Current Opinion in Plant Biology -► Plants derive from ancestors of 5/7 protochromosomes with 10000 protogenes and 50 MB in size. ► Modern diploid plant species are paleopolyploids that went through a diploidization process. ► At the genome level, the diploidization consists in ancestral chromosome fusions/fissions. ► At the gene level, the diploidization consists in deletion, neo- or sub-functionalization. ► The diploidization process follows the gene dosage balance and sub-genome bias rules.