Two-stage patterning dynamics in conifer cotyledon whorl morphogenesis

Size regulation of the lateral organ initiation zone and its role in determining cotyledon number in conifers

Introduction

Unlike monocots and dicots, many conifers, particularly Pinaceae, form three or more cotyledons. These are arranged in a whorl, or ring, at a particular distance from the embryo tip, with cotyledons evenly spaced within the ring. The number of cotyledons, n_c, varies substantially within species, both in clonal cultures and in seed embryos. n_c variability reflects embryo size variability, with larger diameter embryos having higher n_c. Correcting for growth during embryo development, we extract values for the whorl radius at each n_c. This radius, corresponding to the spatial pattern of cotyledon differentiation factors, varies over three-fold for the naturally observed range of n_c. The current work focuses on factors in the patterning mechanism that could produce such a broad variability in whorl radius. Molecularly, work in Arabidopsis has shown that the initiation zone for leaf primordia occurs at a minimum between inhibitor zones of HD-ZIP III at the shoot apical meristem (SAM) tip and KANADI (KAN) encircling this farther from the tip. PIN1-auxin dynamics within this uninhibited ring form auxin maxima, specifying primordia initiation sites. A similar mechanism is indicated in conifer embryos by effects on cotyledon formation with overexpression of HD-ZIP III inhibitors and by interference with PIN1-auxin patterning.

Methods

We develop a mathematical model for HD-ZIP III/KAN spatial localization and use this to characterize the molecular regulation that could generate (a) the three-fold whorl radius variation (and associated n_c variability) observed in conifer cotyledon development, and (b) the HD-ZIP III and KAN shifts induced experimentally in conifer embryos and in Arabidopsis.

Results

This quantitative framework indicates the sensitivity of mechanism components for positioning lateral organs closer to or farther from the tip. Positional shifting is most readily driven by changes to the extent of upstream (meristematic) patterning and changes in HD-ZIP III/KAN mutual inhibition, and less efficiently driven by changes in upstream dosage or the activation of HD-ZIP III. Sharper expression boundaries can also be more resistant to shifting than shallower expression boundaries.

Discussion

The strong variability seen in conifer n_c (commonly from 2 to 10) may reflect a freer variation in regulatory interactions, whereas monocot (n_c = 1) and dicot (n_c = 2) development may require tighter control of such variation. These results provide direction for future quantitative experiments on the positional control of lateral organ initiation, and consequently on plant phyllotaxy and architecture.

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.

Patterning, From Conifers to Consciousness: Turing’s Theory and Order From Fluctuations

This is a brief account of Turing’s ideas on biological pattern and the events that led to their wider acceptance by biologists as a valid way to investigate developmental pattern, and of the value of theory more generally in biology. Periodic patterns have played a key role in this process, especially 2D arrays of oriented stripes, which proved a disappointment in theoretical terms in the case of Drosophila segmentation, but a boost to theory as applied to skin patterns in fish and model chemical reactions. The concept of “order from fluctuations” is a key component of Turing’s theory, wherein pattern arises by selective amplification of spatial components concealed in the random disorder of molecular and/or cellular processes. For biological examples, a crucial point from an analytical standpoint is knowing the nature of the fluctuations, where the amplifier resides, and the timescale over which selective amplification occurs. The answer clarifies the difference between “inelegant” examples such as Drosophila segmentation, which is perhaps better understood as a programmatic assembly process, and “elegant” ones expressible in equations like Turing’s: that the fluctuations and selection process occur predominantly in evolutionary time for the former, but in real time for the latter, and likewise for error suppression, which for Drosophila is historical, in being lodged firmly in past evolutionary events. The prospects for a further extension of Turing’s ideas to the complexities of brain development and consciousness is discussed, where a case can be made that it could well be in neuroscience that his ideas find their most important application.