OSU FLORICULTURE RESEARCH UPDATE By Dr. James Metzger, D. C. Kiplinger Chair Dept. of Horticulture and Crop Science The Ohio State University, Columbus Height control is a major consideration during production of many floriculture crops. Growers have several effective methods of growth regulation at their disposal to control growth during the production of a crop. Although the application of chemical growth regulators and the use of DIF have a proven track record as valuable tools in managing plant growth, both require substantial grower inputs. Innovative technologies such as photoselective filters that selectively remove far-red stretch signals will also require significant economic inputs in order to implement. New technologies are needed to obtain good growth regulation while at the same time reducing grower inputs. In this article I will describe how John Zheng (a former graduate student of mine) and I utilized biotechnology to generate a chrysanthemum plant that is identical to the original cultivar, except that it roughly 15 to 20% shorter. The first step in developing a strategy to genetically engineer shorter plants is the choice of an appropriate gene. We therefore turned to a strategy that exploits a gene that is essential in mediating the growth regulating effect of light. Red light inhibits stem growth while far red light causes stems to stretch. The sensor for these light signals is the pigment phytochrome. Our strategy then was to transfer an extra copy of the gene coding for the red light sensing phytochrome, known as phytochrome B, into a chrysanthemum plant. The underlying logic of our strategy was based on the prediction that a plant producing extra phytochrome would be shorter because it would "see" more of the growth inhibiting red light signals than normal plants. The source of the phytochrome B gene was tobacco; this choice was simply a matter of convenience since we already had this gene in a form ready to be transferred into plants. In reality we could have used a phytochrome B gene from chrysanthemum or any other plant species to achieve the same result. We used Agrobacterium-mediated gene transfer to insert the phytochrome B gene into chrysanthemum. Agrobacterium tumefaciens is a bacterial pathogen that causes witches broom and similar disorders in a number of plant species. During the infection process, the bacterial genes are incorporated into the host plant's genetic material. One can exploit this process for the genetic engineering of plants by using "disarmed" Agrobacterium cells in which the genes that cause the pathogenic symptoms have been removed. We were able to generate a number of chrysanthemum lines containing the tobacco phytochrome B gene. Two lines, which we called LE31 and LE32, appeared especially promising. Both had stems about 15 to 20% shorter than the original cultivar (Photo 1). This reduction in stem length was about the same as what we obtained by applying a drench of Cycocel to the original cultivar at a rate of 1500 ppm or by growing the plants under growth regulating spectral filters that selectively remove far-red light. In addition to shorter stems in the genetically engineered plants, we also noted two unexpected, but potentially useful attributes. First, the transgenic plants contained about 10 to15% more chlorophyll than wildtype plants giving them a greener appearance (Photo 2). As a result, rates of photosynthesis in genetically engineered plants were correspondingly higher. We have yet to test whether the increases in the rates of photosynthesis will result in better growth under low light conditions. We also observed an increase in the longevity of the flowers by 10 to 14 days (Photo 3) (Photo 4). This is significant in view of the fact that chrysanthemum flowers are not particularly sensitive to ethylene, so we may have a new technology to prolong the life span of ethylene insensitive flowers. We are presently evaluating the performance of the genetically engineered plants under commercial greenhouse conditions. We are also attempting to adapt this technology to other floriculture crops as well. The technology might be particularly useful in vegetable transplants in which the application of chemical growth regulators is presently forbidden.