Impact of elevated CO2 on aluminium tolerance of wheat in acid soils

Take home messages

• Response to elevated CO2 of Al-tolerant wheat genotypes appears to be affected by unidentified traits in the breeding parent lines and not by known carboxylate efflux mechanisms.
• The citrate and malate efflux wheat genotypes confer Al-tolerance in acidic soils and a greater overall tolerance is observed when citrate and malate efflux is combined.
• The ET8 wheat genotype showed superior performance under ambient CO2 rather than elevated CO2 for reasons that are not yet identified.

Background

Acidic soils (pH<5.5) are a major constraint to global agricultural production, often due to toxic levels of aluminium (Al) and manganese (Mn) and decreases in the availability of key plant nutrients, most commonly phosphorous (P) (Schroeder et al. 2013). Current estimates suggest that approximately 50% of the world’s arable land is acidic with 77-90 million hectares of topsoil in Australia alone currently acidic (Page et al. 2009). These estimates are predicted to increase in the future due to current farming practices leading to further soil acidification (Scott et al. 2000). While liming of acid soils is an effective ameliorant, it is not economically viable for farmers in developing countries or for ameliorating subsoil acidity (Kochian et al. 2004).

Of all the constraints under acidic soils, Al toxicity is considered the most important (Ma et al. 2001). When soil pH decreases below a pH of 5, Al³⁺ becomes soluble in the soil and its concentration exponentially increases as soil pH decreases (Kinraide 1991). Al³⁺ rapidly inhibits root cell growth and elongation and in the long term, decreases root cell division at the root apex (Ryan et al. 1993). This therefore results in poor water and nutrient uptake, which further exacerbates the problem of decreased nutrient availability in acidic soils. Aluminium tolerant plant species release carboxylates from the roots. These carboxylates such as malate and citrate can complex and thus detoxify Al³⁺ in the rhizosphere (soil directly in contact with the roots) (Kochian et al. 2004). Malate efflux in wheat is regulated by the TaALMT1 gene which is activated in response to Al³⁺, whereas citrate efflux is regulated by the TaMATE1B gene, which has a continuous efflux and should occur regardless of soil pH or Al³⁺ (Ryan et al. 2014).

In addition to these challenges, plants are being exposed to an ever changing climate especially in regards to CO₂, water and temperature. The global atmospheric CO₂ concentration pre-industrial revolution is estimated at ~280 μmol mol^-1 and as of mid-2013 has increased to ~390 μmol mol^-1 (Houghton and Ding 2001; Bala 2013). Climate models predict that global atmospheric CO₂ will rise further to ~550 μmol mol^-1 by the middle of the 21^st century (Houghton and Ding 2001). The growth and productivity of plants are significantly affected by elevated CO₂ (eCO₂), however, the effects on root exudation and rhizosphere interactions is largely unknown (De Graaff et al. 2006; Jin et al. 2012).

This study aimed to examine the effect of eCO₂ on Al tolerance of wheat genotypes that differ in malate (Al³⁺ activated) and/or citrate (continuous) efflux.  

Methodology

A soil column experiment was conducted under ambient CO₂ (aCO₂) (~390ppm) and eCO₂ (550ppm) in a FACE (Free Air CO₂ Enrichment) system known as SoilFACE at the department of Environment and Primary Industries in Horsham, Victoria, Australia (36˚42’S, 142˚11’E). Wheat (Triticum aestivum L.) plants were grown in soil columns (60 cm long, 10cm in diameter), containing an acid Dermosol soil (pH~4.2). Six wheat genotypes differing in malate and/or citrate efflux were used: ES8 (-), Egret x Carazinho (Cz) (-), Egret x Cz (+Citrate), ET8 (+Malate), EGA-Burke x Cz (+Malate) and EGA-Burke x Cz (+Citrate & Malate). Of the six genotypes there are three isogenic pairs (differing in only a limited number of genes) which are ES8 (-) and ET8 (+Malate), Egret x Carazinho (Cz) (-) and Egret x Cz (+Citrate), EGA-Burke x Cz (+Malate) and EGA-Burkex Cz (+Citrate & Malate). Plants were harvested 108 days after sowing.

Preliminary results

Elevated CO₂ significantly increased shoot biomass of EGA-Burke x Cz (+Malate) (24%) and EGA-Burke x Cz (+Citrate & Malate) (60%) (Figure 1). In comparison, eCO₂ decreased shoot biomass of ET8 (+Malate) (-62%), also it did not affect shoot biomass of Egret x Cz (+Citrate) and Al³⁺ sensitive genotypes ES8 (-) and Egret x Cz (-) (Figure 1). There was no genotypic difference in shoot biomass between ambient and elevated CO₂ treatments of Egret x Cz (+Citrate) and the elevated treatment of ET8 (+Malate) (Figure 1). All genotypes that released either malate and/or citrate had greater shoot biomass than corresponding genotypes that did not release malate and/or citrate (Figure 1). Greater shoot biomass was observed for EGA-Burke x Cz (+Citrate & Malate) compared to EGA-Burke x Cz (+Malate) for both CO₂ treatments (Figure 1).

Figure 1. Shoot biomass of six wheat genotypes grown under ambient and elevated CO₂. Cz = Carazinho, (-) = sensitive line, (+Citrate) = citrate efflux, and (+Malate) = malate efflux. Error bars represent ± standard error (n=4).

Similar to shoot biomass, root lengths observed similar trends with eCO₂ significantly increasing the root length of EGA-Burke x Cz (+Citrate & Malate) (78%) (Figure 2). Additionally, decreases in root length were observed for ET8 (+Malate) (-62%) and no change for Al³⁺ sensitive genotypes ES8 (-) and Egret x Cz (-) (Figure 2). In comparison, eCO₂ decreased root length of Egret x Cz (+Citrate) (32%) and did not affect root length of EGA-Burke x Cz (+Malate) (Figure 2). There was no genotypic difference in root length between the ambient CO₂ treatment of Egret x Cz (+Citrate) and the elevated CO₂ treatment of ET8 (+Malate) (Figure 2). All genotypes that released either malate and/or citrate had greater root lengths than corresponding genotypes that did not release malate and/or citrate (Figure 2). Greater root length was observed for EGA-Burke x Cz (+Citrate & Malate) compared to EGA-Burke^A x Cz (+Malate) for both CO₂ treatments (Figure 2).

Figure 2. Total root length of six wheat genotypes grown under ambient and elevated CO₂. Cz = Carazinho, (-) = sensitive line, (+Citrate) = citrate efflux, and (+Malate) = malate efflux. Error bars represent ± standard error (n=4).