Formation of Subero-Lignified Apical Deposit in Root Tip of Radish (Raphanus sativus) as a Response to Copper Stress

Heavy metals induce changes in root metabolism and physiology, which can lead to a complex remodelling of root system. Final morphological responses of radish (Raphanus sativus) roots exposed to toxic concentration of heavy metal (Cu), include root growth inhibition, differentiation of xylem vessels close to the root tip, enhanced suberin lamellae deposition and enhanced lateral root production. Recently, we have found that such changes in root morphology and anatomy are coupled with the formation of subero-lignified apical deposit (SLAD) very close to the root tip. Histochemical analysis of lignin and suberin as well as analysis of spatial-temporal characteristics of SLAD formation was performed. This unique structure, not longer than one hundred µm, consists of modified cell walls of central cylinder that are encircled by a short cylinder of prematurely suberized endodermal cells. SLAD starts to form, in both primary and lateral roots, after cessation of root elongation, and it is coupled with xylem differentiation and root branching close to the root apex. We noticed that deposition of phenolic substances into SLAD, mainly suberin in endodermis, is spatially separated from suberization or lignification in basally located endodermis. Although the main reason for formation of SLAD is elusive, we suggest that it is a part of stress induced responses which relate to decreased root growth or permeability in heavy metals stress.

Acknowledgement: The work was supported by Slovak Grant Agency VEGA by grant VEGA 1/0605/17.