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Identification of the primary lesion of toxic aluminum (Al) in plant roots

Today soil-resources provide 99.7% of human food but in the last 40 year one third of the global arable land has already been lost due to degradation. The continuing degradation of arable soils is an issue of critical importance for maintaining food security of the very fast increasing of human population.
Aluminum is one of the major causes for soil degradation, in particular in acidic soils, where soil minerals release soluble Al that limits the growth of plants. Unfortunately, acidic soils comprise ca. 40% of the world’s arable land and are typically present in the areas that are more favorable for intensive agriculture. Whilst soils naturally acidify over, the problem is greatly exacerbated by agricultural activities. Typically, soil acidity is overcome by the application of lime. However, lime usage in broad-scale agriculture is expensive and is cost-effective only for 5-10% of acid soils.
The route to find acceptable cheaper solution for improving agriculture yields and ensure food security is to identify and breed genotypes with improved Al tolerance. For reaching this goal we need to know the reasons for the aluminum toxicity. However, this understanding is still in its infancy, despite the fact that the importance of toxic aluminum in the soil has been recognized more than a century ago. In order to shed light on the cause of Al toxicity our team studied the aluminum accumulation and distribution in plant roots. The obtained information provides the necessary building-blocks required to breed crops for improved productivity in our degraded lands.
The results obtained in this study have revealed that toxic effects of aluminum are already exerted within the first 5-30 min of exposure in specific cells located in a region approximately 5-10 mm behind the root tip. Since traditional techniques are not sensitive to access accumulation of aluminum in these initial stages we used a combination of synchrotron-based low-energy X-ray fluorescence spectromicroscopy (LEXRF) at the TwinMic beamline (Elettra - Sincrotrone Trieste) and high-resolution secondary ion mass spectroscopy (NanoSIMS) to examine the spatial distribution of Al on a cellular and subcellular level. This integrated approach allowed us to identify the sequence of processes whereby Al reduces the growth of roots in rather short term. The obtained results demonstrate that the majority of Al starts to accumulate within the walls of cells in the outer root tissues (as shown in Figure 1) and this preferential storage in the walls of the outer tissues continues even after exposures of 24 h (Figure 2). This observation is particularly important since the cells located in the region 5-10 mm behind the root tip are the ones responsible for the root growth via walls loosening.


Figure 1. Distribution of Al in 7 µm-thick transverse cross section of soybean roots taken 0.75 mm (a), 2 mm, or 6 mm (c) from the apex, with the exterior of the root on the right and the center of the root cylinder on the left. All roots were exposed to 30 µM Al for 0.5 h. The image in (d) is a similar region indicated by the yellow dotted box in (b) but from subsequent sections, and the image in (e) is similar to the region indicated by the white dotted box in (c). The signal intensity (concentration) is shown as a color scale; relative intensities are used for each individual image. The data for Al in (a) to (c) were collected using µ-XRF whilst the data in (d) and (e) were collected using nanoSIMS.

Figure 2.  Distribution of Al using LEXRF in a 7 μm-thick transverse section of a soybean root 6 mm from the apex (i.e. elongation zone) after exposure to 30 μM Al for 24 h. The signal intensity is shown as a color scale with brighter colors indicating higher concentration. Both (a) and (b) show the distribution of Al, but a logarithmic scale is used to represent the colors in (a) in order to more clearly show the distribution of the lower concentrations of Al in the inner tissues. In (b) a linear scale is used to represent the colors. For (c), the map for Al is overlaid with the map for absorption. Note that the root has ruptured and hence the rhizodermis and outer cortex are torn.

Our findings clearly show that the accumulation of Al is rather rapid process and Al remains concentrated in the cell walls with exposure time. The binding of Al to the cell walls exerts toxic effects, leading to inhibition of cell elongation and growth. The results clearly indicate that for overcoming the deleterious effects of Al it is important to focus on traits related to cell wall composition as well as traits involved in wall loosening.
This research was conducted with the contribution of the Elettra Scientific Computing Team and was lead by the research team of Dr Peter M Kopittke in collaboration with the University of Oxford and the University of South Australia.

 

This research was conducted by the following research team:

  • Peter M Kopittke, Brett J. Ferguson, F. Pax C. Blamey, Neal W. Menzies, Brigid A. McKenna, Peng Wang, Peter M. Gresshoff, Alina Tollenaere, The University of Queensland, School of Agriculture and Food Sciences,St. Lucia, Queensland, Australia
  • Katie L Moore, University of Oxford, Department of Materials, Oxford, OX1 3PH, United Kingdom
  • Enzo Lombi, University of South Australia, Centre for Environmental Risk Assessment and Remediation, Mawson Lakes, South Australia, Australia
  • Alessandra Gianoncelli, George Kourousias, Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
  • Timothy M. Nicholson,The University of Queensland, School of Chemical Engineering, St Lucia, Queensland, Australia
  • Rick I. Webb, Kathryn Green, The University of Queensland, Centre for Microscopy and Microanalysis, St. Lucia, Queensland, Australia.


Contact persons:
Alessandra Gianoncelli: alessandra.gianoncelli@elettra.eu
Peter M Kopittke: p.kopittke@uq.edu.au
George Kourousias: george.kourousias@elettra.eu

 

Reference

Peter M. Kopittke, Katie L. Moore, Enzo Lombi, Alessandra Gianoncelli, Brett J. Ferguson, F. Pax C. Blamey, Neal W. Menzies, Timothy M. Nicholson, Brigid A. McKenna, Peng Wang, Peter M. Gresshoff, George Kourousias, Richard I. Webb, Kathryn Green, Alina Tollenaere, “Identification of the primary lesion of toxic aluminum (Al) in plant roots”Plant Physiology 167, 1402 (2015) 
doi: 10.1104/pp.114.253229

 

 

Last Updated on Tuesday, 28 April 2015 17:27