Explant response and its implication in the TDZ (Thidiazuron)- based regeneration system for eggplant (Solanum melongena L.)

Thidiazuron (TDZ), a very potent auxin, is used for eggplant organogenesis in vitro. In this study, the authors determined the response of cotyledon and hypocotyl of eggplant, cv. Dumaguete Long Purple (DLP) in TDZ-based medium. The eggplants from 11-day old aseptically grown seedlings were cultured in induction medium containing MS salts, 0.44 mg/L, 1000 mg/L myo-inositol, vitamins 30g sucrose, adjusted to pH 5.8 and solidified with combined agar and phytagel at 3 and 2 g/L, respectively. The two ends of the cotyledons were excised leaving 5 mm sections, while the hypocotyls were cut into four 5 mm sections and categorizedn as apical (H1), segment proximal to the cotyledonary node; sub-apical (H2), distal to cotyledonary node; sub-basal (H3), distal to root region: basal (H4), proximal to root region. Within 9 days, the cotyledons enlarged up to three times their original size, and in 17 days, shoot initials formed in cut ends. Shoot initials were obtained in 8.7% (9/103) of the cotyledon, while 91.3% turned brown when sub-cultured in ® MS hormone-free medium (HFM). Only one elongated shoot was obtained in 21 days at TDZ-medium, which did not form roots and eventually died. Shoot initials did not elongate even after two sub-cultures in HFM. In vitro response varied with hypocotyl segment. Within 21 days, polar response was observed. Shoot initials formed in proximal ends (relative to the cotyledon), while callus formed in distal ends of H1, H2 and H3 segments. The frequency of bipolar response was in the order H1 (56.9%) greater H3 (35.5%) greater H2 (31.8%) greater H4 (4.0%). Enlargement accompanied by tissue rupturing was observed in 90% of the H4 segments, 51.8% in H2, 50.7% in H3, and 22.2% in H1. If sub-cultured in appropriate medium, these raptured hypocotyls may yield embryogenic tissues. Shoot elongation was obtained when bipolar hypocotyls were sub-cultured twice, at 21days interval, in HFM. The frequency was in the order of 29.6% (24/81) for H2; 18.1% (17/94) for H1; 17.9% (15/84) for H3; and nil for H4. More shoots were regenerated in bipolar hypocotyls with more growth in shoot initials compared wit callus growth. H1 and H2 segments are the best choice for optimum organogenic response using hypocotyls. Very few regenerants formed roots. The regenerated shoots were hardened on water for two days under light, and in soil mixture for at least one week outside culture room, before keeping them in the green house.