Changes in zinc mobility, physiological responses and gene expression result in differences in zinc tolerance in two Malus rootstocks

To unravel the physiological and molecular regulation underlying the variation in zinc (Zn) tolerance between two contrasting apple rootstocks, namely, Malus baccata Borkh. (Mb) and Malus hupehensis Rehd. (Mh), seedling were exposed to either 1 or 100 μM Zn under hydroponic conditions. Growth inhibition and impairments in leaf anatomical structure were weaker in Mh than in Mb. The Zn concentrations were 14.2 % and 50.25 % lower in the roots and stems of Mh than in those of Mb, respectively. The translocation factor was reduced by 67.89 % and 44.64 % in Mh and Mb, respectively, in response to excess Zn. The Mh roots presented higher proportions of water-insoluble Zn than the Mb roots. The subcellular distribution of Zn revealed that cell walls (CWs) played an important role in Zn detoxification in both rootstocks. Fourier transform infrared spectroscopy analysis revealed that CWs of Mh had a stronger binding capacity for Zn than did those of Mb. The disturbance of the redox balance induced by excess Zn was weaker in Mh than in Mb. Excess Zn induced a greater reduction in the expression of genes involved in Zn uptake and translocation in the Mh roots than in the Mb roots. However, the expression of genes related to Zn detoxification increased more in Mh roots than in Mb roots. Our results suggest that Mh is more tolerant than Mb to excess Zn, which is ascribed largely to the greater inhibition of Zn mobility and activation of physiological responses and the stricter regulation of the expression of key genes involved in Zn uptake, translocation, remobilization, and detoxification.