Topic 20.2

Commercial Uses of Gibberellins

Valerie Sponsel, Biology Department, University of Texas, San Antonio, TX, USA

Gibberellic acid (GA₃) is the GA most often used commercially, since it can be readily obtained in large quantities from fermentations of the fungus Gibberella fujikuroi. The global (excluding China) use of GA₃ per annum is approximately 50 tons. Other GAs, for example GA₄ and/or GA₇, are used for specific crops or specific purposes for which they are more effective than GA₃, though GA_4/7 are produced in lower yields by commercial fermentations and are therefore more expensive than GA₃.

(Click image to enlarge.)

The major commercial uses of GAs are to promote the growth of a variety of fruit crops, to increase sugar yield in sugarcane, and to stimulate the barley-malting process in the beer-brewing industry. Details of additional uses of GAs in horticulture, albeit on a smaller scale than those mentioned below, can be found in Gianfagna (1995).

Many of the table grapes grown in the United States are a genetically seedless variety that would naturally produce small fruit on very compact clusters. Almost all seedless grapes on the market are treated with GA₃. It substitutes for the presence of seeds, which would normally be the source of native GAs for fruit growth. Repeated spraying with GA₃ increases both rachis length (producing looser clusters) and fruit size (Figure 1). The increased rachis length prevents the cluster from being too compact, and this reduces the chance of fungal growth inside the cluster. Two to three additional applications of GA₃ during fruit development are thought to increase berry size by enhancing the import of carbohydrates into the developing fruit. In excess of 8 tons of GA₃ are used in the California grape industry annually.

Figure 1 Gibberellin induces growth in Thompson’s seedless grapes. The bunch on the left is an untreated control. The bunch on the right was sprayed with GA3 during fruit development. (© Sylvan Wittwer/Visuals Unlimited.) (Click image to enlarge.)

Gibberellic acid is also used to boost cherry production. Sweet, bing cherries are sprayed 4 to 6 weeks before harvest to increase fruit size. Application of GA₃ to tart cherries increases yield through enhanced bearing.

Gibberellin A₄ (GA₄) is used to promote the fruit set of apple and pear trees. For example, in some apple cultivars the amount of fruit produced is often limited by biennial bearing, a phenomenon whereby the production of a heavy crop of fruit one year inhibits the subsequent production of flower buds, and hence, the yield of fruit the following year. The alternate bearing of some cultivars can be overcome by applying GA₄ in the "off" year to promote the formation of flower buds, and subsequent fruit set. In regions of Europe where fruit set of apple and pear trees is often reduced by inclement weather at the time of pollination, the application of a hormone mixture can promote the production and subsequent growth of parthenocarpic (seedless) fruit. GA_4/7is also used on Golden Delicious apples to prevent abnormal cell divisions in the epidermal layer that produce "russetting." The russetted appearance of the apple fruit is considered by many consumers to be undesirable.

Gibberellic acid is also applied to citrus crops, though the actual use depends on the particular crop. For example GA₃ is sprayed onto oranges and tangerines to delay or prevent rind-aging, so that fruit can be harvested later without adverse effects on rind quality and appearance. For lemons and limes, GA₃ synchronizes ripening and enhances fruit size.

Gibberellic acid is used extensively to increase the sucrose yield of sugarcane. Sugarcane, a normally fast-growing C₄ member of the Poaceae (grass) family, is sensitive to cooler winter temperatures, which reduce internode elongation and subsequent sucrose yield. The adverse effects of cooler temperatures can be counteracted by the application of GA₃.

As described in Chapter 20, the biochemistry of GA-induced α-amylase synthesis in the aleurone of cereal grains has been studied extensively and is one of the best-understood GA response pathways. Gibberellins from the embryo of germinating grains are necessary for the synthesis of α-amylase by the cells of the aleurone layer, which, in turn is necessary for the hydrolysis of starch within the endosperm (Figure 2). In the brewing industry, the production of beer relies on this hydrolytic breakdown of starch in barley grains to yield fermentable sugars, principally maltose, which are then subjected to fermentation by yeast. During fermentation, glycolytic enzymes from yeast break down the sugars, resulting in ethanol.

Figure 2 Structure of a germinating barley grain and the biochemical processes that occur during the "modification" part of the malting process. (Click image to enlarge.)

In the multistep malting process, mature barley grains are steeped or soaked to allow them to imbibe water. Next, the grains are spread out to germinate, during which time the starch within the endosperm will be hydrolyzed by α-amylase allowing the embryo to begin to grow. This process of starch breakdown is referred to as "modification." Gibberellic acid may be applied during this time and will supplement the native GAs in the grain, enhance the production of α-amylase, and consequently, speed up the hydrolysis of starch. (The molecular aspects of this GA response are shown in Chapter Figure 20.24). The germinated grains, which show a well-developed root and have a shoot (termed an "acrospire"), which is approximately 75% of the length of the grain, are then kilned. This is a two-step process, first to dry the grain, and second, to cure or heat it. The temperature to which the modified malt is cured will determine whether light-colored (low-temperature curing) or dark beer (high-temperature curing) is to be produced. The modified and kilned malt provides the raw material for the fermentation.

Acknowledgements

Many thanks to Dr. Paul Silverman, Valent Biosciences Corporation, who generously contributed information on this Topic.

Reference

Gianfagna, T. J. (1995) Natural and synthetic growth regulators and their use in horticultural and agronomic crops. In P. J. Davies, (Ed.), Plant Hormones: Physiology, Biochemistry and Molecular Biology (pp. 751–773). Kluwer: Dordrecht, Netherlands.