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

Abstract

Seedlings of aluminum (Al)-tolerant <i>Citrus sinensis</i> and Al-intolerant <i>Citrus grandis</i> were fertigated daily with nutrient solution containing 0 and 1.0 mM AlCl₃●6H₂O for 18 weeks. The Al-induced decreases of biomass and root total soluble proteins only occurred in <i>C. grandis</i>, demonstrating that <i>C. sinensis</i> had higher Al-tolerance than <i>C. grandis</i>. Under Al-treatment, <i>C. sinensis</i> roots secreted more citrate and malate than <i>C. grandis</i> ones; less Al was accumulated in <i>C. sinenis</i> than in <i>C. grandis</i> leaves. The Al-induced reduction of phosphorus was lesser in <i>C. sinensis</i> roots and leaves than in <i>C. grandis</i> ones, whereas the Al-induced increase of sulfur was greater in <i>C. sinensis</i> roots and leaves. Using RNA-seq, we isolated 1905 and 2670 differentially expressed genes (DEGs) from Al-treated <i>C. sinensis</i> than <i>C. grandis</i> roots, respectively. Among these DEGs, only 649 DEGs were shared by the two species. Further analysis suggested that the following several aspects conferred <i>C. sinensis</i> higher Al-tolerance: (a) Al-treated <i>C. sinensis</i> seedlings had a higher external Al detoxification capacity <i>via</i> enhanced Al-induced secretion of organic acid anions, a higher antioxidant capacity and a more efficient chelation system in roots; (b) Al-treated <i>C. sinensis</i> 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 <i>C. sinensis</i> than in <i>C. grandis</i> roots; (d) More upregulated than downregulated genes related to fatty acid and amino acid metabolisms were isolated from Al-treated <i>C. sinensis</i> roots, but the reverse was the case for Al-treated <i>C. grandis</i> roots. These results provide a platform for further investigating the roles of genes possibly responsible for citrus Al-tolerance.