A recent study by CRAG researchers reveals that the female reproductive organs of plants, the pistils, maintain circadian rhythms autonomously and independently from the rest of the plant. In the study, led by CSIC researcher at CRAG Paloma Mas and published in the journal Developmental Cell, the authors have separated the different components of the flower and have observed that the pistils are the only floral organs capable of surviving and maintaining circadian rhythms in a very precise way.

Until now, most circadian studies have been done globally using whole plants. Instead, in this work, the development of a new in vivo culture method using laboratory plates has allowed to analyse the response of each floral organ separately. While the rhythms in petals, sepals, and the male reproductive organs (stamens) lose precision once separated from the flower, pistils are able to survive on their own and maintain highly precise rhythms. 

Circadian clock machinery 

Most living organisms, including humans and plants, have an internal biological clock that allows them to anticipate and adapt to environmental changes produced by the Earth's rotation every 24 hours. In plants, the circadian clock is crucial in establishing the timing of germination, growth, and flowering, among many other processes. Circadian rhythms are generated by a network of cellular proteins whose quantity and activity oscillate daily. The regulation of each of these proteins and the interactions between them is crucial to maintaining the accuracy of the clock.

In the present study, researchers have used genetically modified variants of the model plant Arabidopsis thaliana to decipher the exact components and the mechanism of molecular regulation of the pistil clock. Through comprehensive rhythmic analyses with the excised floral organs and subsequent genomic studies, the team has been able to identify the specific function of the clock proteins that confer precision and autonomy to the female reproductive organ.

The female organ of the plant has its own clock

Plants, like mammals, have a master circadian clock, but at the same time, each plant tissue and cell contain a circadian clock comprising all the necessary machinery to adapt its responses to the 24-hour cycle. The same research team already demonstrated in 2020 that the shoots and roots of plants are connected by a protein that moves between the two tissues to synchronize the clock.

The present work has unveiled that even when the pistils are separated from the rest of the plant, and therefore do not receive signals from any other organ or tissue, they are able to generate precise circadian rhythms autonomously. Thus, clear differences have been found between the pistils and the stamens, the female and male floral organs respectively, the latter being incapable of maintaining constant rhythms once separated from the flower. Circadian clock differences in reproductive organs are not unique to plants, as organ-specific rhythmic gene expression has also been described in mammalian ovaries.

The importance of circadian rhythms

The circadian rhythm plays a crucial role in all living beings, as it is responsible for generating the oscillations of biological processes in coordination with the day and night cycle and the associated changes in light and temperature. The Mas’s team demonstrated in 2018 that the circadian clock in plants controls the speed of the cell cycle, hence regulating the growth and division of cells in synchronization with the day and night cycles.

Studying the circadian clock in pistils isolated from the rest of the plant has allowed to determine its function in this organ. Thus, this work has elucidated that the pistil clock controls processes as important as responses to environmental signals, photosynthesis and seed production. Although the study was carried out with the model plant Arabidopsis thaliana, the research team is already thinking of transferring the findings to other crops of interest, such as tomato.

Detailed knowledge about how the circadian clock works in flowers may have biotechnological applications since, as principal researcher Paloma Mas points out, «these results open up interesting possibilities for improving reproduction and productivity in crops of agronomic relevance».

Reference article

Masaaki Okada, Zhiyuan Yang, Paloma Mas Circadian autonomy and rhythmic precision of the Arabidopsis female reproductive organ Developmental Cell, 16 Sept 2022 (DOI: 10.1016/j.devcel.2022.08.013)
*Article selected for the cover of the issue.

About the authors and funding of the study

The laboratory of the CSIC research professor at the CRAG Paloma Mas is financed from the Ministry of Science and Universities – State Research Agency, the Ramón Areces Foundation and the Generalitat de Catalonia (AGAUR). The team also acknowledges support to the Center of the CERCA/Generalitat de Catalunya Program, and the Ministry of Economy and Competitiveness through the Severo Ochoa Program for Centers of Excellence in R&D. Masaaki Okada was the recipient of a fellowship from the Severo Ochoa Postdoctoral Internationalization Program, and Zhiyuan Yang has received a fellowship from the Chinese Scholarship Council (CSC).