Phyllotaxis: from classical knowledge to molecular genetics

Leaves and sporangia developed in rare non-Fibonacci spirals in early leafy plants

Lateral plant organs, including leaves and reproductive structures, are arranged on stems in distinct patterns termed phyllotaxis. Most extant plants exhibit phyllotactic patterns that are mathematically described by the Fibonacci series. However, it remains unclear what lateral organ arrangements were present in early leafy plants. To investigate this, we quantified phyllotaxis in fossils of the Early Devonian lycopod Asteroxylon mackiei. We report diverse phyllotaxis in leaves, including whorls and spirals. Spirals were all n:(n+1) non-Fibonacci types. We also show that leaves and reproductive structures occurred in the same phyllotactic series, indicating developmental similarities between the organs. Our findings shed light on the long-standing debate about leaf origins and demonstrate the antiquity of non-Fibonacci spirals in plants.

Organ Patterning at the Shoot Apical Meristem (SAM): The Potential Role of the Vascular System

Auxin, which is transported in the outermost cell layer, is one of the major players involved in plant organ initiation and positioning at the shoot apical meristem (SAM). However, recent studies have recognized the role of putative internal signals as an important factor collaborating with the well-described superficial pathway of organogenesis regulation. Different internal signals have been proposed; however, their nature and transport route have not been precisely determined. Therefore, in this mini-review, we aimed to summarize the current knowledge regarding the auxin-dependent regulation of organ positioning at the SAM and to discuss the vascular system as a potential route for internal signals. In addition, as regular organ patterning is a universal phenomenon, we focus on the role of the vasculature in this process in the major lineages of land plants, i.e., bryophytes, lycophytes, ferns, gymnosperms, and angiosperms.

Law and order in plants - the origin and functional relevance of phyllotaxis

The regular arrangement of organs (phyllotaxis) in vegetative shoots and flowers is one of the most stunning features of plants. Spiral patterns characterized by Fibonacci numbers have attracted the particular interest of natural scientists and mathematicians. Numerous reviews have dealt with the molecular genetic mechanisms underlying phyllotaxis, and modeling studies have sought to recreate phyllotaxis according to mathematical, biochemical, or physical laws. However, what is the functional significance of regular plant architecture, and how did it evolve? We discuss the developmental constraints and selective forces that may have favored the selection of phyllotaxis, and we argue that a central driver of regular phyllotaxis may have been limitations in the allocation of founder cells and metabolic resources to the different tissues in the shoot apex.

The Search for Geometrical Parameters That Represent the Dynamic Nature of Phyllotaxis in Plants

The shoot apical meristem (SAM) is the main growth centre that produces lateral organs such as leaves in specific and precise symmetrical patterns. The main goal of this commentary is to explore the link between phyllotactic parameters such as the plastochrone ratio (R) as well as Γ (relationship between the size of the SAM and the size of primordia) and growth processes involved in the expression of symmetrical patterns at the level of the SAM. It is also possible to deduce the values of these parameters from measures of the apical area and the radius of the SAM. A comparative analysis of phyllotactic parameters for three species (Begonia scabrida, Euterpe oleracea, and Anagallis arvensis) for which ‘dynamic’ data are available reveals that empirical and theoretical values of R and Γ can differ, indicating that characters defining these parameters are subject to developmental constraints that in turn determine the boundary conditions for the dynamic manifestation of phyllotactic patterns. One of the main challenges that remains is the ability to consistently and effectively observe the dynamic changes taking place at the SAM (in real time) over an extended period.

Fibonacci spirals may not need the Golden Angle

Phyllotaxis, the regular arrangement of plant lateral organs, is an important aspect of quantitative plant biology. Some models relying on the geometric relationship of the shoot apex and organ primordia focus mainly on spiral phyllotaxis, a common phyllotaxis mode. While these models often predict the dependency of Fibonacci spirals on the Golden Angle, other models do not emphasise such a relation. Phyllotactic patterning in Asteraceae is one such example. Recently, it was revealed that auxin dynamics and the expansion and contraction of the active ring of the capitulum (head) are the key processes to guide Fibonacci spirals in gerbera (Gerbera hybrida). In this Insights paper, we discuss the importance of auxin dynamics, distinct phases of phyllotactic patterning, and the transition of phyllotaxis modes. These findings signify the local interaction among primordia in phyllotactic patterning and the notion that Fibonacci spirals may not need the Golden Angle.