Structural Conservation of Phi Thickenings in Diverse Plant Taxa

Phi thickenings are bands of lignified and thickened secondary cell wall that encircle root cortical cells in diverse plant taxa, including both angiosperms and gymnosperms.

We used confocal fluorescence microscopy to investigate phi thickening structure from seventeen different plant families, and investigated how the stress hormone jasmonic acid (JA) regulates phi thickening development.

Phi thickenings from different plant families exhibit strong structural similarities, particularly between species showing Type 1 thickenings in the inner cortex, immediately outside the endodermis. JA treatments rapidly induced phi thickenings in Brassica oleracea, with secondary wall development commencing within 10 h of hormone treatments. JA also induced thickening development in all tested Brassicaceae species with the exception of Arabidopsis spp. This might be due to the loss of the inner cortex in roots from this genus. JA also triggers thickening induction in three further angiosperm families (Fabaceae, Geraniaceae, Resedaceae).

Despite the conservation of the JA-dependent signalling pathways, we suggest that phi thickenings are not a conserved or homologous structure. Instead, phi thickenings may represent an example of convergent evolution.