Around one-third of the world’s soils with potential crop productivity suffer from acidity, further triggered by elevated global temperatures. Mostly distributed in subtropical and tropic areas, acid soils show low crop growth and productivity due to the deficiency of essential nutrients such as P, Ca, Mg and K or the presence of toxic metals such as Al and Mn.
Among them, aluminium toxicity is considered the major limiting factor since solubilised Al ions in acid soils inhibit root elongation and subsequently the uptake of nutrients and water. Therefore, enhancing the aluminium tolerance of crops would help boost crop productivity in acid soils.
Barley – an aluminium-sensitive species
There are large variations in aluminium tolerance between different plant species. Among small grain cereal crops, rice and rye are tolerant to aluminium toxicity, whereas barley is highly sensitive. One reason for this sensitivity is that barley originates from the Fertile Crescent in the Middle East, where soils are predominantly alkaline without aluminium toxicity stress.
Yet barley is the fourth most important cereal crop for human consumption, in the brewing and distilling industry and as animal feed. That’s why it has been cultivated widely on different soils including acid ones. Increasing the aluminium tolerance of barley and subsequently its production on acid soil is thus important for food security.
HvAACT1– a major aluminium-tolerance gene in barley
Fortunately, various mechanisms for aluminium tolerance exist in different barley cultivars. Based on these variations, one major gene controlling aluminium tolerance in barley, HvAACT1, has been identified. Its gene product belongs to the Multidrug And Toxic compound Extrusion (MATE) family.
HvAACT1 is constitutively and mainly expressed in the roots and encodes a plasma membrane-localised transporter for citrate. Upon Al exposure, HvAACT1 is activated to release citrate from the root tips into the rhizosphere. Here, the acid forms a non-harmful complex with Al ions thus detoxifying the metal.
Natural variation of Al tolerance and the underlying mechanism
Further studies show that aluminium-tolerant cultivars constitutively show higher HvAACT1 expression levels in the root tips and thus more citrate secretion than aluminium-sensitive accessions. This higher expression is due to a 1-kb CACTA-like transposon insertion in Asian barley accessions and a 15.3-kb multiretrotransposon-like insertion in European barley accessions upstream of the HvAACT1 coding region.
Interestingly, although the insertions differ between Asian and European barley accessions, both insertions function as promoter-enhancing HvAACT1 expression. This indicates that barley cultivars in East Asia and Europe have developed independent but equivalent strategies to withstand aluminium toxicity in acid soils.
In European accessions, not only the insertion of the multiretrotransposon is required for HvAACT1 expression but also its demethylation. Interestingly, HvAACT1 was shown to be similarly expressed in the stele of root mature region of both aluminium-tolerant and -sensitive cultivars. Here it functions to release citrate to the xylem for the root-to-shoot translocation of iron. Therefore, the insertion alters both the gene expression level and the resulting protein localisation of HvAACT1.
Additionally, evolutionary analysis showed that insertion of the CACTA-like transposon was acquired during the expansion of barley cultivation on acidic soils in the Far-Eastern region. This is in contrast to the multiretrotransposon insertion, which was the result of the post-domestication spread of European barley into acid soil areas.
Breeding aluminium-tolerant barley cultivars through multiple backcrossing
Recently, the lab of Prof. Jian Feng Ma at the Institute of Plant Science and Resources at Okayama University developed a malting barley cultivar with enhanced aluminium tolerance. This was achieved by the introgression of a CACTA-like transposon into the elite malting cultivar Haruna Nijo.
Haruna Nijo is a two-rowed hulled and elite malting barley cultivar in Japan. It has high quality for malting and is often used in Japanese malting barley breeding programmes. However, this cultivar is very sensitive to aluminium toxicity, resulting in low production in acid soils.
By introgression of a 1-kb CACTA-like transposon into this cultivar through backcrossing with Haruna Nijo several times and marker-assisted selection, the BC4F10 line was generated. This line showed higher expression levels of HvAACT1, higher citrate secretion and less accumulation of aluminium in the root tips. When grown on acid soil, the grain yield was 2-3 times higher than the yield of the original cultivar.
Due to global warming, soil acidity is accelerated. Although liming can improve barley growth and productivity on acid soils, this practice is often not economically feasible. Furthermore, surface application of lime cannot alleviate toxic subsoil aluminium, which presents a barrier to deep rooting and the uptake of water and nutrients. Genetic improvement of aluminium tolerance is thus a sustainable way to boost barley productivity on acid soils. Similar approaches could be adapted for other crops in the future.
