Modification of Plant Architecture in Chrysanthemum by Ectopic Expression of the Tobacco Phytochrome B1 Gene

Height control is a major consideration during commercial production of chrysanthemum [Dendranthema ×grandiflora Kitam. (syn. Chrysanthemum ×morifolium Ramat.)]. We have addressed this problem by a biotechnological approach. Plants of ‘Iridon’ chrysanthemum were genetically engineered to ectopically express a tobacco (Nicotiana tabacum L.) phytochrome B1 gene under the control of the CaMV 35S promoter. The transgenic plants were shorter in stature and had larger branch angles than wild type (WT) plants. Reduction in growth caused by the ectopic expression of the tobacco phytochrome B1 gene was similar to that caused using a commercial growth retardant at the recommended rate. Another morphological effect observed in the leaves of the transgenic plants was more intense green color that was related to higher levels of chlorophyll. The transgenic plants appeared very similar to WT plants grown under a filter that selectively attenuated far red wavelengths. Furthermore, when plants were treated either with gibberellin A3 (which promoted growth) or 2-chlorocholine chloride, an inhibitor of gibberellin biosynthesis (which inhibited growth) the difference in the average internode length between the transgenic plants and WT plants was the same in absolute terms. This suggests that reduction of growth by the expressed PHY-B1 transgene did not directly involve gibberellin biosynthesis. The commercial application of this biotechnology could provide an economic alternative to the use of chemical growth regulators, thereby reducing production costs. economic necessity for growers (Gianfagna, 1995). Height control in greenhouse crops is achieved by several approaches. The most common practice is the use of chemical growth regulators (Dole and Wilkins, 1999). Most commercial growth retardants reduce stem growth by inhibiting gibberellin (GA) biosynthesis, thereby reducing the endogenous levels of one or more biologically active GAs (Gianfagna, 1995). However, this method necessitates significant economic inputs by the grower in terms of both the cost of chemicals and the labor required for application. Furthermore, in many countries, synthetic chemical growth regulators for height control are not labeled for use in crops destined for human consumption. Cultural methods are also used to control the height of greenhouse crops. For example, manipulation of the daily greenhouse thermoperiod by maintaining the night temperature higher than the day temperature (DIF) is an effective cultural alternative to the use of chemical growth regulators for many crops (Dole and Wilkins, 1999). However, negative DIF can not be exploited in certain geographical regions, or at certain times of the year when it is impractical or deleterious to plant growth to raise the greenhouse night temperature higher than the day temperature (Dole and Wilkins, 1999). Mechanical stimulation (brushing or impedance) of plants can also be used to limit undesirable stem elongation, but implementing this method of height control is often impractical on a commercial scale (Garner and Bjorkman, 1996). A promising new method to control excessive internode elongation in many greenhouse crops is use of photoselective filters that selectively attenuate far red (700 to 800 nm) wavelengths (McMahon and Kelly, 1995; Rajapakse et al., 1999). Received for publication 31 Jan. 2000. Accepted for publication 8 Sept. 2000. This paper is based on a portion of a PhD dissertation submitted by Z.-L. Z. Research supported in part by the D.C. Kiplinger Endowment for Floriculture. We are grateful to Eva Adam and Ferenc Nagy (Hungarian Academy of Science) for the Nt-PHYB1 cDNA clone, Fang Cheng Gong (Purdue University) for the pJIT117∆2 vector, and Margaret E. Daub (North Carolina State University) for the EHA105 strain of Agrobacterium. Use of trade names does not imply endorsement of the products named or criticism of similar ones not mentioned. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Former graduate research assistant. Currently postdoctoral research associate, Department of Botany and Plant Sciences, University of California, Riverside, CA 92521. Assistant professor, Department of Botany and Plant Sciences, University of California, Riverside, CA 92521. Assistant professor. Professor and corresponding author; e-mail [email protected]. Height control is a major consideration during production of many horticultural crops. Reduction of height by genetic means or cultural practices often provides economic advantages to the producer. Cultivars of many crops that exhibit shorter stems frequently have higher harvest indices. The physiological basis for this is the redirection of assimilate partitioning such that there is an increase in the proportion of shoot biomass accumulating in the harvested unit, thereby resulting in increased yields (Loomis and Conner, 1992). Control of excessive stem elongation is also essential for prevention of lodging in some crops such as vegetable transplants (Garner and Bjorkman, 1996). In addition, shorter stems increase the quality and value of floriculture and other greenhouse-grown crops and therefore height control is an