Contrasting reactions of roots of two coniferous tree species to aluminum stress

Abstract

The formation of cell-wall polysaccharide callose in tree roots has been suggested to detect aluminum (Al) stress at an early stage in forest trees. The objective of this study was to clarify whether the Japanese plantation tree species Cryptomeria japonica, which grows in acid soils, and Pinus thunbergii, distributed in coastal regions, have sensitive or tolerant root reactions against Al stress. We examined the accumulation and distribution of callose and Al, and the exudation of organic acid anion (OA) of the roots of C. japonica seedlings, and compared them with those of P. thunbergii after one day exposure to Al in hydroponic systems. Callose in the root apices of P. thunbergii was significantly induced at 0.5 mM Al or higher. However, no induction of callose was recorded in C. japonica supplied with 1.0 mM Al. Experiments with the callose elicitor digitonin confirmed that both species could have a capacity of callose induction. Aluminum was found to accumulate mainly in the epidermal cell walls of C. japonica. In contrast, in P. thunbergii, Al penetrated into more inner root cells, such as cortical cell walls and lumen, and callose deposition was also evident in these compartments. Exudations of OA, such as oxalate, malate and citrate, were observed in both tree species after the treatment of Al, but the exudation patterns of the species differed. We conclude that roots of C. japonica with lower callose productions are tolerant to Al due to not allowing Al to penetrate readily into its inner cells of the roots, partly via OA exudation, but the detailed mechanism still remains unclear.