Aluminium–silicon interactions in higher plants: an update

Abstract

Aluminium (Al) and silicon (Si) are abundant in soils, but their availability for plant uptake is limited by low solubility. However, Al toxicity is a major problem in naturally occurring acid soils and in soils affected by acidic precipitation. When, in 1995, we reviewed this topic for the Journal of Experimental Botany, it was clear that under certain circumstances soluble Si could ameliorate the toxic effects of Al, an effect mirrored in organisms beyond the plant kingdom. In the 25 years since our review, it has become evident that the amelioration phenomenon occurs in the root apoplast, with the formation of hydroxyaluminosilicates being part of the mechanism. A much better knowledge of the molecular basis for Si and Al uptake by plants and of Al toxicity mechanisms has been developed. However, relating this work to amelioration by Si is at an early stage. It is now clear that co-deposition of Al and Si in phytoliths is a fairly common phenomenon in the plant kingdom, and this may be important in detoxification of Al. Relatively little work on Al–Si interactions in field situations has been done in the last 25 years, and this is a key area for future development.

Interactions of Silicon With Essential and Beneficial Elements in Plants

“…Numerous studies, have shown that Si supply affects aluminum (Al) uptake, and subsequently plant tolerance to Al-toxicity, by either external ( ex planta ) and/or internal ( in planta ) mechanisms (see Hodson and Evans, 2020 and reference therein). Early investigations showed that the ameliorative effect of Si on Al toxicity could mainly be attributed to increased pH of the rhizosphere.…”

“…From chemical studies on Al-Si interaction in solution it was also known that pH influenced the formation of sub-colloidal inert hydroxyaluminosilicate species (HAS) ( Exley and Birchall, 1992 , 1993 ). However, the potential impact of HAS on plant growth was not appreciated until Baylis et al (1994) reported that at low pH, a higher concentration Si was required to ameliorate Al toxicity by soybean; the authors postulated that basis for this higher demand for Si was the higher affinity of Si for Al in HAS formation at low pH (see Hodson and Evans, 2020 ). The beneficial effect of silicic acid was also confirmed in later studies on Al-Si interaction in maize ( Ma et al, 1997 ).…”

“…It is now reasonably well established that Si is essential for only a few species of plants, the FIGURE 1 | Number of publications related to silicon in plants from 2010 to 2020, based on Scopus search with the title words: "Silicon" or "Silicate" and "Plants," refined to "Agricultural and Biological Sciences" (March, 2021). Total number of publications: 351; research articles: 290; reviews: 61. silicophiles, high in Si concentration (see Marschner, 2012;Hodson and Evans, 2020). However, its classification as a beneficial element has been recognized for more than 50 years by those teaching and researching in plant nutrition (e.g., Mengel and Kirkby, 2001;Epstein and Bloom, 2008;Marschner, 2012).…”

“…The concentration of the plant-available form of Si in soil solutions, monosilicic acid (H 4 SiO 4 ), varies between 0.1 and 0.6 mM, which is about two orders of magnitude higher than the concentrations of phosphorus (P) ( Yan et al, 2018 ). Silicon in taken up and translocated through the plant to be deposited as SiO 2 phytoliths in the lumen, cell walls and intercellular spaces (see Hodson and Evans, 2020 ). However, plants species differ greatly in their ability to accumulate Si with values ranging from 0.1% to 10% Si on a dry weight basis ( Epstein, 1994 , 1999 ).…”

“…The essentiality of Si for plants has been the matter of a long debate dating back to the 19th century (e.g., J. von Sachs versus E. Wolff and C. Kreuzhage, see Sreenivasan, 1934 ; Katz et al, 2021 ). It is now reasonably well established that Si is essential for only a few species of plants, the silicophiles, high in Si concentration (see Marschner, 2012 ; Hodson and Evans, 2020 ). However, its classification as a beneficial element has been recognized for more than 50 years by those teaching and researching in plant nutrition (e.g., Mengel and Kirkby, 2001 ; Epstein and Bloom, 2008 ; Marschner, 2012 ).…”