Signal grass is a widely used pasture cover in Australia, southeast Asia, and the New World tropics. The plant is well-suited to the acidic soils of the tropics and sub-tropics, which often have an elevated level of aluminium, making them toxic to the roots of many plants. Signal grass has a high aluminium resistance, though the mechanism of this resistance is not well understood. Furthermore, full resistance occurs only after a 24-48 hour delay, which is preceded by a phase in which the roots are sensitive to the metal.
In a recent study published in Annals of Botany, Zhigen Li and colleagues investigated the mechanism of aluminum resistance in signal grass as well as the source of the delay in its onset. The authors treated seedling roots with Al solution with or without the addition of a metabolic inhibitor that inhibits flux across the plasma membrane, indicating whether the metal is being actively transported. They then evaluated function, morphology, and Al levels in the roots.
The researchers found that Al concentration in the intracellular spaces of the root tissue increased over the first 24 hours, then began to drop, reaching only half of what it had been after 48 hours. The root elongation rate, initially stunted by the Al, also improved at this time. “We also observed that Al toxicity in signal grass was associated with the development of ruptures on the root surface, with these being due to the disintegration of root tissues,” write the authors. “This morphological symptom occurred from 24 h and did not further develop after 48 h, as no obvious new root ruptures were observed at 96 h.”
When treated with the metabolic inhibitor, however, the Al concentration increased 14-fold and there was no drop after 48 hours. “These results indicate that the delayed resistance of signal grass to Al is not due to changes in cell wall properties or the binding of Al to the cell wall, but rather that it is likely to be associated with a delayed ability of signal grass to exclude Al from the intracellular compartment,” write the authors. The exact mechanism of exclusion remains unknown, but this work provides new information regarding which possibilities should be considered.