Plant growth regulators and somaclonal variation in Cavendish banana (Musa AAA cv. Zelig)

Abstract

Cavendish bananas are the most important sub-group of all bananas. They includes more than 30% of the global banana production and almost all bananas exported are of the Cavendish type. This sub-group is also an important food source. Most of the fruit is consumed locally, such that only 35% enters the international market. To meet the regular demand for domestic consumption and market supply there must be a reliable production strategy. The technique of tissue culture is a better option than conventional propagation techniques. However. the high incidence of somaclonal variation among plants derived from tissue culture is a problem for commercial producers. Several factors such as genotype, tissue source, duration in culture, and the tissue culture technique employed, cause somaclonal variation. The impact of plant growth regulators on somaclonal variation was studied on Cavendish banana cv. 'Zelig' obtained from African Biotechnologies Ltd., South Africa. In vitro grown plants at the 4th multiplication cycle were supplied for the investigation. The first component of the investigation dealt with the effect of types of plant growth regulators. Combinations of the auxins IAA, IBA and NAA with the cytokinins BA and TDZ were used to culture the plants for ten multiplication cycles. Plants were then randomly selected to collect leaf material for DNA extraction and RAPD analysis. The second aim of this study was to investigate the effect of the cytokinin BA on plantlets subcultured over 5-10 multiplication cycles. The auxin IAA at the concentration of 2 mgl-1 was combined with BA at concentrations of 2.5, 5.0 and 7.5 mgl-1. Plants were analyzed at each level of subculture from the 5th to the 10th cycle for respective cytokinin concentrations. Plants were then randomly selected for the collection of leaf material for DNA extraction and RAPD analysis. DNA was extracted from leaf tissue of in vitro grown plants using a modified CTAB extraction procedure. DNA amplification products were scored for the presence and absence of bands in a particular locus. Results were clustered according to their similarities. The relationship between multiplication rate and variation was assessed using correlation analysis. Results of the investigation showed that treatments with higher multiplication rates produced higher rates of variation. A variation rate of 55% was recorded for treatments containing IAA and BA. A higher rate of variation (72%) was identified in the treatment with IAA (2mgl-1) plus BA (7.5 mgl-1) over 10 cycles. In all cases the dwarf off-type was the most common type of variant obtained, contributing 87.7% of the total amount of variation. The dwarf specific marker (OPJ-041500) reported previously in Williams Cavendish was identified in cv. 'Zelig' in this study. Another band similar in size was amplified by primer OPC-15 and named OPC-151500 . This band consistently appeared in all the normal plants and was absent in all the dwarf types and hence could be used as a dwarf specific marker.