Spatial characteristics of aluminum uptake and translocation in roots of buckwheat (Fagopyrum esculentum)

Abstract

The detoxification of aluminum (Al) in root tips of the Al accumulator buckwheat by exudation of oxalate leading to reduced Al uptake (Al resistance) is difficult to reconcile with the Al accumulation (Al tolerance). The objective of this study was to analyze resistance and tolerance mechanisms at the same time evaluating particularly possible stratification of Al uptake, Al transport and oxalate exudation along the root apex. The use of a minirhizotron made it possible to differentiate between spatial responses to Al along the root apex with regard to Al uptake and organic acid anion exudation, but also to measure at the same time Al and organic acid transport in the xylem. Al accumulates particularly in the 3-mm root apex. The study showed that Al taken up by the 10-mm root apex is rapidly transferred to the xylem which differentiates in the 10 to 15-mm root zone as revealed by a microscopic study. Al induces the release of oxalate from the root apex but particularly from the subapical 6-20 mm root zone even when Al was applied only to the 5-mm root apex suggesting a basipetal signal transduction. Citrate proved to be the most likely ligand for Al in the xylem because Al and citrate transport rates were positively correlated. In conclusion, the data presented show that the Al-induced release of oxalate, and Al uptake as well as Al accumulation are spatially not separated in the root apex.

Differences in Aluminium Accumulation and Resistance between Genotypes of the Genus Fagopyrum

“…This assumption is supported by own data on the oxalate and citrate contents in surgically partitioned cortical and stele tissues [25]. In contrast, in our study a significant positive correlation between xylem-sap citrate and Al concentrations existed when the Al supply to substrate grown buckwheat cultivar Lifago was increased ( Figure 3B) and across the Fagopyrum genotypes ( Figure 5A) confirming previous results by references [24,25]. Also in the Al accumulator H. macrophylla Al and citrate xylem-sap concentrations were positively correlated [38].…”

“…Another day later the pH of the nutrient solution was reduced in three steps resulting in at least 12 h for adaptation to pH 4.3 before the beginning of the Al treatment. Afterwards, the plants were transferred to 10 mL of a simplified continuously aerated nutrient solution (500 μM CaCl2, 8 μM H3BO3; 100 μM K2SO4, pH 4.3) supplemented either with 0 μM or 75 μM AlCl3 inhibiting root growth by 50%-60% (Klug and Horst, 2010b) and activating Al exclusion and tolerance mechanisms [24]. The pH was controlled frequently and, when necessary, re-adjusted to 4.3 using 0.1 M HCl or 0.1 M KOH.…”

“…Al accumulation in shoots requires radial transfer of Al from the root surface to the xylem where Al is transported primarily as Al-citrate to the shoot [22,23]. Based on sophisticated physiological approaches references [24][25][26] The aim of the present study was to enhance the physiological understanding of Al exclusion on the one hand and Al accumulation on the other hand by exploiting differences in Al resistance and Al accumulation within a large set of genotypes of the species F. esculentum (buckwheat). To even widen the genetic background we included into the genotype selection also genotypes from further species of the Fagopyrum genus namely F. tataricum and F. acutatum.…”

Magnesium mobility in soils as a challenge for soil and plant analysis, magnesium fertilization and root uptake under adverse growth conditions

“…However, acid soils bear also the risk of element toxicities, particularly Mn and Al. Manganese is an essential plant micronutrient, whereas Al is not required by plants, even though some plant species accumulate Al in their tissue (Klug and Horst 2010;Ma et al 2001 [Fagopyrum esculentum], Matsumoto et al 1976 [Camellia sinensis], Ma et al 1997 [Hydrangea macrophylla]) This section is included due the substantial progress made in the last years in clarifying the role of Mg in metal toxicity stress alleviation.…”

Aluminium speciation and internal detoxification mechanisms in plants: where do we stand?

“…Three transport pathways may be involved in Al mobilization: apoplastic transport in the cortical cell walls, symplastic transport and xylem transport. 29 Al has been detected in the xylem of the three most widely studied hyperaccumulators (buckwheat, tea and M. malabathricum) and is usually bound to citrate in the xylem rather than oxalate (as in the root cytosol), 9,12 suggesting that this is a major Al transport route (Fig. 1C).…”