Plants produce flowers to complete their life cycle and to reproduce. Flower development requires genetic and structural changes in the tip of the plant shoot as the plant transitions from its vegetative to its reproductive stage. During this time, gene expression patterns also determine the overall arrangement of the flowers, called the inflorescence, into single organs, unbranched or branched clusters, or into compound structures. In most, but not all plant species, the reproductive branches terminate their growth after flowers have given rise to fruits. Our recent study of the oil seed plant Swedish thale-cress (Arabidopsis suecica), a species that normally produces single flower spikes, showed that under certain light conditions this termination is reversed and multiple compound flower spikes are instead produced. The central aim of this project is to understand the genetic mechanisms for this abnormal behavior. Using molecular techniques, including in situ hybridization, gene expression will be compared in normal and abnormal floral tissues, to determine if and how the different distribution of flower development genes leads to the differences in floral architecture. The hypothesis tested is that both strength of gene expression and the distribution of the gene products (the proteins) within the developing inflorescence are responsible for these changes. Understanding the genetic pathways controlling inflorescence development in commercially important oil seed plants could be applied to efforts to enhance seed yield. Creating types with more favorable inflorescence architectures might be useful in biofuel production, for example.
This project will provide outstanding training opportunities for undergraduate students. The project will be led by the University of Puget Sound, a primarily undergraduate institution, in collaboration with the University of Washington, and Heritage University, a minority-serving college in central Washington state.
Arabidopsis suecica is a close relative of the model plant Arabidopsis thaliana and a member of the mustard family. Flowers in mustard plants are generally arranged along the flowering spike (the inflorescence) as single flowers. We noticed that occasionally A. suecica produces individual flowers that can give rise to multiple additional flowers, a process called floral reversion. In this project we aimed to describe this phenomenon in detail, find out under what conditions floral reversion is increased in A. suecica, begin to understand the molecular reason for this type of flower morphology, and assess if a deliberate change in flower morphology from the general single flower inflorescence to multiple flower inflorescences would increase plant seed yield. Using various types of microscopy we were able to show that the floral structure, from which the additional flowers arise, is most likely the replum, a tissue that originates from the flower stem and that connects the two valves of the fruit. We found that A. suecica plants are more likely to revert in short daylight conditions than in long daylight conditions, and that reverting flowers are not randomly distributed along the inflorescence stem but are more likely to occur in the lower half of the stem. We found evidence that suggests that floral reversion and time to flowering are positively correlated. We conducted molecular experiments to see which types of genes might be involved in the formation of these unusual flowers and found 83 candidates genes with functions including hormone responses, organismal development, and reproduction. We also found that reverting flowers produce more seeds than regular flowers, but that plants with more reverting flowers per plant do not necessarily produce overall more seeds than plants without any reverting flowers. Our data lead us to focus our future efforts on two aspects: first, determine which of the 83 candidate genes have direct effects on the formation of reverting flowers, and second investigate if changes in the level of gene expression of these genes will lead to the production of more reverting flowers per plant when grown in conditions most conducive to floral reversion. We see one possible long-term impact of this project on agriculture in particular: if we succeed in understanding which genes are involved in changing the floral architecture of a plant from a single to a multiple flower while increasing overall seed mass per flower, it might be possible in the future to breed plants, particularly in the mustard family, with higher seed yields. This project was carried out at an undergraduate college with the help of undergraduate student researchers. A second major impact of this work thus was the training opportunities the project afforded the participants, some of whom have subsequently found work in the biotech industry, or have continued their studies in graduate and professional schools.
