Abstract
The major limitation of cereal production in acidic soils is aluminium (Al) phytotoxicity which inhibits root growth. Recent evidence indicates that different genotypes within the same species have evolved different mechanisms to cope with this stress. With these facts in mind, root responses of two highly Al tolerant Portuguese bread wheat genotypes—Barbela 7/72/92 and Viloso mole—were investigated along with check genotype Anahuac (Al sensitive), using different physiological and histochemical assays. All the assays confirmed that Barbela 7/72/92 is much more tolerant to Al phytotoxicity than Viloso Mole. Our results demonstrate that the greater tolerance to Al phytotoxicity in Barbela 7/72/92 than in Viloso Mole relies on numerous factors, including higher levels of organic acid (OAs) efflux, particularly citrate efflux. This might be associated with the lower accumulation of Al in the root tips, restricting the Al-induced lipid peroxidation and the consequent plasma membrane integrity loss, thus allowing better root regrowth under Al stress conditions. Furthermore, the presence of root hairs in Barbela 7/72/92 might also help to circumvent Al toxicity by facilitating a more efficient uptake of water and nutrients, particularly under Al stress on acid soils. In conclusion, our findings confirmed that Portuguese bread wheat genotype Barbela 7/72/92 represents an alternative source of Al tolerance in bread wheat and could potentially be used to improve the wheat productivity in acidic soils.
Highlights
Aluminium (Al) phytotoxicity has been recognized as one of the most detrimental factors for plant growth and productivity under acidic soil conditions which forms a large portion of the earth’s land area [1]
The results of the present study suggest that the Al tolerance of Barbela 7/72/92 is clearly outstanding compared to Viloso Mole, probably due to the difference in both level as well as kind of organic acids (OAs) secreted by its roots (Figures 3 and 4)
We analyzed the promoter of TaALMT1, the gene controlling malate efflux in bread wheat [39], in the selected Portuguese bread wheat genotypes. We found that both Al tolerant genotypes—Barbela 7/72/92 and Viloso Mole—contain the type VI promoter which is associated with the higher basal expression of TaALMT1 transcript in wheat, whereas the genotype Anahuac with the type I promoter showed the lowest level of expression and Al tolerance [35,40]
Summary
Aluminium (Al) phytotoxicity has been recognized as one of the most detrimental factors for plant growth and productivity under acidic soil conditions which forms a large portion of the earth’s land area [1]. In both monocots and eudicots, the exclusion of Al through the efflux of organic acid anions from root apices has been demonstrated to be a main mechanism to overcome Al stress [4,5]. Malate exudation is considered as the main mechanism of Al tolerance in bread wheat, whereas in other cereals such as rice, barley, oat, maize and sorghum, citrate exudation is mainly responsible for their Al tolerance. Some cereals such as rye and triticale, secrete both malate and citrate to cope with Al phytotoxicity [4,5]. The maintenance of longer root hairs is highly important for the efficient uptake of water and nutrients, including effective phosphorus acquisition in acid soils [8]
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