ABSTRACT

The concept of totipotency proposed in the 19 century has been demonstrated in a wide range of plant species over the last 50 years. However, the mechanism that underlies cell differentiation leading to plant morphogenesis in vitro is not well understood. Although phytohormones, predominantly auxin and cytokinin, have along been known to play an important role in various morphogenic events of cultured cells and tissues, evidence from several lines of study in recent years indicates that somatic embryogenesis (SE) and organogenesis in vitro have been associated with ethylene and polyamines (PAs). Ethylene and PAs are linked metabolically because they share a common precursor, 5-adenosylmethionine (SAM), for their synthesis. Both ethylene inhibitor and transgenic studies have demonstrated that inhibition of ethylene synthesis or action of cultured tissues resulted in promotion of morphogenic events of several plant species. These results have led to a conclusion that ethylene accumulation in culture may be responsible for poor morphogenesis. However, ethylene inhibition has resulted in increased PA content in cultured explants. This, together with the promoting effect of exogenous PAs on plant regeneration in vitro, suggest that poor regeneration may also be attributed to low levels of endogenous PAs of the cultured tissues. These findings indicate that plant morphogenesis in vitro may be regulated through precursor competition between ethylene and PA biosynthesis.