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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - RiceStyle (Carpel evolution: a walk in the rice side)

Teaser

Summary

Flowering plants (angiosperms) protect their female reproductive structure, the ovule, within a carpel. After the embryo sac is fertilized by the pollen tube, the ovule gradually turns into a seed, and the carpel becomes a fruit. The carpel is thus an evolutionary conserved structure, but showing an outstanding variability in shapes and function. Several transcription factors directing the main aspects of carpel formation and morphogenesis, are quite well characterized in the model eudicot plant Arabidopsis thaliana. More recent evo-devo studies suggest that some of these transcription factors are functionally conserved, at least in eudicot plants. However, this knowledge is far to be complete, especially if we consider the basal angiosperm taxa and monocots. This proposal aims to investigate more in depth these phenomena, using as main model species rice (Oryza sativa), and subsequently to do comparative studies in other evolutionary significant species. The final goal is to achieve a broader knowledge about the transcriptional machinery ruling carpel development, and its degree of functional conservation among the most relevant angiosperm taxa, by means of innovative techniques. The proposal will also allow us to address new evidence about the molecular pathways acting downstream this transcriptional network. Currently, we have already demonstrated the importance of several candidate genes in determining the identity and development of the rice carpel and of each part that compose it, ovary, style and stigma thus showing a significant conservation of the proposed molecular network among flowering plants, and confirming its ancestral common origin.

Work performed

With approximately 352,000 living species on Earth, the flowering plants or angiosperms constitute the largest and most diverse extant group of the plant kingdom, and about 90% of land plant species. The rapid appearance and diversification of angiosperms which is documented by fossils have been defined as â€˜abominable mysteryâ€™ by Charles Darwin. Certainly, the evolution of new reproductive structures like the flower and its female part, the pistil, had a major role in the extraordinary evolutive success of angiosperms. How these structures evolved, also at molecular level, is however still unknown. In addition, after pollination the pistil and the ovule(s) within develop into fruit and seed(s) respectively, which are the edible part of most crops. Therefore, molecular studies on pistil development can both help clarify one of the bigger â€˜secretsâ€™ of plant evolution, and bring significant advantages and tools for targeted crop improvement. Despite information about genetic control of pistil identity and patterning is already available, these data mainly pertain to Arabidopsis and few other eudicot model plants. Therefore, these genetic mechanisms could either explain the evolution of the pistil in the earliest ancestors of all flowering plants, or represent more recent novelties reflecting the adaptation of eudicots. Therefore, comparative studies are needed in the other largest and most evolved group of flowering plants, the monocots, and also in basal angiosperms. This Marie Curie action â€˜Ricestyleâ€™ aims to clarify part of the â€˜abominable mysteryâ€™ by the comparative study of pistil development and its genetic basis in rice, which is by far the main monocot model plant and, together the other cereals, the main source of calories for human diet.

During the first phase of the project, in the Host Laboratory of Prof. Dabing Zhang (Shanghai Jiao Tong University, China) we have developedall the tools necessary for the functional characterization of the candidate genes which might be involved in pistil tissue identity and patterning. Based on the homology with genes known in Arabidopsis thaliana and other eudicot model plants, we have search in the rice genome for phylogenetically related and functionally equivalent genes. Our work has yielded a complete study of the evolutionary history of these gene families that will be the base for our functional studies.

In addition, the collaboration between the two Host Labs allowed the characterization of three other transcription factors belonging to the SEPALLATA MADS-box subfamily. These factors confer the identity of all the floral organs including the pistil, thus they are master regulators acting upstream also of the moleculart network for carpel development. The work has been published recently on Plant Physiology (Wu D, Liang W, Zhu W, Chen M, Ferrandiz C, Burton RA, Dreni L, Zhang D. Plant Physiol. 2017 Dec 7. pii: pp.00704.2017. doi: 10.1104/pp.17.00704).
Moreover, we have isolated a gene encoding for a glutaredoxin which is important for the pistil development in rice. Loss of function mutants cannot produce pistils in about half of the spikelet, otherwise produce reduced pistils. These results are somehow unexpected and may represent a monocot evolutionary innovation or, otherwise, be also important in dicots, only unexplored so far. Thus,they may provide a two-way pathway to improve our knowledge in pistil morphogenesis in angiosperms. A first paper describing the function of this glutaredoxin and its genetic interaction with floral and carpel identity genes will be submitted soon.

In the second phase of the project we have continued the phenotyping and the characterization of the above mentioned mutants, and also created higher order mutants. The data from these experiments indicated a number of genes with essential function in determining the correct carpel morphology in rice. We are deeply characterizing the molecular networks regulated by these genes and

Final results

Major conclusions and advances are expected towards the end of the second phase. We can say that progress beyond of the art is significant, mainly around the undescribed role of some key genes in rice carpel development, confirming one of our initial hypothesis. In addition, the work on SEP genes and glutaredoxin has uncovered new interactions and opened new ideas to incorporate in our model of pistil development.

At the end of the project, we plan to perform some additional experiments, and incorporate the results in a new model for pistil development in monocots that until now has been poorly addressed. 2-3 more publications in high impact journals are expected. Understanding how pistils and therefore grains form and develop in monocots may help to increase yield in cereal crops.

The impact on the fellow\'s career has been significant. It has already produced one publication and other in preparation, it has provided a extensive and solid network of collaborations in Asia and Australia, with access to resources of importance, and it has endowed him with an array of leadership abilities. All these has allowed the fellow to apply for several tenure-track positions in Spain, like the programa Ramon y Cajal, and more.

The main contribution to the host lab and the institute is the implementation of rice research in developmental genetics, a long sougth goal of the group. Since Valencia is a rice-growing region, socio-economic implications might be important also in the development of new links to growers and breeders of local varieties.