Abstract
Toxic concentrations of aluminium cations and low phosphorus availability are the main yield-limiting factors in acidic soils, which represent half of the potentially available arable land. Brachiaria grasses, which are commonly sown as forage in the tropics because of their resilience and low demand for nutrients, show greater tolerance to high concentrations of aluminium cations (Al3+) than most other grass crops. In this work, we explored the natural variation in tolerance to Al3+ between high and low tolerant Brachiaria species and characterized their transcriptional differences during stress. We identified three QTLs (quantitative trait loci) associated with root vigour during Al3+ stress in their hybrid progeny. By integrating these results with a new Brachiaria reference genome, we identified 30 genes putatively responsible for Al3+ tolerance in Brachiaria. We observed differential expression during stress of genes involved in RNA translation, response signalling, cell wall composition, and vesicle location homologous to aluminium-induced proteins involved in limiting uptake or localizing the toxin. However, there was limited regulation of malate transporters in Brachiaria, which suggests that exudation of organic acids and other external tolerance mechanisms, common in other grasses, might not be relevant in Brachiaria. The contrasting regulation of RNA translation and response signalling suggests that response timing is critical in high Al3+-tolerant Brachiaria.
Highlights
Acidic soils constitute ~30% of the world’s total land area and 50% of the potentially available arable land (Von Uexküll and Mutert, 1995)
Root morphology in Brachiaria species in different aluminium cation concentrations We demonstrated the superior aluminium tolerance of the B. decumbens accession CIAT 606 compared with B. ruziziensis
A whole-genome assembly (WGA) of CIAT 26162 was produced using Platanus v.1.2.4 (Kajitani et al, 2014), from Illumina paired-end and Nextera MP reads with a coverage of ~100× and 7×, respectively (Supplementary Table S2)
Summary
Acidic soils constitute ~30% of the world’s total land area and 50% of the potentially available arable land (Von Uexküll and Mutert, 1995). Acidic soils are predominant in a northern ‘temperate belt’ and a southern ‘subtropical belt’. The adverse effects of soil acidity are mostly associated with several mineral toxicities and deficiencies, increased concentrations of soluble forms of manganese, iron, and aluminium, and reduced levels of available forms of phosphorus (P), calcium, magnesium, and potassium. Lower P solubility and aluminium toxicity are considered the main limiting factors on productivity (Eswaran et al, 1997; Kochian et al, 2015). As soil pH decreases below 5, aluminium becomes soluble as the aluminium trivalent cation (Al3+), a form highly toxic to plants. Soluble Al3+ effect on root apices results in diminished ion and water uptake
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