Genome doubling confers distinct root architecture and salt tolerance in Citrus and Poncirus by activating ethylene biosynthesis. Citrus

Almost all flowering plants have undergone one or more rounds of polyploidy during evolution. Once undergone polyploidy, how to adapt to the environment and maintain growth vitality is vital for plants in the early stage of polyploidy; however, little is known regarding its mechanisms. Here, we found three autotetraploids of different genotypes in genera Citrus and Poncirus have distinct root architecture and enhanced salt tolerance relative to the diploid parents. The autotetraploid root system confers greater salt tolerance to the scion relative to the diploid. Surprisingly, an ethylene biosynthesis gene encoding 1-aminocyclopropane-1-carboxylate oxidase (ACO1) is evaluated in the roots of all the autotetraploids by transcriptome analysis. Correspondingly, the autotetraploid roots accumulate more ethylene. Salt stress tolerance in Citrus and Poncirus is enhanced by the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC), but is decreased by treatment with the ethylene inhibitor aminoethoxyvinylglycine (AVG). Furthermore, increasing expression of ACO1 promotes ethylene production and mimics the root architecture and enhanced salt tolerance observed in the autotetraploids, whereas the deletion of ACO1 leads to decreased salt tolerance. Increasing ethylene production activates the biosynthesis of phenylpropanoid and flavonoid, which can act as ROS scavenging antioxidants. In addition, DNA methylation of ACO1 gene body is decreased in all the autotetraploids. DREB2c as a core salt-responsive factor binds to and activates ACO1 to promote ethylene production under salt stress. Collectively, these results indicate that genome doubling confers distinct root architecture and salt tolerance by activating ethylene biosynthesis.