Agrobacterium rhizogenes-Mediated Transformation for Generation of Composite Sugar Beet with Transgenic Adventitious Roots

Sugar beet (<i>Beta vulgaris</i> L.), a biennial sugar crop, provides about 16% of the world's sucrose production. PEG and <i>Agrobacterium tumefaciens</i>-mediated transformation have been established for sugar beet. However, the traditional transformation of sugar beet is time-consuming, low efficiency, and dependent on tissue regeneration. Recently, the use of <i>Agrobacterium rhizogenes</i> for genetic transformation without tissue culture has become a new possibility. Here, we describe an optimized <i>A. rhizogenes</i>-mediated transformation for the generation of composite sugar beet without tissue culture. By dipping <i>A. rhizogenes</i> K599 colonies onto a wound of hypocotyl and petiole, about 81.7% and 51.1% of shoots and leaves could be induced to produce adventitious roots. Of these, more than 60% of the explants contained transformed adventitious roots. Specifically, we discovered that the transformation efficiency was significantly improved when the <i>MAS</i> promoter was employed instead of the <i>CaMV35S</i> promoter. The transformation in adventitious roots was also validated by qRT-PCR and Western blot at the transcriptional and translational levels. The transformed adventitious roots have great potential for the study of taproot development, sugar accumulation, and resistance to root diseases, which is closely related to sugar beet yield and quality.