Phi thickenings in Brassica roots – an adaption to water stress?

Collings DA, Aleamotu'a M and McCurdy DW

The University of Newcastle, Callaghan NSW 2308 Australia.

With the Earth’s population to pass 9 billion by 2050, and with climate change increasing rainfall variability, breeding crop varieties more efficient in water usage and resistant to water stress is essential. We have investigated phi thickenings, unusual secondary cell wall thickenings found in the root cortex where only thin, primary cell walls normally occur. These bands extend around radial cell walls, with synthesis coordinated between adjacent cells so that in transverse view they look similar to the Greek letter phi. Surprisingly little research into phi thickenings has been conducted, despite the observations that they can be induced by abiotic stresses in a wide variety of evolutionarily-diverse species. Currently, their function(s) remain unclear. We investigated phi thickenings in Brassica roots grown on agar plates. Confocal microscopy demonstrated the lignified cell walls of phi thickenings forming a complex network of thick reinforcements surrounding the inner cortical cells of the root, and a more delicate, reticulate network on the inner face of these cells adjoining the endodermis. Quantification of phi thickenings showed induction in response to water stress caused by salt (40 mM or higher) or sucrose (1% or higher). However, induction strongly depended on cultivar: of more than 20 commercial Brassica oleracea cultivars, some such as "golden acre" lacked inducible phi thickenings whereas others such as "marathon F1" induced strongly. Similar strong cultivar differences occur in B. napus roots where the winter varieties (for example "edimax") lacked phi thickening induction whereas spring varieties such as "hyola474CL" induced strongly. These observations provide a platform to test phi thickening functions, and discover the genetic pathways leading to their formation. Such information might be applicable to crop breeding strategies in Brassica, and other crops where phi thickenings occur, to develop varieties with improved resistance to water stress.