Rapid Uptake of Aluminum into Cells of Intact Soybean Root Tips (A Microanalytical Study Using Secondary Ion Mass Spectrometry)

Abstract

A wide range of physiological disorders has been reported within the first few hours of exposing intact plant roots to moderate levels of AI3+. Past microanalytic studies, largely limited to electron probe x-ray microanalysis, have been unable to detect intracellular AI in this time frame. This has led to the suggestion that AI exerts its effect solely from extracellular or remote tissue sites. Here, freezedried cryosections (10 pm thick) collected from the soybean (Glycine m a ) primary root tip (0.3-0.8 mm from the apex) were analyzed using secondary ion mass spectrometry (SIMS). The high sensitivity of SIMS for AI permitted the first direct evidence of early entry of AI into root cells. AI was found in cells of the root tip after a 30-min exposure of intact roots to 38 p~ AI3+. The accumulation of AI was greatest in the first 30 fim, i.e. two to three cell layers, but elevated AI levels extended at least 150 pm inward from the root edge. Intracellular AI concentrations at the root periphery were estimated to be about 70 nmol g-' fresh weight. After 18 h of exposure, AI was evident throughout the root cross-section, although the rate of accumulation had slowed considerably from that during the initial 30 min. These results are consistent with the hypothesis that early effects of AI toxicity at the root apex, such as those on cell division, cell extension, or nutrient transport, involve the direct intervention of AI on cell function. ~~

Aluminum Accumulation at Nuclei of Cells in the Root Tip. Fluorescence Detection Using Lumogallion and Confocal Laser Scanning Microscopy

“…The results are consistent with others in the literature from experiments with higher Al concentrations in solution. Collectively, they indicate that Al is able to penetrate the cell symplasm relatively fast (Lazof et al, 1994a;Vitorello and Haug, 1996) and bind to nuclear molecules (Matsumoto, 1991), presumably leading to observed decreases in mitotic activity (Clarkson, 1965; Matsu- Table I. Al accumulation in root apices of soybean cv Young (Al-sensitive) and PI 416937 (Al-tolerant) exposed to 1.45 M Al 3ϩ for 48 h Following exposure to Al, root tips (approximately 5 mm) were excised and washed in ice-cold 10 mM citrate for 30 min.…”

“…Furthermore, the combination of images from DIC and from DAPI and Al fluorescence at the cell wall and nucleus allowed much better cellular definition and Al localization compared with other methodologies used for Al localization previously, e.g. energy-dispersive X-ray microanalysis (Delhaize et al, 1993a) and secondary ion mass spectrometry (Lazof et al, 1994a). Confocal laser scanning microscopy coupled with double staining of Al with morin and polynucleotides with propidium iodide was successfully employed to assess the magnitude of Al accumulation and its distribution in yeast cells (Ezaki et al, 1999).…”

“…Relatively direct disruptions associated with Al binding to DNA or other nuclear material seem possible. It appears that Al can accumulate inside cells at the root tip within 30 min to several hours of exposure (Delhaize et al, 1993a;Lazof et al, 1994a;Vasquez et al, 1999), and it has been shown in many experiments by Matsumoto (1991) and others that intracellular Al binds to cell nuclei and DNA (for a summary of results, see Matsumoto, 1991). Nonetheless, the importance of Al binding at the nucleus in the root growth response remains in question, because experiments typically involved exposure to very high Al concentrations (0.1-1.0 mm Al T ).…”

Aluminium effects on microtubule organization in dividing root‐tip cells of Triticum turgidum. I. Mitotic cells

“…However, shortly afterwards small quantities of Al, which in the case of soyabean root-tip cells vary between 30 and 70 mmol g −" f.wt, enter the cytoplasm (Lazof et al, 1994), interfering with many signal-transduction pathways (Kochian, 1995 ;Kochian & Jones, 1997). It is likely that in this way the Al ions indirectly affect the organization of the cytoskeleton in both plant cells (Kochian, 1995 ;Kochian & Jones, 1997 ;Grabski & Schindler, 1995 ;Blancaflor et al, 1998) and animal cells (Strong et al, 1997).…”

Direct Evaluation of the Ca2+-Displacement Hypothesis for Al Toxicity

“…These investigations often require reliable estimates of AI uptake into the symplasm, but the capacity for the cell wall to accumulate high concentrations of cations makes the resolution of apoplasmic and symplasmic fractions difficult (Dainty and Hope, 1959; Zhang and Taylor, 1990; Reid and Smith, 1992). Some research indicates that A1 can readily cross the plasma membrane (Lazof et al, 1994) and that a large proportion of the A1 in roots resides in the symplasm (Tice et al, 1992; see Taylor, …”