*4.3. QTLs Associated with Regeneration Rate*

Candidate gene approaches have emerged as a way to merge QTL analysis with extensive data on traits involved in the regeneration rate. Plant regeneration is an important step in the success of a plant improvement program using tissue culture technology. The regeneration of plants is achieved through embryogenesis or tissue development [41]. The results of QTL analyses on plant regeneration rate were reported for several crops, such as rice [42,43], wheat [44], and barley [45–47], and a QTL analysis on plant regeneration rate was also reported in *B. oleracea* [48]. However, most of these or other plants are the results of QTL analyses involved in regeneration rates via anther culture [49,50] or tissue culture [51–54]. The QTL analysis of the regeneration rate in *Raphanus sativus* has not been reported to date, but a single marker analysis to find candidate genes involved in the e fficiency of microspore culture has recently been reported [23]. There have been many reports on experiments on heat treatment, plant growth regulators, temperature controls, and light conditions to find suitable culture conditions for microspore culture for the regeneration of plants in Brassicaceae [55–60]. In the present paper, based on a review paper published by Ikeuchi [61], the genes related to regeneration were identified as candidate genes. The genes Rs426380 and Rs426400 have the same function as AT4G32540, which is known to function as the flavin-binding monooxygenase family protein and is known to act as an enzyme in auxin biosynthesis [62]. It also plays a key role in *de novo* root formation [63,64]. Auxin biosynthesis makes a contribution to root regeneration from leaf explants as well, because taking root is repressed by the chemical inhibition of auxin biosynthesis or YUC1, YUC2, YUC4, and YUC6 quadruple mutants that are faulty in auxin production [65]. The Rs465100 gene has the same function as AT5G51230, which is known as the PRC2 subunit of VEFS-Box of polycomb protein. In PRC2 mutants, these PRC2-targeted genes are ectopically expressed, resulting in spontaneous somatic dedi fferentiation, callus formation, and embryonic development [66]. The genes Rs479580 and Rs479680 have the same functions as Arabidopsis's AT4G02020, which is known as a SET domain-containing protein, also known as the PRC2 subunit. PRC2, one of the polycomb-group proteins, has histone methyltransferase activity and primarily trimethylates histone H3 on lysine 27 (H3K27me3). In Arabidopsis, PRC2 is known to inhibit the process of allowing embryos to mature during plant development [67].
