Mrs Maketalena Aleamotu’a1, Associate Professor David W McCurdy1, Associate Professor David A Collings1
1School of Environmental and Life Science, The University Of Newcastle, Newcastle, Callaghan, Australia
Phi thickenings are peculiar secondary cell wall thickenings found in radial walls of cortical cells in plant roots, where only thin, primary walls normally occur. Although first described in the 19th century, research into phi thickening development has been lacking, and their roles within roots remain unknown. While we speculate that thickenings strengthen the root tip, possibly helping the root penetrate through soil, the lack of appropriate induction systems precludes characterisation of their roles.
We developed a simple system for rapid phi thickening induction in primary roots of Brassica in which four-day old seedlings are transferred from control agar plates to new plates containing osmotica such as salt. Phi thickenings develop within a narrow region of the differentiation zone in levels proportional to osmotic stress, with cellulose deposition and lignification starting after twelve and fifteen hours respectively. Osmoprotectants such as glycine-betaine, however, inhibited induction when tested in combination with thickening-inducing osmotica. An independent biomechanical pathway regulating phi thickening induction is also present within Brassica roots, with root elongation rates and substrate texture important for thickening induction. Phi thickening development is also controlled by stress-related plant hormones, most notably jasmonic acid (JA). As ibuprofen, a JA biosynthesis inhibitor, blocks phi thickening induction by both osmotic and mechanical effects, we suggest that JA plays a critical role in controlling phi thickening induction. Our research not only provides the first understanding of the developmental pathways controlling phi thickening induction, but provides tools with which the functions of these enigmatic structures might be clarified.
Maketalena Aleamotu’a is a finishing PhD student at the University of Newcastle working on phi thickening, a secondary cell wall structure in Brassica root. She’s an aspiring student determining to uncover the genes involve in phi thickening formation with the ambition that the outcome will be directly applicable to crop breeding strategies in Brassica.