Gynogenesis Induction in Sugar Beet (Beta vulgaris) Improved by 6-Benzylaminopurine (BAP) and Synergized with Cold Pretreatment

For sugar beet (Beta vulgaris) breeding, producing homozygous lines through haploid and doubled haploid techniques are preferred over conventional and time-consuming methods. Doubled haploid sugar beet production necessitates inducing ovules to develop into haploid plants, referred to as gynogenesis. The protocol involves an interaction between cold pretreatment of six genotypes of sugar beet inflorescences at 4 °C for 1 week or more and 6-benzylaminopurine (BAP) concentrations (1 or 2 mg L⁻¹) to increase the response rate of haploid embryo induction. Compared with freshly cultured ovules (6.49%), cold pretreatment for 1 week almost doubled the mean of haploid plantlet induction rate (11.3%), whereas pretreatment for more than 1 week was not as effective as the control. Addition of 2 mg L⁻¹ BAP to the culture medium nearly doubled the induction rate of the cultured ovules (10.75%), followed by 1 mg L⁻¹ BAP (7.78%) in comparison with hormone-free medium (5.69%). The highest gynogenesis rate (37.8%) was achieved when ovules were cultured on medium containing 2 mg L⁻¹ BAP following 1-week cold pretreatment. This combination approximately tripled the mean total haploid embryo induction rate of all the genotypes to 16.3% in comparison with the control (5.74%). However, the addition of BAP resulted in vitrification proportionately. As a result, 2 mg L⁻¹ BAP decreased the normal plantlet emergence (NPE) to one-third (7.59%) while 1 mg L⁻¹ BAP had a moderate effect (NPE: 18.98%) in comparison with hormone-free treatment (NPE: 24.35%). The results indicate that the combination of cold pretreatment and BAP is very effective in inducing haploid plants from recalcitrant genotypes of sugar beet, but BAP can have both advantages and disadvantages.