Thidiazuron Promoted Microspore Embryogenesis and Plant Regeneration in Curly Kale (Brassica oleracea L. convar. acephala var. sabellica)

Curly kale (Brassica oleracea L. convar. acephala var. sabellica), the most common type of edible kale, characterized by providing rich nutrition and health care functions, is sought after and has been listed as top of the healthiest vegetables in recent trends, and has aroused the interest of breeders in cultivating new varieties. However, it usually takes more than six years to obtain a homozygous kale inbred line for commercial seed production through conventional breeding procedures due to its long growth and development period. The isolated microspore culture (IMC) technique could be a time-saving alternative method for producing doubled haploid (DH) lines that are genetically homozygous. In this study, we successfully utilize the efficient cytokinin thidiazuron (TDZ) to promote microspore embryogenesis and plant regeneration in two curly kale cultivars (‘Winterbor F2’ and ‘Starbor F2’). Compared with the control (0 mg/L TDZ), all tested TDZ concentrations (0.1, 0.2, 0.3, 0.4 mg/L) had no adverse effects on embryogenesis, and 0.2 mg/L TDZ had an optimal effect on embryo survival and plant regeneration of the two genotypes. For ‘Starbor F2’, 0.2 mg/L TDZ treatment achieved the highest embryogenesis rate (1.83-fold higher than the control group) and direct seeding rate (1.61-fold increase), and the lowest mortality rate. Likewise, 0.2 mg/L TDZ increased the embryogenesis rate of ‘Winterbor F2’ by 1.62 times, the direct seeding rate by 1.61 times, and the mortality rate fell to the lowest. A 1/2 Murashige and Skoog (MS) medium with 0.2 mg/L 1-Naphthaleneacetic acid (NAA) can significantly promote the rooting of the regenerated seedlings. These results provide new insights into the practical application of the IMC technique in shortening the breeding cycle of kale.