Metabolomic analyses of highbush blueberry (Vaccinium corymbosum L.) cultivars revealed mechanisms of resistance to aluminum toxicity

Aluminum (Al) is an important factor that limits plant growth under acidic soil conditions. However, several plant species developed distinct mechanisms that limit the damage caused by high Al concentrations. In highbush blueberry (Vaccinium corymbosum), the Al resistance mechanisms are not fully understood. This study was designed to evaluate the effect of Al toxicity on roots and leaves of highbush blueberry genotypes with contrasting Al resistance [Star (Al-sensitive) and Camellia and Cargo (Al-resistant)] and identify the main molecular and physiological strategies underpinning adaptive Al stress responses in nutrient solution. After 48 h of Al treatment, the reduced form of ascorbate (ASC) was higher in roots, but unchanged in leaves of Cargo and Camellia genotypes compared to the control. We also observed decreased root exudation of oxalate in the Al-treated sensitive cultivar Star throughout the treatment period. However, in the resistant cultivar (Camellia), the exudation of oxalate increased 2.4- and 2.8-fold at 24 and 48 h, respectively. Al treatment differentially affected the enzyme activity and gene expression of the tricarboxylic acid (TCA) cycle enzymes. NAD-dependent malate dehydrogenase (NAD-MDH) expression in roots of cultivar Cargo was reduced at 24 h and increased at 48 h, whereas in leaves the expression was higher at 24 h and decreased at 48 h compared to the control. Citrate synthase (CS) activity in Al-resistant Cargo roots diminished at 24 h, increasing afterwards, without variation in the CS gene expression, compared with the initial time point (t = 0). In Al-resistant Camellia roots, the gene expression and the activity of CS decreased during Al exposure. NADP-dependent malate dehydrogenase (NADP-MDH) activity showed increased activity and gene expression at 24 h, in the leaves of cultivar Cargo, whereas in roots the gene expression decreased, but the activation state of NADP-MDH increased. The expression of genes encoding TCA cycle enzymes did not differ significantly in the Al-sensitive cultivar Star during Al exposure. In conclusion, the exudation of organic acid anions, particularly oxalate, plays an important role in Al resistance of highbush blueberry genotypes whilst elevated levels of ASC in roots, also contribute to the Al-resistance mechanisms exhibited by genotypes Camellia and Cargo.This work was supported by FONDECYT Initiation N° 11160355 and FONDECYT Regular N° 1171286 projects (FONDECYT, Chile). Research fellowships granted by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil) to R.P.O.G., J.R.V. and A.N.N. are also gratefully acknowledged.Peer reviewe