The evolution and ecology of plant architecture

Ecological factors affect the distribution of plant architectural diversity and shape its evolution, and future work integrating phylogenetically informed architectural data with ecological variables will continue to unveil how plant architecture is shaped at a global scale.
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In contrast to most animals, plants have an indeterminate body plan, which allows them to add and accumulate new body parts during their lifetime. A plant’s realized modular construction is the result of external constraints and internal processes.

Main concepts in plant architecture.
Main concepts in plant architecture. (A) Growth pattern determinate (i) or indeterminate (ii). (B) Monopodial (i) vs. sympodial (ii) branching. (C) Types of sympodial branching related to the number of relay meristems: monochasium (i), dichasium (ii) and polychasium (iii). (D) Orientation growth of a stem can be orthotropic, when the growth is upright and stem symmetry is typically radial (i), or plagiotropic, when it is horizontal (as a result of endogenous processes, not environmental effects) and often associated with bilateral symmetry (ii). (E) Plagiotropic branches can be monopodial (i) or sympodial, and in this case result from mixed axes with either axillary inflorescences: plagiotropy by apposition (ii) or with terminal inflorescences, implying that each new module is needed to maintain the reproductive and vegetative function: plagiotropy by substitution (iii). (F) Rhythmic growth with periods of endogenous growth cessation where the apical meristem rests in a bud (i) vs. continuous growth where there is no endogenous growth cessation (ii). Scale leaf scars show clear growth units (GUs). (G) Branching can be lateral (axillary), as in almost all seed plants (i), or terminal, involving meristem dichotomy as in lycopsids and various extinct plant groups (ii). (H) Examples of monopodial (i) and sympodial (ii) architectural units. Note that the branching order (BO) increases rapidly in the sympodial architectural unit, while the apparent branching order (AO) mirrors the branching order of the monopodial unit. X denotes apex death. (I) In species with rhythmic shoot growth, branching can be delayed (i) when the axillary meristem outgrowth is delayed compared with apical meristem outgrowth, or immediate (ii), when the apical and axillary meristem branch concurrently. Immediate branching is often characterized by extensive primordium neoformation (Cremer, 1972), and a long first internode in axillary branches termed a hypopodium (H). (J) In species with rhythmic branching, a small number of condensed internodes per GU associated with little intermodal extension defines short shoots (i), which often have specialized functions (e.g. brachyblasts in Pinus), as opposed to long shoots (ii). (K) On a vertical axis or GU, the preferential repartition branches can be towards the apex (acrotony) (i), towards the middle (mesotony) (ii) or towards the base (basitony) (iii). (L) On a horizontal stem or growth unit, a sibling shoot can occur in an upper position (epitony) (i), lateral position (ii) or basal position (hypotony) (iii).

The phylogenetic distribution of plant growth forms across the phylogeny implies that body architectures have originated and been lost repeatedly, being shaped by a limited set of genetic pathways. Chomicki et al.:

  1. synthesize concepts of plant architecture, so far captured in 23 models.
  2. extend them to the fossil record.
  3. summarize what is known about their developmental genetics.
  4. use a phylogenetic approach in several groups to infer how plant architecture has changed and by which intermediate steps.
  5. discuss which macroecological factors may constrain the geographic and ecological distribution of plant architectures.

This synthesis highlights the architectural diversity of dichotomously branched Paleozoic plants, and the subsequent replacement of dichotomy by axillary branching. Plotting the frequency of branching types through time based on an analysis of 58 927 land plant fossils revealed a decrease in dichotomous branching throughout the Devonian and Carboniferous, mirrored by an increase in other branching types including axillary branching. The authors suggest that the evolution of seed plant megaphyllous leaves enabling axillary branching contributed to the demise of dichotomous architectures.

It also pinpoints the gaps in our understanding of the molecular control, ecology and evolution of plant architecture.

Reference

Chomicki, G., Coiro, M., & Renner, S. S. (2017). Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies. Annals of Botany, 120(6), 855–891. https://doi.org/10.1093/aob/mcx113


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