determined. Additionally, cell fate and lineage alterations during de novo formation of new meristems were examined by mechanical perturbations. These quantitative analyses lead to the conclusion that in Ceratopteris gametophytes, initiation and proliferation of multicellular meristems relies on a few marginal cell lineages. Once established, the meristem maintains an actively dividing zone during gametophyte development. Within the meristem, cell division is independent of cell lineages and marginal cells are more actively dividing than inner cells. The meristem also triggers differentiation of adjacent cells into egg-producing archegonia in a position-dependent manner.
In summary, this work provides insight into the evolution of key stem-cell regulators and advances the understanding of diversified meristem development in land plants.