LAUSR

The mechanisms of extreme Al-resistance in Urochloa decumbens are not established. Full resistance expression requires a lag time of 72-96h and is preceded by a sensitive phase (24-48h) with Al-induced root growth inhibition. The aim here was to identify key processes of the activation phase of Al-resistance analysing both root exudates and comparative root proteome. Samples were taken after 0, 24 and 96h exposure to 0 or 200μM Al. Al-induced stimulation of citrate and oxalate efflux was limited to the sensitive phase. Only 11 proteins revealed Al-induced abundance differences; six were identified. After 24h, phenylalanine ammonium lyase (PAL), methionine synthase (MS), and deoxymugineic acid synthase (DMAS) decreased, while acid phosphatase (APase) abundance increased. Coincident with growth recovering, PAL and MS, but not DMAS, returned to initial levels. After 96h, γ‑carbonic anhydrase (γ‑CA) and adenylate kinase (AK) along with two unidentified proteins were more abundant. In conclusion, few protein changes characterize the initial response to Al in signalgrass. During the alarm phase, changes are related to P-mobilization, downregulation of Fe-acquisition, reduction of phenolic biosynthesis, and small stimulation of organic acid exudation. After recovering (resistant phase), biosynthesis of phenolics and methionine, but not Fe-mobilization are re-established. Full expression of Al-resistance is characterized by enhanced γ‑CA mediating mitochondrial complex I assembly and increased AK abundance indicating higher root respiration and better provision of ADP and Mg2+ to ATP synthase, respectively. The unidentified proteins and the specific role of γ‑CA in Al resistance of U. decumbens will centre future research.