Applications of Biotechnology in Floriculture Crops

Commercial production of chrysanthemum requires that the plants be of a certain size before flowering. Mums are short day plants, which means that they will flower only when the days are shorter than a specific length, termed the critical daylength. For many mum cultivars, the critical daylength is around 14.5 hours. In other words, these plants will flower only when the days are 14.5 hours and shorter. For the state of Ohio, daylengths are conducive for flowering in mums for much of the year-from July 21 to May 21! In order to maintain mums in a vegetative state until they are ready to be induced to flower, growers provide plants with a nightbreak of light from low intensity lamps, usually between the hours of 10:00PM and 2:00AM. The expense of electricity for the nightbreaks is significant, so the development of plants that require fewer hours of the nightbreak to remain vegetative would benefit growers by reducing production costs.

Our strategy to address this need was to genetically engineer mum plants to be more sensitive to the light of nightbreaks. Plants sense light through a special pigment called phytochrome. However, phytochrome occurs in extremely low levels in plants- about one ten-billionth that of chlorophyll, the green pigment that uses sunlight in photosynthesis. By creating plants that produce more phytochrome, less light will be necessary for a nightbreak to be effective. We accomplished this by incorporating an extra copy of the gene that codes for the phytochrome into a mum's existing genetic material. The extra phytochrome gene was transferred into a mum plant using a special strain of the bacterial plant pathogen Agrobacterium carrying the phytochrome gene. This strain of Agrobacterium is disarmed in the sense that it causes no pathological symptoms, and is simply used to deliver genes of interest in a variety of plant species. Just as we expected, mum plants containing the extra phytochrome gene were more sensitive to nightbreaks than those with the normal amount of phytochrome. In fact, only a 1-hour night break was necessary to maintain the genetically engineered mum plants vegetative, compared to the four hours required for the normal plants (Figure 1) (Figure 2). In addition, we found two additional positive attributes associated with the genetically engineered plants: First, the plants were shorter but had the same number of leaves as the non-engineered plants. Second, the genetically engineered plants were darker green than their non-engineered counterparts. In fact the plants looked as though they were treated with a normal amount of growth regulator. This suggests that besides reducing the duration of light required for the nightbreak to be effective, plants that produce extra phytochrome may lessen the need for applications of growth regulators (Figure 3). This obviously will benefit growers by reducing the cost of productions.

This specific use of biotechnology has obvious potential applications in other floriculture crops as well. However, the positive results that we obtained do not guarantee that its use will become widespread because there may be unforeseen deleterious effects on the plant associated with the extra production of phytochrome. For example, the genetically engineered plants may exhibit increased susceptibility to disease or a reduction in stress tolerance, and such effects may outweigh the benefits gained by having plants with reduced nightbreak and growth regulator requirements. Currently we are in the process of examining the performance of our genetically engineered mum plants under commercial production conditions.