Shoot and root proliferation in 'Williams' banana: are the topolins better cytokinins?

The effects of five topolins (meta-Topolin = mT; meta-Topolin riboside = mTR; meta-Methoxy topolin = MemT; meta-Methoxy topolin riboside = MemTR and meta-Methoxy topolin 9-tetrahydropyran-2-yl = MemTTHP) on shoot and root regeneration of micropropagated 'Williams' bananas were compared to benzyladenine (BA). Sterile shoot-tip explants were cultured on modified Murashige and Skoog (MS) media containing 10, 20 or 30 mu M of the above cytokinins (CKs) for 42 days. Using shoot regenerated from optimum CK concentration, rooting experiments involving the use of indole-3-acetic acid, indole-3-butyric acid, naphthalene acetic acid, smoke-water (SW) and karrikinolide (KAR(1)) were conducted. Apart from 10 mu M BA and 30 mu M MemTTHP treatments, the number of shoots produced in all the CK treatments were significantly higher than the control. In addition, 30 mu M mT resulted in the highest number of shoots (7.3 +/- A 1.0). Unlike other CK treatments requiring higher concentrations, optimum mean shoot number per explant rate was attained at the lowest concentration in MemT and MemTTHP (10 mu M) treatments. The use of 10 mu M MemTTHP had the least root inhibitory effect during the shoot proliferation phase. In terms of abnormality index, mTR-regenerated plantlets were of the best quality across all the CKs tested. In mT- and BA-derived shoots, SW and KAR(1) significantly increased the number and length of roots compared to the control. Overall, when compared to BA, the use of topolin demonstrated higher mean shoot number per explant (MemT and MemTTHP) at lowest CK concentrations and the ease of rooting during the shoot proliferation phase (MemTTHP). During the rooting phase, topolin treatments produced more off-shoots than BA-treated ones. The current finding is a further demonstration of the increasing importance of topolins in micropropagation.