The presence of hormones (auxins and cytokinins or substances of similar action) is required for the induction, proliferation and differentiation of cells in in vitro cultures (Maheshwari et al. 1995). The dynamic development and the introduction of in vitro techniques to micropropagation and to the study of mechanisms of physiological processes have resulted in the need for the search for new groups of substances playing a role similar to those of plant hormones. Fusicoccine, cotynine, helmintosporine, pestalocine and some other metabolites isolated from fungi belong to this group (Muller et al. 1991).
In tissue culture of wheat and rape, the influence of zearalenone greatly resembled that of 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic analogue of auxin (Biesaga-Koscielniak 2001; Biesaga-Koscielniak et al. 2003). Zearalenone completely replaced 2,4-D or increased its effect under in vitro conditions. It increased the percentage of wheat calli capable of regenerating shoots by more than 2,4-D, and especially the process of effectively regenerating shoots from poorly differentiated wheat calli. Zearalenone enabled the breaking of the blockade of the regeneration of shoots from callus of winter rape cv. "Gorczanski". Additionally, the application of thidiazurone to theses media increased the percentage of plant regenerated from callus for both wheat and rape. Therefore, it is possible to use zearalenone as an alternative to auxin or as a supplementary hormone analogue in in vitro culture of plants. This could be especially important when indirect regeneration of plants via callus induction is planned (Biesaga-Koscielniak et al. 2003; Szechynska-Hebda et al. 2007).
Moreover, zearalenone stimulated the growth of cell suspension of winter wheat and winter rape (in aqueous media) by more than 2,4-D, contributing to the increment in the volume and dry weight of cells during the culture period (Biesaga-Koscielniak 2001). In the suspension culture of wheat, the addition of zearalenone to a medium containing 2,4-D caused not only an increase in the dry weight of cells, but also an increase in the population of living cells in the culture.
The maize pollination system was used as a model to compare the activity of zearalenone with 2,4-D (Biesaga-Koscielniak 1998). Zearalenone in a concentration 50 times lower than that of 2,4-D demonstrated a similar effectiveness in stimulating the development of haploid embryos in wheat flowers after pollination with maize pollen (Biesaga-Koscielniak et al. 2003). The concentration of zearalenone (6 mM) was most effective in inducing ovary swelling (84 swollen ovaries/100 pollinated florets) and increasing the frequency of embryo induction (18.9 embryos/100 pollinated florets), but these embryos were severely deformed. They had low capability to germinate in vitro, while callus was easily formed and indirect regeneration of plants was possible. The results showed that zearalenone had some of the properties of an auxin analogue, while other effects of its actions were unique.
Zearalenone was also found to be more effective than cytokinin treatment in inducing shoots in in vitro winter wheat production. Moreover, both zearalenone and cytokinins increased the activity of antioxidant enzymes in wheat callus undergoing regeneration, and it is very likely that they also stimulated the plant regeneration process (Szechynska-Hebda et al. 2007). The effectiveness of regeneration on media containing zearalenone shows the possibility of using zearalenone as an alternative hormone also to cytokinins in winter wheat callus culture.
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