Phi thickenings in roots: novel secondary wall structures responsive to biotic and abiotic stresses

Abstract

Phi thickenings are specialized secondary walls found in root cortical cells. Despite their widespread occurrence throughout the plant kingdom, these specialized thickenings remain poorly understood. First identified by Van Tieghem in 1871, phi thickenings are a lignified and thickened cell wall band that is deposited inside the primary wall, as a ring around the cells’ radial walls. Phi thickenings can, however, display structural variations including a fine, reticulate network of wall thickenings extending laterally from the central lignified band. While phi thickenings have been proposed to mechanically strengthen roots, act as a permeability barrier to modulate solute movement, and regulate fungal interactions, these possibilities remain to be experimentally confirmed. Furthermore, since temporal and spatial development of phi thickenings varies widely between species, thickenings may perform diverse roles in different species. Phi thickenings can be induced by abiotic stresses in different species; they can, for example, be induced by heavy metals in the Zn/Cd hyperaccumulator Thlaspi caerulescens, and in a cultivar-specific manner by water stress in Brassica. This latter observation provides an experimental platform to probe phi thickening function, and to identify genetic pathways responsible for their formation. These pathways might be expected to differ from those involved in secondary wall formation in xylem, since phi thickening deposition in not linked to programmed cell death.

The Induction and Roles Played by Phi Thickenings in Orchid Roots

“…Permeability experiments showed that in this orchid, phi thickenings did not limit the apoplastic diffusion of tracers. Thus, in their induction by stress and their apparent function in stabilising the root, the phi thickenings in Miltoniopsis behave in a similar manner to phi thickenings in other angiosperms and gymnosperms [1].…”

“…Phi thickenings have been observed in more than 100 different plant species, including several monocots, diverse dicot families, and numerous gymnosperms [1,4]. That similar structures are present in so many diverse species suggests either that the phi thickenings have arisen multiple times through convergent evolution, or that they are an ancestral characteristic that has been conserved.…”

“…Because these isolated bands often appear near circular in cross section, containing a thin primary cell wall running down their centre, they resemble and are named for the Greek letter phi (φ). However, the term is now used to encompass all patterned bands of secondary thickenings in the root cortex [1]. Initially observed in gymnosperms [2], these structures have been described in more than 100 plant species from 16 families, including both monocots and eudicots (reviewed in [1,3,4]).…”

“…However, the term is now used to encompass all patterned bands of secondary thickenings in the root cortex [1]. Initially observed in gymnosperms [2], these structures have been described in more than 100 plant species from 16 families, including both monocots and eudicots (reviewed in [1,3,4]). 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.…”

“…The induction of phi thickening formation as a response to environmental stresses and/or changes in growth conditions also suggests that the new, lignified secondary cell walls might be able to mechanically support cortical cells [1], a suggestion regarding phi thickenings initially made by Van Tieghem [13]. In cherry (Prunus avium), roots grown in culture on MS-based agar medium lacked phi thickenings, but these developed when the plants were transplanted to soil [14], with similar changes also observed in Prunus crosses [15].…”