Abstract
Aluminum (Al) is one of the most important crust elements causing reduced plant production in acidic soils. Barley (Hordeum vulgare L.) is considered to be one of the crops that is most sensitive to Al, and the root cell wall is the primary target of Al toxicity. In this study, we evaluate the possible involvement of specific pectic epitopes in the cells of barley roots in response to aluminum exposure. We targeted four different pectic epitopes recognized by LM5, LM6, LM19, and LM20 antibodies using an immunocytochemical approach. Since Al becomes available and toxic to plants in acidic soils, we performed our analyses on barley roots that had been grown in acidic conditions (pH 4.0) with and without Al and in control conditions (pH 6.0). Differences connected with the presence and distribution of the pectic epitopes between the control and Al-treated roots were observed. In the Al-treated roots, pectins with galactan sidechains were detected with a visually lower fluorescence intensity than in the control roots while pectins with arabinan sidechains were abundantly present. Furthermore, esterified homogalacturonans (HGs) were present with a visually higher fluorescence intensity compared to the control, while methyl-esterified HGs were present in a similar amount. Based on the presented results, it was concluded that methyl-esterified HG can be a marker for newly arising cell walls. Additionally, histological changes were detected in the roots grown under Al exposure. Among them, an increase in root diameter, shortening of root cap, and increase in the size of rhizodermal cells and divisions of exodermal and cortex cells were observed. The presented data extend upon the knowledge on the chemical composition of the cell wall of barley root cells under stress conditions. The response of cells to Al can be expressed by the specific distribution of pectins in the cell wall and, thus, enables the knowledge on Al toxicity to be extended by explaining the mechanism by which Al inhibits root elongation.
Highlights
Aluminum (Al) is the third most abundant element in the Earth’s crust and the most common metal
The mean length of the root cap of a control root at pH 6.0 was 277 μm ± 19.5 (SD), while in the roots that had been treated with aluminum, it decreased by almost 50% and was 160 μm ± 31.7 (SD)
The obtained results indicate that Al alters the growth parameters of roots and their histology, which is manifested by an increase in the root diameter, a significant increase in the diameter of the rhizodermal cells, and an acceleration of xylem differentiation and fiber maturation
Summary
Aluminum (Al) is the third most abundant element in the Earth’s crust and the most common metal. At neutral pH, it is present in soils as insoluble aluminosilicates and oxides which are harmless to living organisms [1]. In pH below 5.5, Al becomes soluble and available to plants in the form of phytotoxic Al3+ ions [2]. Al-induced changes in the root system reduce nutrient uptake, which results in a nutritional deficiency that leads to a reduction of overall plant growth and yield. It is estimated that about 50% of the world’s arable lands are acidic, predominantly in South America, Central Africa, and Southwest Asia, and in eastern North America and throughout Europe [3]. Industrial pollution and the application of ammonium-containing fertilizers promote soil acidification [4]. In developing countries, where the use of amendments in order to raise the soil pH is limited by economic constraints, the problem of soil acidity is most prevalent, but Al toxicity is considered to be one of the most important limiting factors in agricultural production worldwide [5]
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