The Induction and Roles Played by Phi Thickenings in Orchid Roots.

Nurul A Idris,David A Collings

doi:10.3390/plants8120574

Abstract

Phi thickenings are specialised secondary wall thickenings present in the root cortex of many plant species, including both angiosperms and gymnosperms. While environmental stresses induce phi thickenings, their role(s) in the root remain unclear. Suggested functions include regulation of transport through the apoplast in a manner similar to the Casparian strip, limiting fungal infections, and providing mechanical support to the root. We investigated phi thickening induction and function in Miltoniopsis sp., an epiphytic orchid. As movement of a fluorescent tracer through the apoplast was not blocked by phi thickenings, and as phi thickenings developed in the roots of sterile cultures in the absence of fungus and did not prevent fungal colonisation of cortical cells, the phi thickenings in Miltoniopsis did not function as a barrier. Phi thickenings, absent in roots grown on agar, remained absent when plants were transplanted to moist soil, but were induced when plants were transplanted to well-drained media, and by the application of water stress. We suggest that it is likely that phi thickenings stabilise to the root during water stress. Nevertheless, the varied phi thickening induction responses present in different plant species suggest that the phi thickenings may play multiple adaptive roles depending on species.

Highlights

Phi thickenings are specialised secondary cell walls that form bands around the cortical cells in roots that normally only have a primary cell wall
As plants grown on agar lacked phi thickenings, and as these structures were present in the cortex of soil-grown plants (Figure 1b) viewed both in section (φ) and at the ends of cells, it was possible to manipulate the development of phi thickenings through the use of culture conditions
While it has been suggested that phi thickenings can block the penetration of endomycorrhizal fungi in roots [5,6], we found no correlation between the presence of phi thickening and obstruction of fungal penetration, with phi thickenings formed in the cortex of the roots regardless of the presence or absence of a mycorrhizal interaction, and with fungal infections being independent of the presence or absence of phi thickenings

Summary

Introduction

Phi thickenings are specialised secondary cell walls that form bands around the cortical cells in roots that normally only have a primary cell wall. As phi thickenings are present in diverse taxa, the basic capacity to generate a phi thickening has either evolved multiple times during the evolution, or it has been conserved for a very long period of time In either case, this conservation indicates the fundamental importance of phi thickenings for plant growth. The roles played by phi thickenings, and why such a wide variety of plants develop them, remain unclear, three possible roles for phi thickenings have been proposed. These are to regulate solute uptake, to defend against penetration by fungal hyphae, and to mechanically stabilise the root, either as a response to environmental stresses

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