Priming of environmental responses in Arabidopsis thaliana by cytokinin

Principal Investigator: Prof. Dr. Thomas Schmülling

During phase I of CRC 973 we have shown that the developmental plant hormone cytokinin regulates in the r-selected organism Arabidopsis thaliana the responses to different stresses, in particular to high light, circadian and cold stress, and unravelled part of the signalling chains mediating these activities.
Analysis of cytokinin-related transcriptomic data and functional studies led to the identification of a novel cytokinin- and cold-regulated transcription factor gene of the AP2/ERF family named ERF105, which is required for priming the cold stress response in different settings. Further, cytokinin receptor mutant analysis has indicated that cytokinin is a positive regulator of the cold stress response, but suppresses primability, which is the ability of an organism to be primed for a better response to future stress. Thus primability appears to be a specific feature of the priming process being at least partly under hormonal control.

In phase II we aim to carry out further functional studies of the ERF105 gene and three evolutionary closely related and similarly regulated ERF genes in order to place these into the known context of the cold stress response. This work will include studies of loss- and gain-of-function mutants and investigation of upstream and downstream signalling processes, which includes in particular the identification of target genes. The kinetics of the priming process and involvement of cytokinin and the ERF105 signalling modules in preserving priming information (memory) will be investigated as well with a focus on epigenetic changes. Further, we will study the role of cytokinin in determining primability. Last but not least, we plan to establish quantitative analysis of shoot growth and seed yield following stress and the influence of priming as a readout of the plants' stress response. This should enable a more appropriate assessment of the hypothesis that the role of cytokinin in priming reflects its function in regulating the distribution of the plants' resources between growth and defence.

A second type of priming by cytokinin that has been studied during phase I is its role in priming seed germination, which is a very important event in the life history of a plant influencing fitness and population structure of future generations. We have analysed the phyA-mediated very low fluence response of germination, which is dramatically increased in seeds from plants with a low cytokinin status. Several genes of the cytokinin circuitry that are relevant for the altered germination phenotype have been identified including signalling and metabolism genes. Genetic analyses indicated that there is a significant contribution of the maternal genome regulating germination behaviour, most likely acting through changes occurring in the endosperm. Further we have shown that the environmental conditions of parental plants such as the red/far red wavelength ratio prime seed germination behaviour. This indicates that information is stored in the seed determining the response to a later triggering stimulus, which is light.

In phase II we will continue to explore the functional context of seed priming by cytokinin, in particular in relation to the action of other hormones (ABA, gibberellin) and PIF transcription factors. A more general aspect in phase II will be to explore how the endosperm acts in detecting, memorising and executing environmental information. It is hypothesised that transcriptional and epigenetic changes in the endosperm provide a mechanistic basis to store information during the memory phase. An important new direction is the use of genome-wide association mapping exploiting the available genotypic and phenotypic variation among Arabidopsis accessions to identify novel genes regulating the germination response.

References

• Brenner, W. and Schmülling, T. 2012. Transcript profiling of cytokinin action in Arabidopsis roots and shoots discovers organ-specific cytokinin responses. BMC Plant Biol. 12: 112.
• Brenner, W., Ramireddy, E., Heyl, A. and Schmülling, T. 2012. Gene regulation by cytokinin in Arabidopsis. Front Plant Sci. 3: 8.
• Brenner, W. and Schmülling, T. 2015. Summarizing and exploring data of a decade of cytokinin-related transcriptomics. Front. Plant Sci. 6: 29.
• Cortleven, A., Nitschke, S., Klaumünzer, M., AbdElgawad, H., Asard, H., Grimm, B., Lurz, R., Riefler, M. and Schmülling, T. 2014. A novel protective function for cytokinin in the light stress response is mediated through the AHK2 and AHK3 receptors. Plant Physiol. 164: 1470-1483.
• Heyl, A., Riefler, M., Romanov, G.A. and Schmülling, T. 2012. Properties, functions and evolution of cytokinin receptors. Eur. J. Cell Biol. 91: 246-256.
• Hilker, M., Schwachtje, J., Baier, M., Balazadeh, S., Bäurle, I., Geiselhardt, S., Hincha, D.K., Kunze, R., Mueller-Roeber, B., Rillig, M.C., Rolff, J., Romeis, T., Schmülling, T., Steppuhn, A., van Dongen, J., Withcomb, S.J., Wurst, S., Zuther, E. and Kopka, J. 2015. Priming and memory of stress responses in organisms lacking a nervous system. Biol. Rev. doi: 10.1111/brv.12215.
• Nishiyama, R., Watanabe, Y., Fujita, Y., Kojima, M., Werner, T., Yamaguchi-Shinozaki, K., Shinozaki, K., Kakimoto, T., Sakakibara, H., Schmülling, T. and Phan Tran, L.-S. 2011. Analysis of cytokinin metabolism during drought and salt stresses reveals important regulatory roles of cytokinins in drought, salt and ABA responses, and ABA biosynthesis. Plant Cell 23: 2169-2183.