Abstract

Phyllotactic diversity and developmental transitions between phyllotactic patterns are not fully understood. The plants studied so far, such as Magnolia, Torreya or Abies, are not suitable for experimental work, and the most popular model plant, Arabidopsis thaliana, does not show sufficient phyllotactic variability. It has been found that in common verbena (Verbena officinalis L.), a perennial, cosmopolitan plant, phyllotaxis differs not only between growth phases in primary transitions but also along the indeterminate inflorescence axis in a series of multiple secondary transitions. The latter are no longer associated with the change in lateral organ identity, and the sequence of phyllotactic patterns is puzzling from a theoretical point of view. Data from the experiments in silico, confronted with empirical observations, suggest that secondary transitions might be triggered by the cumulative effect of fluctuations in the continuously decreasing bract primordia size. The most important finding is that the changes in the primary vascular system, associated with phyllotactic transitions, precede those taking place at the apical meristem. This raises the question of the role of the vascular system in determining primordia initiation sites, and possibly challenges the autonomy of the apex. The results of this study highlight the complex relationships between various systems that have to coordinate their growth and differentiation in the developing plant shoot. Common verbena emerges from this research as a plant that may become a new model suitable for further studies on the causes of phyllotactic transitions.

Highlights

  • The regular distribution of lateral organs in plant axial systems, called phyllotaxis, has intrigued scientists for a long time

  • This paper introduces common verbena (Verbena officinalis L.), which meets these requirements and exhibits multiple developmental changes in phyllotaxis that occur in a seemingly non-random manner

  • The sequences of multiple phyllotactic transitions in Verbena officinalis shoots suggest a constant change in the ratio between primordia size and the size of the organogenic surface of the apical meristem, and this has been confirmed in the case of primary transitions

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Summary

Introduction

The regular distribution of lateral organs in plant axial systems, called phyllotaxis, has intrigued scientists for a long time. Interesting examples of this universal phenomenon have been found among brown algae (Peaucelle and Couder 2016), leafy bryophytes (Zagórska-Marek et al 2018), clubmosses (Gola 1996; Yin and Meicenheimer 2016), and seed plants (Yin et al 2011; Zagórska-Marek 1985; for review see Gola and Banasiak 2016). Phyllotactic transitions may occur even when organ identity does not change, as in Journal of Plant Research (2021) 134:441–456 the vegetative shoots of Abies balsamea and in the gynoecia of Magnolia (Zagórska-Marek 1985, 1994). The last two cases indicate that the genetic mechanism specifying primordia distribution acts independently from that responsible for their identity

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