Phi Thickenings: Their History, Current Status and Role(s) in Mechanically Strengthening the Plant Root

Abstract

Phi thickenings, lignified bands of secondary cell wall encircling root cortical cells, align between adjacent cells to form a complex network that frames the endodermis and central stele. Since their description in numerous angiosperms and gymnosperms in the two decades from the 1860s, there has been little research into the functions of these enigmatic structures. Their cage-like organisation led to speculation that phi thickenings mechanically strengthen the root, but more recent ideas include that they regulate biotic interactions or control ion movements in a manner similar to the Casparian strip. There is, however, sparse direct evidence supporting these suggestions. In this review, we focus on the roles that phi thickenings might play within roots, and although we conclude that the primary function of phi thickenings is to mechanically stiffen the root apex, we emphasise that phi thickenings need not have a single role and that they can be “re-tooled” to perform other functions. We describe several experimental systems for studying phi thickening functions. Geranium and Pelargonium roots, in which phi thickenings form in cortical cells immediately under the epidermis, are well suited for cell biology investigations, whereas the large aerial roots of epiphytic orchids are ideal for investigating root biomechanics. For genetic analysis, however, the differential induction of phi thickenings in Brassica roots in response to water stress or hormones provides a powerful experimental platform to identify regulatory mechanisms and directly test our models of phi thickening functionality. Since phi thickenings typically form in the root apex, we propose that these structures function primarily to strengthen or stiffen the plant root. We reject the concept that phi thickenings function to block ion flows in a manner analogous to the Casparian strip, but instead see the potential for limiting ion flows within the root cortex as drawback to the presence of thickenings. This negative outcome due to the presence of phi thickenings may explain why phi thickenings are only induced in response to specific biotic or abiotic challenges in some species and are not formed constitutively. In some instances, however, phi thickenings may have been “re-tooled” to perform other roles including blocking uptake of ions, notably in the case of Brassicaceae species growing under extreme conditions.