A new genome allows the identification of genes associated with natural variation in aluminium tolerance in Brachiaria grasses

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 aluminiuminduced
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.