1. Field of the Invention
This invention relates generally to the field of plant genetic engineering and more specifically to genes involved in the regulation of plant reproductive development.
2. Background Information
A flower is the reproductive structure of a flowering plant. Following fertilization, the ovary of the flower becomes a fruit and bears seeds. As a practical consequence, production of fruit and seed-derived crops such as grapes, beans, corn, wheat, rice and hops is dependent upon flowering.
Early in the life cycle of a flowering plant, vegetative growth occurs, and roots, stems and leaves are formed. During the later period of reproductive growth, flowers as well as new shoots or branches develop. However, the factors responsible for the transition from vegetative to reproductive growth, and the onset of flowering, are poorly understood.
A variety of external signals, such as length of daylight and temperature, affect the time of flowering. The time of flowering also is subject to genetic controls that prevent young plants from flowering prematurely. Thus, the pattern of genes expressed in a plant is an important determinant of the time of flowering.
Given these external signals and genetic controls, a relatively fixed period of vegetative growth precedes flowering in a particular plant species. The length of time required for a crop to mature to flowering limits the geographic location in which it can be grown and can be an important determinant of yield. In addition, since the time of flowering determines when a plant is reproductively mature, the pace of a plant breeding program also depends upon the length of time required for a plant to flower.
Traditionally, plant breeding involves generating hybrids of existing plants, which are examined for improved yield or quality. The improvement of existing plant crops through plant breeding is central to increasing the amount of food grown in the world since the amount of land suitable for agriculture is limited. For example, the development of new strains of wheat, corn and rice through plant breeding has increased the yield of these crops grown in underdeveloped countries such as Mexico, India and Pakistan. Unfortunately, plant breeding is inherently a slow process since plants must be reproductively mature before selective breeding can proceed.
For some plant species, the length of time needed to mature to flowering is so long that selective breeding, which requires several rounds of backcrossing progeny plants with their parents, is impractical. For example, perennial trees such as walnut, hickory, oak, maple and cherry do not flower for several years after planting. As a result, breeding of such plant species for insect or disease-resistance or to produce improved wood or fruit, for example, would require decades, even if only a few rounds of selection were performed.
Methods of promoting early reproductive development can make breeding of long generation seed plants such as trees practical for the first time. Methods of promoting early reproductive development also would be useful for shortening growth periods, thereby broadening the geographic range in which a crop such as rice, corn or coffee can be grown. Unfortunately, methods for promoting early reproductive development in a seed plant have not yet been described. Thus, there is a need for methods that promote early reproductive development. The present invention satisfies this need and provides related advantages as well.
The present invention provides a recombinant nucleic acid molecule containing an inducible regulatory element operably linked to a nucleic acid molecule encoding a floral meristem identity gene product. An inducible regulatory element can be, for example, a copper inducible regulatory element, tetracycline inducible regulatory element, ecdysone inducible regulatory element or heat-shock inducible regulatory element, and a floral meristem identity gene product can be, for example, APETALA1 (AP1), CAULIFLOWER (CAL) or LEAFY (LFY). The invention further provides a transgenic seed plant, such as an angiosperm or gymnosperm, that contains a recombinant nucleic acid molecule of the invention.
The invention also provides a method of converting shoot meristem to floral meristem in an angiosperm by introducing a recombinant nucleic acid molecule of the invention into the angiosperm to produce a transgenic angiosperm; and contacting the transgenic angiosperm with an inducing agent, thereby increasing expression of the floral meristem identity gene product and converting shoot meristem to floral meristem in the transgenic angiosperm. The methods of the invention can be practiced with an inducible regulatory element such as a copper inducible regulatory element, tetracycline inducible regulatory element, ecdysone inducible regulatory element or heat-shock inducible regulatory element.
Also provided herein is a method of promoting early reproductive development in a seed plant by introducing a recombinant nucleic acid molecule of the invention into the seed plant to produce a transgenic seed plant; and contacting the transgenic seed plant with an inducing agent, thereby increasing expression of the floral meristem identity gene product and promoting early reproductive development in the transgenic seed plant. The methods of the invention can be practiced with a seed plant such as an angiosperm or gymnosperm using a floral meristem identity gene product such as AP1, CAL or LFY.
In addition, the present invention provides a nucleic acid molecule encoding a chimeric protein, which contains a nucleic acid molecule encoding a floral meristem identity gene product such as AP1, CAL or LFY linked in frame to a nucleic acid molecule encoding a ligand binding domain. A transgenic seed plant, such as an angiosperm or gymnosperm, that contains a nucleic acid molecule encoding a chimeric protein of the invention also is provided.
The invention further provides a method of converting shoot meristem to floral meristem in an angiosperm by introducing a nucleic acid molecule encoding a chimeric protein of the invention into the angiosperm to produce a transgenic angiosperm, where, under appropriate conditions, the chimeric protein containing a floral meristem identity gene product fused to a ligand binding domain is expressed; and contacting the transgenic angiosperm with cognate ligand, where, upon binding of cognate ligand to the ligand binding domain, floral meristem identity gene product activity is increased, thereby converting shoot meristem to floral meristem in the transgenic angiosperm. A floral meristem identity gene product useful in converting shoot meristem to floral meristem can be, for example, AP1, CAL or LFY, and a ligand binding domain can be, for example, a glucocorticoid receptor ligand binding domain or an ecdysone receptor ligand binding domain
The invention additionally provides a method of promoting early reproductive development in a seed plant by introducing a nucleic acid molecule encoding a chimeric protein of the invention into the seed plant to produce a transgenic seed plant, where, under appropriate conditions, the chimeric protein containing a floral meristem identity gene product fused to a ligand binding domain is expressed; and contacting the transgenic seed plant with cognate ligand, where, upon binding of the cognate ligand to the ligand binding domain, floral meristem identity gene product activity is increased, thereby promoting early reproductive development in the transgenic seed plant. A floral meristem identity gene product such as AP1, CAL or LFY and a ligand binding domain such as a glucocorticoid receptor ligand binding domain or an ecdysone receptor ligand binding domain are particularly useful in the methods of the invention for promoting early reproductive development.