The mechanisms of zinc-induced root growth inhibition in the zinc hyperaccumulator Noccaea caerulescens and the non-accumulator Microthlaspi perfoliatum

This study aimed at revealing the mechanisms of zinc(Zn)-induced root growth inhibition in the Zn hyperaccumulator Noccaea caerulescens and the excluder Microthlaspi perfoliatum to shed light on the intriguing question whether there is any selectivity in the Zn effects on root cell division and elongation.

Zinc effects on various parameters characterizing root cell division and elongation were studied. Total Zn uptake, accumulation, root-to-shoot translocation, and distribution over the root tip tissues as well as metal-induced oxidative stress were also assessed.

Similar degrees of root growth inhibition were achieved when the Zn concentration in the medium and Zn content in the roots were 150 and 5 times higher, respectively, in the hyperaccumulator, compared to the excluder. Zinc accumulated in the cell walls and protoplasts in the root meristem and elongation zone. Although Zn negatively affected both root cell division and elongation, at a similar degree of root growth inhibition, the contribution of the inhibition of cell division was greater in N. caerulescens compared to M. perfoliatum, as the decrease in the meristem length, the number of meristematic cells in a file, and the mitotic index were more prominent in the hyperaccumulator.

Greater contribution of the inhibition of cell division in N. caerulescens compared to M. perfoliatum at a similar degree of root growth inhibition seemed to be partly determined by a stronger degree of oxidative stress and disturbance of mineral nutrition in the roots of the hyperaccumulator, owing to a much higher Zn accumulation.