A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage of development of the gametophyte (the

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

phase) in the life cycle of

moss Mosses are small, non-vascular plant, non-vascular flowerless plants in the taxonomic phylum, division Bryophyta (, ) ''sensu stricto''. Bryophyta (''sensu lato'', Wilhelm Philippe Schimper, Schimp. 1879) may also refer to the parent group bryo ...

es. When a moss first grows from a

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

, it starts as a ''germ tube'', which lengthens and branches into a filamentous complex known as a ''protonema'', from which a leafy

gametophore Gametophores are prominent structures in seedless plants on which the reproductive organs are borne. The word gametophore and ‘-phore’ (Greek Φορά, "to be carried"). In mosses, liverworts and ferns (Archegoniata), the gametophores suppor ...

, the adult form of a

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the se ...

bryophyte Bryophytes () are a group of embryophyte, land plants (embryophytes), sometimes treated as a taxonomic Division (taxonomy), division referred to as Bryophyta ''Sensu#Common qualifiers, sensu lato'', that contains three groups of non-vascular pla ...

s, grows. Protonemata are characteristic of all mosses, are present in some liverworts under certain conditions Kohchi, T., Yamato, K. T., Ishizaki, K., Yamaoka, S., & Nishihama, R. (2021). Development and Molecular Genetics of Marchantia polymorpha. Annual Review of Plant Biology, 72(1), 677–702. https://doi.org/10.1146/annurev-arplant-082520-094256 but are absent from hornworts. The protonemata are composed of two cell types: chloronemata, which form upon germination of the spore, and caulonemata, which later differentiate from chloronemata under the influence of plant hormone

auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...

. Reski, R. (1998). Development, Genetics and Molecular Biology of Mosses. Botanica Acta, 111(1), 1–15. https://doi.org/10.1111/j.1438-8677.1998.tb00670.x Jang, G., & Dolan, L. (2011). Auxin promotes the transition from chloronema to caulonema in moss protonema by positively regulating PpRSL1and PpRSL2 in Physcomitrella patens. _New Phytologist_, _192_(2), 319–327.https://doi.org/10.1111/j.1469-8137.2011.03805.x The chloronema cells are visually characterised by presence of high number of chloroplasts, relatively short cells, and cross walls (cell walls separating the cells along the filament) that are angled perpendicular to the growth axis. In comparison, caulonema cells are longer, have less chloroplasts and have cross walls that are situated at an oblique angle to the growth axis. The transition from chloronema to caulonema cells along a filament is gradual. Later in the development of the plant, caulonema cells can form new branches of chloronema cell type, called ''secondary chloronema''. The protonema cells grow ''apically'', meaning that the growth of the filament happens by the division of the cells at the tip of branches. This is distinct from the ''three-faced apical growth'' of the mature gametophyte, which similarly divides at the tip, but forms three daughter cells. The transition from protonema to mature gametophyte happens with the formation of a bud, a single cell that branches out from the protonema filaments, giving rise to mature gametophytic structures like the stem and leaves. A bud typically forms on caulonema cells triggered by the plant hormone

cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in Cell (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...

(particularly 6-(Δ²isopentenyl)adenine, the native cytokinin of mosses), but they can also form on chloronema if the hormone is present at lower concentrations.

References

Bryophytes Plant morphology {{bryophyte-stub