Gibberellins are a classic example of the production of plant growth regulators by microorganisms.They are important biotechnological products and are increasingly used in agriculture and horticulture.This article intends to assemble information on the hisotry of the identification of gibberellins (GA) and producing microorganisms, especially Gibberella fujikuroi (Saw.) Wr. Furthermore, the biosynthesis of gibberelins through the isoprenoid biosynthetic pathway will be described. The main product of GA biosynthesis in Gibberella fujikuroi is gibberellic acid (GA,), which is formed from GA, via GA,. Both the amount and the type of gibberellins produced by the fungus are dependent on the genetic constitution of the strain and the fermentation conditions. Mutation and selection for increased product formation are probably the most important factors in improving the yield of gibberellins. Some publications concerning methods of parasexual recombination will also be summarized. Beside strain improvement of wild strains, medium development and appropriate cultivation techniques (batch, fed-batch-, continuous-, and solid state-fermentation) are very important prerequisites for successful economy of gibberellin production. Furthermore, the most important ways of gibberellin recovery and purification are described. Continuing reductions in the costs make gibberellins more attractive for existing applications and open possibilities for further applications of GA, and some other active gibberellins like GA,, Ga,, and GA,. The plant growth promoting properties of gibberellic acid, a metabolic product of the fungus Gibberella fujikuroi, J . Sci. Food Agric., 5 , 602, 1954. Lang, A., Bolting and flowering in biennial Hyoscyamus niger, induced by gibberellic acid, Plant Physiol., 31(Suppl.), 35, 1956. 1 87 Critical Reviews in Biotechnology Downloaded from informahealthcare.com by Mcgill University on 02/03/15 For personal use only.

In Gibberella fufikuroi, ammonium (NH~-) interfered with the production of gibberellic acid (GA3). Optimal production occurred at 19 mM (NH4)2SO4 and the synthesis of GA3 was reduced threefold in a medium with 38 mM (NH4)2SO4. Using a resting cell system with mycelia previously grown on two concentrations (19 mM and 38 mM) of (NH4)2SO4, it was found that NH+depressed synthesis of the gibberellin-synthesizing enzymes. Furthermore, addition of NH + to a producing system shut off gibberellin formation, indicating that the negative effect of NH2-ions is also due to inhibition of one or more enzymes in the gibberellin biosynthesis pathway. The onset of gibberellin biosynthesis in media with high (38 mM) and low (19 mM) concentrations of (NH4)ESO4 was studied by addition of cycloheximide to batch cultures of various ages.

A new method of optimization of the medium composition by biostatistical methods is reported. The well-known BOX-WILSON scheme combined with the method of optimum curves was applied for an exploration of the response surface. The primary goal has been to optimize the concentration of nutrients for the production of gibberellic acid by the fungus Gibberella fujikuroi with a minimum number of experiments. As the result, a nearly fivefold improvement yield of the was achieved performing only two experiments.Gibberellic acid (GA,) and some other biological active gibberellins are produced by the phytopathogenic fungus Gibberella fujikuroi (SAW) WOLL. (Fusarium moniliforme SHELD) when grown in submerged culture in nitrogen-limited media.Gibberellins are typical secondary metabolites. The phases of growth can be discerned, described and related to the nutritional and environmental state of the fermentation (BORROW etal., 1961, 1964, VASS and JEFFERYS, 1979. -Significant production ,of gibberellins starts only after nitrogen exhaustion. On the other hand, production is greater the higher the value of initial nitrogen over a wide range. Further increases in nitrogen concentrations lead to a decrease of the mycelium productivity (BORROW et al., 1964). Therefore, the selection of both the initial concentration of nitrogen and of an optimal C/N ratio are the most important factors.The object of this paper was the improvement of the medium composition by mathematical methods for planning the experiment.In an unconventional manner the Box-WILSON scheme is used for the optimization of a nutrient medium composition consisting of four nutrients -corn steep liquor (CL), ammonium sulfate (AS), sunflower oil (SO), and potassium dihydrogen phosphate (PP). Originally, the BOX-WILSON method was developed for the optimization of chemical processes (Box and WILSON 1951). To optimize a biological response such as the biosynthesis of gibberellins and other secondary metabolites, some modifications according design and analysis are needed to make the BOX-WILSON procedure more effectively. A combination with the method of optimum curves developed by SCHRODER and WEIDE (1973) seems to be very helpful to avoid an overdimensional expansion of experimental effort. In the following by the example of gibberellic acid formation an appropriate methodical basis is given for the application of factorial designs in fermentation. As the rule a consulting statistician solving problems in fermentation is confronted with four problems which are typical for the optimization of biological response. I The great variation of experimental results after repetition; I1 The bad reproducibility of fermentation experiments over the time; I11 The overdimensional effort of the factorial design (2k or 2k-p) as a consequence of the high-dimensional parameter space; IV The violation of the smoothness hypothesis underlying the response surface technique. 16'

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