Regeneration of fertile plants from Helianthus nuttallii T&G and Helianthus giganteus L. mesophyll protoplasts.

Interspecific hybridisation in the genus Helianthus via somatic cell fusion is thought to play an important role in future sunflower breeding programs. The establishment of this technique requires, however, the development of single-cell-regeneration protocols. For this purpose, we applied a regeneration protocol recently developed for Helianthus annuus L. to mesophyll protoplasts of two wild sunflowers (H. nuttallii T&G, H. giganteus L). Protoplasts of both species were embedded in agarose droplets and covered by liquid mKM medium. After 4-5 weeks, callus was transferred onto solid differentiation medium yielding plating efficencies of 1.5% (H. nuttallii) and 2.5% (H. giganteus). Emerging shoots were elongated on hormone-free medium, and root formation was induced by an NAA treatment. Regenerated plants were transferred to the greenhouse where they grew up to a height of 2 m and flowered after 3 months. Seeds were harvested from regenerated plants of both species.