Ascorbate and glutathione in organogenesis, regeneration and differentiation in plant in vitro cultures

Abstract

The prerequisite for shoot, root or somatic embryo formation in plant in vitro culture is the development of meristem from dedifferentiated cells of the explant tissue. Auxin and cytokinin levels and their relative ratios play a decisive role in inducing the morphogenetic pathways leading to shoot, root or somatic embryo formation in plant in vitro cultures. Exogenous auxin is required to maintain the high rate of an unorganised growth in plant cell suspension cultures. On the other hand, the proliferation of hairy root cultures is usually dependent on endogenous hormonal factors. Auxin and cytokinin execute their regulatory role by being involved in a cross-talk with numerous endogenous factors affecting cell division and differentiation. Among them, ascorbate/dehydroascorbate (ASC/DHA), glutathione/glutathione disulphide (GSH/GSSG) redox pair, H2O2 and other components of cellular redox systems play an important role in triggering developmental responses in plant in vitro culture. Ascorbate, glutathione and related enzymes participate in the responses to auxin/cytokinin treatments. In addition, they can even directly affect hormone metabolism in tissue. Ascorbate and glutathione have important regulatory roles in the process of cell-cycle progression within the meristems, where they participate in redox-dependent determination of proliferation and quiescence patterns. The mechanism underlying the regulatory effects of ascorbate and glutathione in cell divisions is not fully elucidated; however, it seems to be related to the regulation of nucleotide synthesis. Ascorbate-levels in apoplast modulate the rate of organ elongation by increasing cell wall extensibility. Besides the effects on cell proliferation and growth, ascorbate and glutathione concentrations as well as the enzymes of their metabolism protect the in vitro cultured tissues against oxidative stress. This function is of particular importance during root regeneration and the elicitation of metabolite production by hairy root cultures, where increased levels of oxidising agents are often required to stimulate both processes. In this review, we report recent studies on the involvement of ascorbate and glutathione in the processes of regeneration and proliferation in plant tissue culture.