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Scientists Find "Master Switch" that Initiates FloweringNational Science Foundation (NSF)-funded plant geneticists at The Salk Institute for Biological Studies in La Jolla, California, have generated plants in which flowering has been dramatically accelerated. The genetic engineering feat was achieved in diverse plants, ranging from a tiny weed, the small mustard-plant relative Arabidopsis, to a long-lived tree, the aspen. The scientists' success shows that the activity of a single gene, "LEAFY," is sufficient to initiate the formation of flowers. This ability to single-handedly cause a plant to skip entire growth phases and proceed directly to a flowering state identifies LEAFY as a developmental master switch. The results of the study are reported in the October 12, 1995, issue of the journal Nature, in an article titled "A Developmental Switch Sufficient for Flower Initiation in Diverse Plants." The precocious flowering results achieved by scientist Detlef Weigel contribute significantly to the scientific understanding of how plants switch over from a vegetative to a flowering state. But it is the practical applications made possible by his findings that are expected to create excitement. The discovery should allow for dramatic shortening of the vegetative phase in any plant amenable to genetic engineering, says Weigel. During their initial vegetative phase, plants are normally unable to produce flowers. The discovery now shows that this block in flowering can be overcome by artificially switching on the LEAFY gene, which is normally silent during vegetative growth. Weigel is the first to report the acceleration of initial flowering in a tree so that it happens in a matter of months instead of the eight to 20 years in nature (and when the aspen is only inches tall, instead of the 30 feet in height to which it would normally have grown). His success with several unrelated plants suggests that this may well be a universally applicable strategy. "While a lot of breeding has taken place in smaller plants, tree breeding has been very limited because the life cycle of trees is so long," says Weigel. "It takes several plant generations before certain desirable characteristics can be achieved and verified. When you're dealing with a tree that first flowers at age 8 or 20, then this can take longer than one human lifetime. This genetic engineering scheme collapses this timetable to make tree breeding feasible for the first time." By reducing the time to flowering, the researcher's scheme will also simplify breeding of plants other than trees, as most crop plants take several months to produce their first flowers. Since successful breeding normally requires a series of crosses over six generations or more, even cutting flowering time in half will speed up breeding significantly. The elegance of the method lies in the fact that the early flowering is conditioned by a single known gene, rather than by the combination of several unknown genes, as is the case in more traditional schemes for early flower induction. Thus, after the desired breeding effects have been achieved by a series of accelerated-flowering crosses, the scheme makes it very easy to return the improved plants to their normal flowering habits by removing this single gene. The discovery also underlines the importance of basic science for applied research, says Weigel. The LEAFY gene was initially identified because it is necessary for the formation of flowers in Arabidopsis, a plant without economic value, but easily manipulated in the laboratory. Weigel has shown that LEAFY is sufficient to induce flowers in Arabidopsis and tobacco. The breakthrough with aspen came when Weigel was joined by Ove Nilsson, a recent graduate of the Swedish University of Agricultural Sciences. There, he was part of a group that developed genetic engineering procedures for aspen. With a patent on their discovery applied for, the researchers are eager for practical applications to be made. One matter they will soon address is how to slow down the speed with which their transgenic plants begin to flower. "Usually an effect is not pronounced enough and must be strengthened, while here we have the opposite case. We have virtually turned the aspen tree into a weed not unlike Arabidopsis. So we need to find less powerful versions of LEAFY that will allow the aspen flowering to come at a time when the tree is a little older and larger," says Weigel. One of their long-term goals is to be able to induce flowering at will in crop plants. Weigel's research is funded by NSF's genetics and nucleic acids program.
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