IOS-0744874, Regulation of flower maturation, PI Jason W. Reed, co-PI Punita Nagpal
Flowers are the reproductive structures of plants, and have both male and female parts that produce pollen and seeds. Male and female development in flowering plants must be coordinated in order for successful pollination and seed production to occur. This project is to study mechanisms of late flower development and pollination in the model plant Arabidopsis. Several regulators of gene expression are known that regulate flower opening and male and female reproductive competence. Two of these also regulate responses to the plant hormone auxin, suggesting that auxin may regulate flower maturation. The functions of hypothesized negative regulators of these proteins will be determined using genetic and molecular biology approaches. Downstream effectors of these transcription factors will be identified by analyzing global gene expression patterns. These studies will lead to a more comprehensive understanding of how late stages of flower development shortly before fertilization are regulated. It is likely that the mechanisms discovered will be applicable in other plants, and may therefore lead to greater understanding of mating systems in natural populations, and to practical applications such as in designing ways to reduce outcrossing of transgenic crops, or to optimize seed and fruit production. In addition, this work may also suggest the molecular basis for the closed florets of domesticated broccoli. In addition to this experimental part, the project will incorporate an educational component. A multimedia presentation on flower opening and pollination and how scientists study it will be designed, and presented to schoolchildren and the general public as part of the Science 360 program at the Morehead Planetarium and Science Center at the University of North Carolina at Chapel Hill. The Morehead Center hosts 130,000 annual visitors and summer camp participants.
Intellectual merit Perfect flowers have both male organs that produce and release pollen, and female organs that make seeds and later form the part of the fruit that harbors seeds. Flowers also often attract pollinators using visual or chemical signals. So that male, female, and pollinator attraction functions occur at the right time, flower organs must grow and mature in a coordinated fashion. In the model self-pollinating plant Arabidopsis thaliana, between developmental stages 12 (just before flowers open) and 13 (just after flowers open), the flower organs grow rapidly, anthers release pollen, the gynoecium (female organs) becomes competent to support pollination, and nectaries enlarge and begin to produce nectar (see flower sketch, diagram A). At the same time, volatile chemicals are produced that may attract pollinators or repel pathogens. A regulatory network controls these processes by changing gene expression in developing flowers. The plant hormones auxin (acting through the transcription factors ARF6 and ARF8) and gibberellins promote biosynthesis of a third class of hormone, jasmonates (see diagram B). This hormonal cascade in turn induces expression of transcription factors called MYB21 and MYB24 that promote gynoecium growth, petal growth, stamen filament elongation, and pollen release from anthers. To understand the dynamics of this flower maturation regulatory network, we characterized morphological, chemical, and global gene expression phenotypes of arf, myb, and jasmonate pathway mutant flowers. arf and myb mutations delayed flower growth (see flower photographs). Similarly, jasmonate pathway mutants such as aos-2 had decreased MYB21 expression and delayed stamen and petal growth at the stage when flowers normally open. However, jasmonate deficiency caused increased MYB21 expression in petals of older flowers, resulting in renewed and persistent petal expansion at later stages. Global gene expression analyses revealed that both auxin response and jasmonate synthesis promoted positive feedbacks which may ensure rapid and coordinated petal and stamen growth as flowers open. At a later stage, MYB21 also fed back negatively on expression of jasmonate biosynthesis pathway genes to decrease flower jasmonate level, which correlated with termination of growth after flowers had opened. These dynamic feedbacks may promote timely, coordinated, and transient growth of flower organs. Our results provide a framework to understand how flower opening and reproduction are coordinated in Arabidopsis and other flowering plants. Broader impacts Together with the Morehead Planetarium and Science Center and the University of North Carolina, we developed a presentation on plant pollination called "Flower Power." This is presented to the public by planetiarium staff, and is now entering its fourth summer in rotating repertory.
