Genetic and hormonal feedbacks defining tissue polarity by broad brushes and fine PINs

Multicellular organisms such as animals and plants develop specialised organs, which are composed of different types of tissues. The structure - or pattern - of organs is determined by the polarity within tissues such as for example radial or medio-lateral symmetry. Polarity is established when cells are provided with a sense of direction. Although developmental biologists have successfully identified genes required to specify individual cell types of an organ, and even how they interact in genetic networks, we know very little about the mechanisms that regulate tissue polarity.
Flowering plants evolved about 140 million years ago and today comprise more than 90% of plants of the plant kingdom. The main reason for their enormous success is the development of fruits as the reproductive organ containing the developing seeds. After fertilisation, the fruit nurtures, protects and mediates the efficient dispersal of seeds to ensure success of future generations. Fruits occur in a range of shapes and sizes, but common to all fruits is that they develop from structures called carpels in the centre of the flower that fuse to form a gynoecium.
Interestingly, the German poet and scientist Johann Wolfgang von Goethe hypothesised more than 200 years ago that all floral organs are in fact modified leaves. Modern genetics and molecular biology has confirmed Goethe's prediction, and revealed that also carpels are evolutionarily derived from leaves. The gynoecium is therefore formed from a simpler basic plan and modified over time to optimise its function as a reproductive organ. These modifications have for example allowed development of ovules in the ovary at the centre of the gynoecium that will be fertilised to develop seeds. They were also responsible for producing a structure at the tip of the gynoecium called stigma to facilitate pollination, and modifications furthermore resulted in the formation of a style just below the stigma to support the development of pollen tubes, which will guide the pollen to the ovules for efficient fertilisation. Whereas the ovary has medio-lateral symmetry reflecting the origin as two fused leaves, the style and stigma adopts radial symmetry. The modifications involved in creating the gynoecium from two leaves, therefore involved changing the polarity of tissues.
Mobile signals such as hormones are likely to coordinate growth of the different domains and structures in the gynoecium. Our recent work suggests that the plant hormone auxin has such a prominent role. In this proposal we will build on these preliminary data and take advantage of newly developed genetic and molecular resources, mathematical modelling as well DNA-deep-sequencing technology to:
1) understand how interaction between a known set of transcription factors and auxin activity regulate polarity of specific tissues.
2) identify pathways and key components regulated by the auxin/transcription factor module.
3) study how the auxin distribution pattern is established.
Through these studies we aim to provide a unified understanding of tissue polarity establishment during gynoecium development and to reveal the key importance of auxin in allowing the transition from vegetative leaves to a complex reproductive organ.

In the past 2-3 decades, developmental biologists have made tremendous progress in identifying genes required for the specification of individual cell types of an organ and in describing how they interact in genetic networks. In comparison, very little is known about the mechanisms that regulate tissue polarity and overall organ patterning.
The Arabidopsis gynoecium provides an excellent system to study organ patterning and tissue specification with its partition into distinct domains. Interactions among key regulators of Arabidopsis gynoecium and fruit development have revealed a network of upstream transcription factor activities required for this division. Regulation of the plant hormone auxin is emerging as an immediate downstream output from these activities, and here we aim to understand the spatiotemporal information that is defined through interactions between a set of transcription factors and auxin during the patterning process.
Using the Arabidopsis gynoecium as a model system, we will test the hypothesis that auxin is recruited by the upstream network to direct organ patterning and tissue polarity through its regulated distribution, thereby allowing the formation of a functionally reproductive structure. We will use established reporter lines to monitor auxin distribution throughout wild-type gynoecium development as well as in mutants with defects in polarity establishment. We will also take a global transcriptomic approach to identify additional components and pathways involved in gynoecium patterning. Finally, computational simulations will be used to demonstrate how auxin fluxes are established to ensure appropriate gynoecium patterning.
Through these studies, we aim to reveal the mechanism by which tissue polarity is established in the Arabidopsis gynoecium to ensure the formation of a functional reproductive structure.

Who will benefit from this research and how?
A striking difference between fruits from the model plant Arabidopsis and its close crop relative oilseed rape is the length of the apical style. Whereas the Arabidopsis style is short and barely noticeable with the naked eye, the style of an oilseed rape fruit makes up 20-25% of the entire fruit length. In addition to a waste of energy in producing this extended structure, long styles pose a serious problem for seed dispersal (pod shatter) as they often get entangled and rip the fruits open under windy conditions. Furthermore, precocious style emergence is a common problem for farmers of oilseed rape, where the style elongates prematurely and the stigma is separated from its own anthers. If this occurs during cold and humid conditions when natural pollinators such as insects are absent, these flowers will fail to produce seeds altogether. Attempts to reduce style development in oilseed rape therefore have great potential to minimise seed loss. The results expected from this proposal will point out directions for achieving this through regulation of auxin dynamics.

The agricultural industry: The industry will benefit from technology development to improve yield and yield predictability and to modify pod shape to minimise seed loss due to unsynchronised seed dispersal (pod shatter). The data obtained here may also point out directions for increasing pod size, seed size and seed number through alteration of hormone levels in specific tissues.

Public: The public would benefit from greater predictability of yields, through greater stability in production costs, which would impact on prices in the shops. There are also obvious environmental benefits using the technology described here. Oilseed rape has emerged as the second largest oilseed crop with an annual worldwide production of 38 million tons of oil and demand is increasing. For this to be sustainable, seed yield needs to be dramatically increased through more efficient breeding programmes while at the same time minimising the amount of fertiliser input in order to protect the environment. We believe that the data obtained here will set out strategies to optimise fertility and reduce dispersal, thus contributing significantly towards such a goal.

What will be done to ensure that they have the opportunity to benefit from this research?

Publications: Results will be published in high-impact scientific journals and the breeding/farming press in a timely fashion. It will also be presented at national and international conferences and trade shows.

Collaborations: The PI has strong connections to the international auxin research community. This is reflected in the access to the novel and unique resources described in Case for Support, part 2. We also have strong links to the breeding industry and Brassica crop improvement programmes. The data that we obtain will be of immediate use to these interest groups for example via the Defra-funded Oilseed Rape Genetic Improvement Network (OREGIN). This network brings together academic researchers and breeding companies to generate pre-breeding material, and have established a number of populations with the aim to improve traits with relevance to fruit morphology as described in this proposal.

Commercialisation: We are dedicated to promote the use of our results for crop improvement purposes. Informal contacts with industrialists, biotechnologists and related umbrella organisations will be made as soon as any exploitable results are generated. We have tight links with relevant industries and will present results to them either when they visit JIC, at joint meetings or when visiting the companies.