Expansion protein DcEXP22 regulates taproot enlargement via mediating root cell extension in carrot

Carrot (Daucus carota L.) is an important root vegetable crop of the Daucus genus in the Apiaceae. As the main product organ of carrot, the taproot has high nutritional and economic value. Expansins, a class of proteins involved in plant cell wall relaxation and cell extension, are mainly found in growing tissues and organs. Expansins play an important role in plant root development. Here, the DcEXP22 gene with a length of 789 bp was cloned from the carrot cultivar 'Kurodagosun'. Based on the stable genetic transformation system and CRISPR/Cas9 gene-editing technology, the DcEXP22 gene was overexpressed and knocked out in carrots. The results indicated that overexpression of the DcEXP22 gene increased carrot root fresh weight, root diameter, and root-shoot ratio, and enlarged the perimeter and area of taproot phloem cells. In contrast, knockout of the DcEXP22 gene inhibited the development of carrot taproots and the extension of phloem cells, suggesting that the DcEXP22 gene might promote the enlargement of carrot taproots by regulating the size of phloem cells. RNA-seq analysis identified several genes that were co-expressed with DcEXP22, including DcCYP734A1, DcERF1, DcMAP2K1, and DcSAD9. It was hypothesized that the DcEXP22 gene might influence the enlargement of carrot taproot by participating in the signal transduction of phytohormones such as brassinosteroids, cell wall synthesis and modification, and fatty acid metabolisms. These findings will advance our knowledge of the molecular mechanisms of carrot taproot enlargement.