Aluminium-induced changes in root epidermal cell patterning, a distinctive feature of hyperresistance to Al in Brachiaria decumbens

Brachiaria, a genus of forage grasses of African origin, is gaining considerable importance because of both its nutritional value and its high stress resistance. An extraordinary resistance to Al toxicity has been reported in B. decumbens. The mechanisms of this hyperresistance are still unknown. This study explores the localization of Al in two contrasting Brachiaria species, the hyperresistant B. decumbens and the less resistant B. brizantha. Scanning Electron Microscope/Energy Dispersive Spectrometry, confocal fluorescence microscopy and optical microscopy of lumogallion or morin-stained roots was performed. Species differences in Al resistance were evident at 32 μM Al3+ activity in low ionic strength full nutrient solution containing Si. Roots of B. decumbens accumulated less Al than those of B. brizantha. Moreover, location and Al form seemed different. In B. decumbens Al accumulation was localized in hot spots of high Al concentrations. These sites with high Al accumulation mainly correspond to root hairs. B. brizantha exhibited a more even distribution of Al in cell walls of the root tip. Analysis of soluble phenolic substances in roots revealed species differences in response to Al. An Al-induced increase of chlorogenic acid concentrations was found in B. decumbens but not in B. brizantha. Taken together the results suggest a possible role for chlorogenic acid as a primer for changes in root epidermal cell patterning that may contribute to the Al hyperresistance in B. decumbens.

Graphical abstractAluminium hyperresistance in Brachiaria decumbens is related to Al-induced changes in epidermal cell patterning and confinement of Al in root hairs. In the less resistant B. brizantha a more even distribution of morin-stainable Al is observed. Presence of a multiseriate exodermis may further contribute to restriction of apoplastic Al transport.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Higher aluminium tolerance in Brachiaria decumbens than in B. brizantha. ► Hypertolerance to Al in B. decumbens requires a lag time of several hours to be fully expressed. ► Al-induced root hair development close to the tip indicates change in root epidermal cell patterning in B. decumbens. ► Morin staining revealed hot spots of Al concentrations in root hairs of B. decumbens.