| In vitro culture is an important approach for genetic improvement. However, genetic improvement by in vitro culture in mustard (Brassica juncea Coss.), especially in Chinese mustard, is restricted by the poor frequency of shoot regeneration, and no regeneration system is established yet. The main target of this study is to obtain a high-frequency regeneration system in various mustard varieties, which is the first step towards genetic improvement.In the present studies, cotyledon and hypocotyl explants of five varieties, including tillering mustard (B. juncea Coss. var. gemmifera Lee et Lin cv. Jia-xing Xue-li-hong), stem mustard (B. juncea Coss. var. crassicaulis Chen et Yang cv. Zhong-dou Bang-cai), brussels mustard (B. juncea Coss. var. multiceps Tsen et Lee cv. Zhong-dou No.l) and tuber mustard (B. juncea Coss. var. tumida Tsen et Lee cv. CMS of Zhe-tong No. 1 and its maintainer line), were cultured in various media. The results are showed as followings:The frequency of shoot regeneration from cotyledon explants was significantly affected by the genotype and concentration of plant growth regulators (PGR). Cotyledon of all cultivars cultured on MS medium without PGR exhibited no shoot. While cultured in the MS medium containing 6-benzyladenine (BA) 1.0?.0mg/L and oc-naphthalenecetic acetic acid (NAA) 0?l.Omg/L, the regeneration frequency of tillering mustard, stem mustard, brussels mustard and tuber mustard was 6.9?5.2%, 0?7.5%, 3.8?7.76% and 0?0.0%, with the average regeneration frequency of 23.04%, 21.31%, 24.58% and 21.26% respectively. To obtain the highest frequency of shoot regeneration, the most suitable medium was the MS medium containing BA 3.0mg/L and NAA 0.5mg/L, though the frequency of shoot regeneration varied with cultivars.The shoot regeneration was also obviously affected by the types of explants. The result showed that cotyledons of CMS and its maintainer line of tuber mustard had higher shoot regeneration frequency than that of hypocotyls. The average frequency of shoot regeneration in the CMS line was 30.0% and 2.34% respectively and that in maintainer line was 30.0% and 3.54% respectively. On the other hand, explants of different sections of cotyledon resulted in different shoot regeneration abilities. Cotyledon without petiole showed a significantly lower frequency of shoot regeneration (16.7%) compared with that of entire cotyledon (33.8%), but the averagenumber of shoots per explant (4.2) was more than that of entire cotyledon (2.5). No shoot was obtained when the petiole alone was cultured. The frequency of shoot regeneration (8.3%, 12.5%) and average number of shoots per explant (2.3, 1.7) from the upper section of lamina or one side of lamina was lower than those of entire cotyledon.In addition, no obvious effect of medium volumes on shoot regeneration was found in the present study. But nursing culture, i.e. regenerated shoots used as nursing materials, not only advanced the day of the shoot formation for 5-6days, but increased the frequency of shoot regeneration as well.The supplementation of the medium with 5-30mg/L AgNOa significantly enhanced the shoot regeneration. Both the frequency and the average number of shoots regeneration in the treatment with 15.0mg/L AgNOa were significantly higher than that of the other five treatments including the control. The supplementation of the medium with AgNOs decreased the ethylene production significantly during in vitro culture. Statistical analysis revealed that a significantly negative correlation existed between the ethylene production and the frequency or the average number of shoot regeneration.It is easy to induce callus derived from hypocotyls and cotyledons, but callus derived from root tip was rather difficult to be done. The most suitable medium for callus induction was the MS medium containing BA 0.5 mg/L and l.Omg/L NAA. The frequency of shoot regeneration from callus derived from cotyledons was higher than that from hypocotyls. No shoot was obtained from the callus derived from root tip.
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