Transgene-free genome editing in poplar

Precise gene-editing methods are valuable tools to enhance genetic traits. Gene editing is commonly achieved via stable integration of a gene-editing cassette in the plant's genome. However, this technique is unfavorable for field applications, especially in vegetatively propagated plants, such as many commercial tree species, where the gene-editing cassette cannot be segregated away without breaking the genetic constitution of the elite variety. Here, we describe an efficient method for generating gene-edited Populus tremula × P. alba (poplar) trees without incorporating foreign DNA into its genome. Using Agrobacterium tumefaciens, we expressed a base-editing construct targeting CCoAOMT1 along with the ALS genes for positive selection on a chlorsulfuron-containing medium. About 50% of the regenerated shoots were derived from transient transformation and were free of T-DNA. Overall, 7% of the chlorsulfuron-resistant shoots were T-DNA free, edited in the CCoAOMT1 gene and nonchimeric. Long-read whole-genome sequencing confirmed the absence of any foreign DNA in the tested gene-edited lines. Additionally, we evaluated the CodA gene as a negative selection marker to eliminate lines that stably incorporated the T-DNA into their genome. Although the latter negative selection is not essential for selecting transgene-free, gene-edited Populus tremula × P. alba shoots, it may prove valuable for other genotypes or varieties.