A possible mechanism and sequence of events that lead to the Al3+-induced [Ca2+]_cyt transients and inhibition of root growth

Figures & data

Figure 1 A model that describes a possible mechanism and sequence of events that lead to the [Ca²⁺]_cyt transients and inhibition of root growth. (1) Al³⁺ interacts with Ca²⁺ channels in the plasma membrane of root cells in the root transition zone. The Ca²⁺channels open and external Ca²⁺ enters the cytosol. (2) [Ca²⁺]_cyt rises producing the first peak of the biphasic [Ca²⁺]_cyt signature. (3) Increased [Ca²⁺]_cyt activates internal Ca²⁺ channels located in membranes of internal Ca²⁺ stores (e.g., tonoplast, ER, mitochondria or plastids) producing the second peak of the [Ca²⁺]_cyt signature. (4) Al³⁺ permeates the PM through Ca²⁺- and non-selective cation channels. (5) Al³⁺ opens internal Ca²⁺ channels in the tonoplast, ER, mitochondria or plastids and as a result more Ca²⁺ is released into the cytosol. (6) The overall [Ca²⁺]_cyt elevation stimulates mechanisms that inhibit root growth.

Figure 2 Al³⁺ uptake by PM vesicles isolated from 5 mm root tips of both the Al-sensitive cultivar Scout 66 and the Al-tolerant cultivar Atlas 66. (A) Rate of Al³⁺ uptake by PM vesicles incubated in increasing concentrations of Al³⁺. The PM vesicles from the Al sensitive cultivar Scout 66 were more permeable to Al³⁺ than those of the Al-tolerant cultivar Atlas 66. The values are means ± SD. Rates of Al³⁺ uptake are expressed in Fluorescence Intensity Units (FIU) mg⁻¹ protein min⁻¹. (B) Effect of Ca²⁺ channel blockers on the rate of Al³⁺ uptake by PM vesicles s percent of the control. All Ca²⁺ channel blockers tested inhibited the rate Al³⁺ uptake by the PM vesicles in both cultivars. The accumulation of Al³⁺ in the PM vesicles was monitored by measuring the fluorescence emitted by the Al-morin complex as described in the text. Both experiments were repeated three times in triplicate (n = 9). The PM vesicles were pooled from multiple independent membrane isolations in order to obtain enough membrane protein for the assays.