Genetic Stability of Cassava Plants Regenerated Through Organogenesis Using Microsatellite Markers

Abstract

Tissue culture technology of cassava (Manihot esculenta Crantz) is a viable alternative to currently adopted techniques for mass propagation, germplasm conservation and genetic improvement. However, somaclonal variation is a common phenomenon in tissue culture which makes it mandatory to monitor the genetic stability of plants. Therefore, the objective of this study was to evaluate the genetic stability of cassava plants regenerated from axillary bud explants through direct organogenesis using simple sequence repeat (SSR) markers. High shoot regeneration (81.2 – 90.0%) occurred in MS medium supplemented with 10 mg/L 6-bnzylaminopurine (BAP) and multiple shoots (2 – 4 shoots per from axillary bud explant) were formed for all the three cultivars (TME14, TMS60444 and Kibandameno) tested. High frequency of rooting (100%) was obtained after transferring the plantlets to cassava basic medium (CBM) and the rooted plants were successfully hardened and acclimatized in the glasshouse with 100% survival rate. Three-month old plants exhibited normal morphological characters comparing with the mother plant. A total of 10 SSR markers were used to validate the genetic homogeneity amongst five randomly selected plants along with the donor mother plants. DNA fingerprints of axillary bud regenerated plants displayed monomorphic bands similar to mother plant, indicating homogeneity among the regenerated plants with donor mother plant. The effect of subculture frequency on genetic stability of axillary bud-derived regenerants and micropropagated plants was also assessed using SSR markers. All the SSR profiles from axillary bud regenerants and micropropagated plants were monomorphic and comparable to mother plants from 1st to 5th subculture, confirming the genetic stability among clones and mother plants. At the 6th subculture, similarity indicators between the progenies and the mother plants ranged from 0.95 to 1.0 and such a similarity indicated a very low polymorphism. The dendrograms generated through Unweighted Pair Group Method with arithmetic mean (UPGMA) analysis of the 6th subculture revealed 96% similarity amongst axillary bud regenerants and micropropagated plants with donor mother plant. This low polymorphism ratio between mother plants, axillary bud regenerants and micro-propagated plants indicates the little effect of somaclonal variations, the high genetic similarity between mother plants and progenies and demonstrates the reliability of this propagation system for cassava. These results suggest that direct organogenesis from the axillary buds is the safest method for regeneration of true-to-type plants and this system can be used for clonal mass propagation, germplasm conservation and genetic transformation of cassava.