Gibberellin signalling - new opportunities for crop management

The potential for exploitation of the gibberellin (GA) plant growth hormones in agriculture was a major factor driving research on the physiological function of these substances following their introduction to the West from Japan in the 1950s. As a consequence of its vital role in promoting organ growth as well as in stimulating seed germination and reproductive development, this class of hormone has assumed considerable importance as a plant growth regulator (PGR) and a wide range of applications of GAs are known. However, in terms of treated area and sales, the commercial utilisation of GAs is dwarfed by that of the growth retardants, which, as inhibitors of GA biosynthesis, function by reducing the bioactive GA content of treated plants, thereby reducing shoot elongation. Several inhibitors of GA biosynthesis are of practical relevance with important uses. The annual global PGR market accounts for approximately €700 million, of which some 50% can be assumed to be represented by growth retardants. Area- and value-wise, stem shortening in small grains and in rice production to reduce the risk of lodging is the main application of PGRs worldwide. It is estimated that some 30% of global PGR sales (equalling approximately €210 million) are represented by stem stabilisers. In addition to inhibitors of GA biosynthesis, ethylene-releasing ethephon is also used for this purpose, particularly in barley. The usage of such products is general practice in countries with intense production of wheat, barley, rye, triticale, and oats such as France, Germany and Great Britain. For instance, 92% of the winter wheat, 81% of the winter barley, 67% of the oats, and 100% of the rye acreage were treated with anti-lodging products in Great Britain in 2008. Despite this, it has been estimated that losses due to lodging cost the British wheat industry about €50 million per year and these costs are likely to be even higher to date. The application of a PGR allows for an active regulation of developmental processes. For example, control of stem elongation in cereals may be “fine-tuned” according to need, both in terms of affecting the right growth stage (by timing of treatment) and the intensity of growth reduction (by dosage). Also, but within limits, distinct parts of a plant may be targeted. It can be counter-productive in cereal production if, in addition to shoot growth, root growth is also reduced. This can be avoided by spray-applying non-systemic compounds such as prohexadione-Ca. Thus, in terms of flexibility, immediate growth control and potential specificity, PGRs can offer advantages over conventional breeding and genetic engineering approaches. On the other hand, there is wide-spread concern about the use of chemicals in crop production. In order to comply with the principles of integrated crop production better, it would be preferable if problems solved with presently existing PGRs could be overcome by introducing improved genotypes and/or by making available new and more specific PGRs with an even further reduced risk to the grower, environment and consumer. The introduction of semi-dwarfing genes into wheat and rice in the 1960s played an important role in the Green Revolution that resulted in dramatic increases in crop yield. Shorter, more stable stems were necessary to avoid lodging of the crop under high levels of nitrogenous fertiliser, but semidwarfism also improved yields by increasing grain numbers, probably by allowing more assimilate to partition into the developing spike. The dwarfing genes in both rice and wheat act on the GA-signalling system: semi-dwarf rice contains mutations in a GA-biosynthesis gene that result in reduced GA content while the reduced height (Rht) mutation in wheat compromises the ability of GA to stimulate growth. In view of the importance of the GA signalling pathway in regulating physiological processes of relevance to agriculture, it is a prime target in crop improvement programmes. To date, most interest has been focussed on the GA-biosynthetic and inactivation pathways that determine GA concentration since they have been more clearly understood than the mechanisms for GA perception and signal transduction. A number of studies describing the genetic modification of GA metabolism, either to enhance or restrict plant growth, have been published. For example, increasing GA content by ectopic expression of GA 20-oxidase genes, which encode a rate-limiting enzyme in GA biosynthesis, has been shown to result in higher biomass in tobacco and in longer fibres in aspen, resulting in improved paper-making quality. Conversely, ectopic expression of GA 2-oxidase genes, which encode GA-inactivating enzymes, provides a particularly efficient method for introducing dwarfism. These approaches are clearly effective and even though breeding by genetic engineering is not publicly acceptable in a number of countries, they highlight potential gene targets for breeding to improve plant architecture or yield. Recently, the range of targets has been potentially broadened by impressive advances in our understanding of GA signal transduction.