Genetic and epigenetic aspects of somaclonal variation: flower colour bud sports in azalea, a case study

Bud sporting is the consequence of sudden variations in gene expression of somatic cells, leading to the occurrence of phenotypically altered shoots on plants. This phenomenon can be observed in most vegetatively propagated plant species and finds its origin in genetic as well as epigenetic events. In azalea the frequent occurrence of flower colour sports is appreciated as a valuable additional source of variation; more than half of the commercial assortment consists of flower colour bud sports. However, when clonal uniformity is desired for registration (Distinctness Uniformity Stability, DUS) and mass propagation, this somaclonal variation is perceived as a hindrance. Insight into the molecular mechanism(s) underlying the induction of bud sporting might improve breeding strategies, towards the appropriate choice of cultivars in hybridisation experiments. The complex beauty of sport phenotypes clearly indicates interactions within and between genetic and epigenetic mechanisms. Therefore, using azalea somaclonal variation as a model, a review of the literature on various genetic and epigenetic phenomena that regulate or influence gene expression, as well as on the links that exist between them is presented.