Root Adaptive Responses to Aluminum-Treatment Revealed by RNA-Seq in Two Citrus Species With Different Aluminum-Tolerance.

Seedlings of aluminum (Al)-tolerant and Al-intolerant were fertigated daily with nutrient solution containing 0 and 1.0 mM AlCl●6HO for 18 weeks. The Al-induced decreases of biomass and root total soluble proteins only occurred in , demonstrating that had higher Al-tolerance than . Under Al-treatment, roots secreted more citrate and malate than ones; less Al was accumulated in than in leaves. The Al-induced reduction of phosphorus was lesser in roots and leaves than in ones, whereas the Al-induced increase of sulfur was greater in roots and leaves. Using RNA-seq, we isolated 1905 and 2670 differentially expressed genes (DEGs) from Al-treated than roots, respectively. Among these DEGs, only 649 DEGs were shared by the two species. Further analysis suggested that the following several aspects conferred higher Al-tolerance: (a) Al-treated seedlings had a higher external Al detoxification capacity enhanced Al-induced secretion of organic acid anions, a higher antioxidant capacity and a more efficient chelation system in roots; (b) Al-treated seedlings displayed a higher level of sulfur in roots and leaves possibly due to increased uptake and decreased export of sulfur and a higher capacity to maintain the cellular phosphorus homeostasis by enhancing phosphorus acquisition and utilization; (c) Cell wall and cytoskeleton metabolism, energy and carbohydrate metabolism and signal transduction displayed higher adaptative responses to Al in than in roots; (d) More upregulated than downregulated genes related to fatty acid and amino acid metabolisms were isolated from Al-treated roots, but the reverse was the case for Al-treated roots. These results provide a platform for further investigating the roles of genes possibly responsible for citrus Al-tolerance.