Fluorescence resonance energy transfer-sensitized emission of the yellow cameleon 3.60 was used to study the dynamics of cytoplasmic calcium ([Ca 2+

Studies of AI partitioning and accumulation and of the effect of AI on the growth of intact wheat (Trificum aesfivum 1.) roots of cultivars that show differential AI sensitivity were conducted. The effects of various AI concentrations on root growth and AI accumulation in the tissue were followed for 24 h. At low externa1 AI concentrations, AI accumulation in the root tips was low and root growth was either unaffected or stimulated. Calculations based on regression analysis of growth and AI accumulation in the root tips predicted that 50% root growth inhibition in the AI-tolerant cv Atlas 66 would be attained when the AI concentrations were 105 p~ in the nutrient solution and 376.7 pg AI g-' dry weight in the tissue. In contrast, in the AI-sensitive cv Tam 105, 50% root growth inhibition would be attained when the AI concentrations were 11 p~ in the nutrient solution and 546.2 pg AI g-' dry weight in the tissue. The data support the hypotheses that differential AI sensitivity correlates with differential AI accumulation in the growing root tissue, and that mechanisms of AI tolerance may be based on strategies to exclude AI from the root meristems. ~ A1 is a major growth-limiting factor of plants in acid soils (Foy, 1988;Kochian, 1995) because A1 solubility in the soil solution increases as the soil p H decreases. A1 inhibits plant growth by interfering with the regulatory processes of root growth and development (for reviews see Foy, 1988;Taylor, 1988a;Kochian, 1995). Nonetheless, varieties of the same species have developed strategies to avoid or tolerate A1 stress. These strategies are genetically controlled (Foy, 1988) and severa1 genes may be involved (Berzonsky, 1992). The mechanism(s) of differential A1 sensitivity is a subject of much discussion and debate and has been reviewed recently (Taylor, 1988a, 198813;Kochian, 1995).To better understand the principles of AI tolerance mechanisms and A1 sensitivity, it is necessary to elucidate whether the concentration of AI in the tissue is responsible for the onset of root growth inhibition and to understand how A1 is taken up and transported by the roots at both the cellular and tissue levels. Although AI binds mainly to the components of the cell wall Taylor, 1990,1991) plasma membrane after a short exposure of the tissue to A1 (Lazof et al., 1994). A1 accumulation inside the cell may be required for growth inhibition (e.g. binding to DNA, microtubules, enzymes, etc.); however, A1 may inhibit growth by disrupting the signal transduction pathways without entering the protoplast. The primary site of A1 toxicity is the root meristem (Foy, 1988; Bennet and Breen, 1991;Ryan et al., 1993), and recently, Rincón and Gonzales (1992) and Delhaize et al. (1993a) have found that the major site of A1 accumulation in wheat is the growing root region. Their observations indicate that a differential AI accumulation between the root tips of sensitive and tolerant wheat cultivars correlates with the differential sensitivity to AI.The objective of this study was to spatially ...

Fluorescence resonance energy transfer (FRET)- sensitized emission imaging of Arabidopsis thaliana roots expressing the yellow cameleon 3.60 calcium (Ca(2+)) reporter showed that the concentration of calcium in the cytosol ([Ca(2+)]cyt) increased upon aluminum ion (Al(3+)) treatment in root cells from the transition zone within seconds. The Al(3+)-induced [Ca(2+)]cyt transients were biphasic and were modified by Ca(2+) channel blockers and by an antagonist of neuronal glutamate receptors, 2-amino-5-phosphonopentanoate (AP-5), and by the anion channel blocker, 5-nitro-2-(3'-phenylpropyl-amino) benzoate (NPPB). The [Ca(2+)]cyt transients were not uniquely associated with Al(3+) toxicity mechanisms since lanthanum (La(3+)) and gadolinium (Gd(3+)) also elicited [Ca(2+)]cyt transients that were similar to those induced by Al(3+). Here a testable model that describes a possible mechanism and sequence of events that lead to the Al(3+)-induced [Ca(2+)]cyt transients and inhibition of root growth is proposed. This model can be applied to study also the signal-response coupling of the trivalent ions La(3+) and Gd(3+).