Abstract
Phi thickenings are specialized bands of secondary wall deposited around radial walls of root cortical cells. These structures have been reported in various species from the Brassicaceae, including Brassica oleracea, where previous reports using hydroponics indicated that they can be induced by exposure to salt. Using roots grown on agar plates, we show that both salt and sucrose can induce the formation of phi thickenings in a diverse range of species within the Brassicaceae. Within the genus Brassica, both B. oleracea and B. napus demonstrated the formation of phi thickenings, but in a strongly cultivar-specific manner. Confocal microscopy of phi thickenings showed that they form a complex network of reinforcement surrounding the inner root cortex, and that a delicate, reticulate network of secondary wall deposition can also variously form on the inner face of the cortical cell layer with phi thickenings adjacent to the endodermal layer. Results presented here indicate that phi thickenings can be induced in response to salt and water stress and that wide variation occurs in these responses even within the same species.
Highlights
Phi thickenings are peculiar secondary cell wall structures found in the cortex of plant roots in diverse species ranging from gymnosperms to angiosperms and from monocots to eudicots
Our definition would include the more complex wall thickenings in the roots of some epiphytic orchids, a series of structures sometimes referred to as a ‘pseudovelamen’ [3,4]. Another related type of cell wall thickening in the root cortex has been described as a ‘crescent thickening’ because the thickening is limited to the inner faces and sides of the cortical cells [5,6]
Following previous work in B. oleracea in which 80 mM salt treatment administered through hydroponics induced phi thickenings [15,18], we confirmed that salt treatment can induce the formation of phi thickenings in cultivar ‘Marathon F1’
Summary
Phi thickenings are peculiar secondary cell wall structures found in the cortex of plant roots in diverse species ranging from gymnosperms to angiosperms and from monocots to eudicots. They were first reported in transverse sections in the ring of cells surrounding the outside of the endodermis in roots of Taxus baccata by Van Tieghem [1], who described the cell wall thickenings with the French term “reseau sus-endodermique” which might be translated to “super-endodermal” or “peri-endodermal”. Another related type of cell wall thickening in the root cortex has been described as a ‘crescent thickening’ because the thickening is limited to the inner faces and sides of the cortical cells [5,6]
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