Accumulation of Al in Root Mucilage of an Al-Resistant and an Al-Sensitive Cultivar of Wheat

Abstract

To estimate rates of AI accumulation within the symplasm, all apoplastic pools of AI need to be eliminated or accounted for. We have developed a revised kinetic protocol that allows us to estimate the contribution of mucilage-bound AI to total, nonexchangeable AI, and to eliminate the mucilage as an apoplastic pool of AI. By comparing the AI content of excised root tips (2 cm) of wheat (Trificum aestivum L.) with and without the removal of the mucilage (using a 1 O-min wash in 1 M NH,CI), we found that AI bound to the mucilage accounted for approximately 25 to 3 5 % of AI remaining after desorption in citric acid. The kinetics of AI uptake into mucilage were biphasic, with a rapid phase occurring in the first 30 min of uptake, followed by a linear phase occurring in the remainder of the experimental period (1 80 min). By adopting a step for removal of mucilage into our existing kinetic protocol, we have been able to isolate a linear phase of uptake with only a slight deviation from linearity in the first 5 min. Although we cannot unambiguously identify this phase of uptake as uptake into the symplasm, we believe this new protocol provides us with the most accurate quantitative estimate of symplastic AI yet available.A number of recent studies have emphasized the importance of the root tip in the expression of A1 toxicity and resistance in plants. This was perhaps most elegantly demonstrated by Ryan et al. (1993), who showed that A1 must be supplied to the terminal 2 to 3 mm of the root apex of Zea mays for symptoms of A1 toxicity to be expressed. This observation is consistent with an array of less direct evidente, which also supports the role of the root tip as the primary site of Al-related lesions. For example, in Allium cepa and Vigniu unguiculata, decreased rates of mitosis have been associated with accumulation of A1 in the root apex (Clarkson, 1965;Horst et al., 1982Horst et al., , 1983. A1 has also been shown to bind to cell nuclei in root tips of Z. mays (Galsomies et al., 1992) and, more specifically, to DNA in roots of Pisum sntivum and A. cepa (Matsumoto et al., 1976;Morimura et al., 1978). If the root tip is indeed the site where toxicity is most clearly expressed, we would expect potentia1 resistance mechanisms to be most clearly expressed in this region as well. Although mechanisms of A1 resistance are poorly understood, Delhaize et al. (1993b) demonstrated that the terminal 3 to 5 mm of root tips of an Al-resistant cultivar of Triticum aestivum L. were the pri-

Aluminum tolerance measured by root growth and mucilage protection inUrochloa brizanthaandUrochloa decumbens

“…Saturation of Al 3 ' -binding sites present in mucilage led to a higher exposure of roots to this metal. This finding corroborates those results obtained by Archambault et al (1996). Indeed, in Vigna unguiculata, 50% of the total Al 3 ' in the root apex was bound to mucilage and removal of the mucilage dramatically reduced root growth (Horst et al 1982).…”

“…Indeed, in Vigna unguiculata, 50% of the total Al 3 ' in the root apex was bound to mucilage and removal of the mucilage dramatically reduced root growth (Horst et al 1982). In resistant plants under Al 3 ' stress, continual synthesis of mucilage should permit maintenance of linkage capacity, thereby providing protection for the growth region (Archambault et al 1996). Detoxification of Al 3 ' by means of root exudates does not only explain apoplastic protection, but also explains reduced uptake of Al 3 ' by the simplast (Huang et al 2009;Yu et al 2009;Horst et al 2010), in which small quantities of Al 3 ' may be associated with irreversible damage to cell function (Jan et al 2001).…”

“…The effect of mucilage through its protective function has been suggested as a tolerance mechanism in some species, such as wheat (Horst et al 1982;Archambault et al 1996) and pea (Yu et al 2009). For aluminum to reach the meristem, it must first penetrate and cross the mucilage layer.…”

“…Other roots of a second subset of the plants were treated with 1 M NH 4 Cl for 10 min in order to remove mucilage, washed in distilled water and cultured in Al 3 ' (Archambault et al 1996). A third subset was treated with NH 4 Cl and cultured in the reverse order of the second subset.…”

Aluminium absorption by intact roots of the Al-accumulating plant Camellia sinensis L.

“…Nevertheless, the possibility of other factors contributing to the non-exchangeable Al cannot be excluded. Some researchers found that long exposures (48 h) created conditions conducive to the accumulation of non-exchangeable Al in the cell wall, which has been attributed to mucilage binding of Al and/or Al precipitation/polymerization in the root surface [14,15]. Thus, the compartmentation of Al in the roots of tea plants remains a topic of future investigation.…”