PROPAGATION OF PLANTS IN BIOREACTORS: PROSPECTS AND LIMITATIONS

Abstract

Bioreactors provide a rapid and efficient plant micropropagation system but as yet are not fully exploited commercially for agricultural crops and forest trees. The culture system was applied to herbaceous and woody plant species utilizing liquid media to overcome intensive manual handling. Large-scale liquid cultures were used for propagation through organogenesis or somatic embryogenesis pathways depending on the species. Bioreactors have several advantages over agar cultures with a better control of the culture conditions. Direct contact of the plant tissue with the medium, optimal nutrient and growth regulators supply, continuous aeration and circulation, filtration of the medium for controlling exudates and contamination and the production of clusters of buds, meristems or protocorms for automated dispensing. The major issue imposed by liquid media in bioreactors is the phenomenon of hyperhydricity, morphogenic shoot and leaf malformation, due to the continuous immersion of the tissue in the medium. The malformations are manifested in glossy hyperhydrous leaves with distorted anatomy. The very nature of liquid tissue culture is an imposing stress to which the plants respond to the environmental signals in developmental aberration. The submerged tissue was found to exhibit oxidative stress symptoms, with elevated levels of reactive oxygen species that were associated with a change in anti-oxidant enzyme activity. These changes greatly affected the anatomy and physiology of the plants and their survival after transplanting. Two major solutions for malformation control were proposed: the use of growth retardants to control rapid proliferation and temporary immersion bioreactors. Growth retardants reduced water uptake during cell proliferation, decreased vacuolation and rapid growth, shortened stems and inhibited leaf expansion, inducing the formation of clusters. In tuber, bulb and corm producing plants growth retardants were found to enhance storage organs formation. Temporary immersion bioreactors were used to provide an improved aeration system to reduce hyperhydricity, by shortening plant duration in the liquid phase. The frequency of immersion and the medium composition improved plant development. Future prospects of micropropagation in bioreactors for optimal plant production will depend on both basic and applied studies. The former contributing to our understanding of plant responses to microenvironment signals to overcome the limitations of liquid cultures and the latter to provide specific culture manipulation to control the morphogenesis of plants in vitro improving survival ex vitro.