Acquisition of high embryogenic potential in regenerated plants of Helianthus annuus x H-tuberosus

Differentiated tissues of the tetraploid interspecific hybrid N. annuus x H. tuberosus (2n = 4x = 68) respond to growth regulator stimuli chiefly by forming callus, while somatic embryogenesis and/or organogenesis are only occasionally induced. However, a high embryogenic potential is displayed by explants of regenerated plants subjected to a second culture cycle in vitro, even in absence of growth regulators. The embryogenic potential is not restricted to a particular organ, but is widespread in the plant. It is maintained throughout plant development, and is not lost after repeated sub-culture. Only minor differences in chromosome numbers were found between embryogenic and non-embryogenic hybrids. Furthermore, the course of mitotic index, analyzed during culture of either embryogenic or non-embryogenic leaf explants, seems to exclude that embryo differentiation in clones of the former is due to the presence of cells which remain meristematic during the leaf differentiation process. The results indicate that plant regeneration could induce a stable epigenetic modification in hybrid cells that acquire the ability to differentiate somatic embryos at high frequency or allow the selection of rare cells with genetic changes leading to increased embryogenic potential.