The life of phi: the development of phi thickenings in roots of the orchids of the genus Miltoniopsis


Phi thickenings, bands of secondary wall thickenings that reinforce the primary wall of root cortical cells in a wide range of species, are described for the first time in the epiphytic orchid Miltoniopsis. As with phi thickenings found in other plants, the phi thickenings in Miltoniopsis contain highly aligned cellulose running along the lengths of the thickenings, and are lignified but not suberized. Using a combination of histological and immunocytochemical techniques, thickening development can be categorized into three different stages. Microtubules align lengthwise along the thickening during early and intermediate stages of development, and callose is deposited within the thickening in a pattern similar to the microtubules. These developing thickenings also label with the fluorescently tagged lectin wheat germ agglutinin (WGA). These associations with microtubules and callose, and the WGA labeling, all disappear when the phi thickenings are mature. This pattern of callose and WGA deposition show changes in the thickened cell wall composition and may shed light on the function of phi thickenings in plant roots, a role for which has yet to be established.

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Wheat germ agglutinin


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We thank Manfred Ingerfeld for microscopy technical support, David Conder (School of Biological Sciences) for help maintaining the orchids, Dr John Clemens (Christchurch Botanic Gardens) and John Clements (Canterbury Orchid Society) for supplying the orchids. NAI acknowledges the Malaysian Education Ministry for the PhD scholarship (SLAB) and funding by School of Biological Sciences at the University of Canterbury.

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Correspondence to David A. Collings.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary movie 1. Confocal 3D reconstruction of berberine hemisulfate stained longitudinal root section with phi thickenings aligned in long bands between cells running through the root length and parallel to the vascular tissue. (WMV 677 kb)

Supplementary movie 2. Confocal 3D reconstruction of a cross sectioned root stained with berberine hemisulfate showing some phi thickenings perfectly aligned between adjacent cells (Φ) and others which do not (Þ). (WMV 1052 kb)

Supplementary movie 1. Confocal 3D reconstruction of berberine hemisulfate stained longitudinal root section with phi thickenings aligned in long bands between cells running through the root length and parallel to the vascular tissue. (WMV 677 kb)

Supplementary movie 2. Confocal 3D reconstruction of a cross sectioned root stained with berberine hemisulfate showing some phi thickenings perfectly aligned between adjacent cells (Φ) and others which do not (Þ). (WMV 1052 kb)

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Idris, N.A., Collings, D.A. The life of phi: the development of phi thickenings in roots of the orchids of the genus Miltoniopsis . Planta 241, 489–506 (2015).

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  • Callose
  • Cellulose deposition
  • Orchids
  • Microtubule organization
  • Miltoniopsis
  • Mycorrhizae
  • Phi thickenings
  • Wheat germ agglutinin