News and Publications

Coding and Decoding Calcium Signals in Plants

J. Harper, S. Romanowsky, M. Gehl, L. Poulsen, W.S. Chung, C. Dammann, I. Baxter, J. Frietsch

Calcium signals have been implicated in the regulation of all aspects of plant growth and development, including responses to cold, heat, salt, drought, and pathogens. We are investigating the mechanism by which calcium signals are coded and decoded in Arabidopsis, the first plant to have its genome sequenced. We use disruptions in genes to delineate the functions of more than 100 genes involved in creating and sensing calcium signals, including calcium pumps, glutamate receptor-like channels, cyclic nucleotide-gated channels, calcium-dependent protein kinases, and Snf1-related kinases. Our long-term goal is to understand the structure and biological function of the calcium-signaling machinery in plants. From the perspective of animal research, plants provide an insightful comparison for identifying common and unique features of signal transduction that arose during the evolution of multicellular organisms.

A specific area of focus is a unique family of calcium sensors called calcium-dependent protein kinases (CDPKs). This kinase family has been detected only in plants and protists, and therefore the enzymes are potential targets for new herbicides or antiprotozoan drugs. CDPKs are defined by a unique structure: they contain, in a single polypeptide, both a kinase domain and a calmodulin-like regulatory domain. This "fused" structural arrangement makes CDPKs distinct from the calcium-regulated protein kinases found in animal systems. Evidence indicates that CDPKs are activated by intramolecular binding between their calmodulin-like domain and an autoinhibitory domain. Arabidopsis has 34 CDPK genes.

A second area of focus is calcium pumps. These pumps remove from the cytoplasm the calcium that enters through calcium channels. Arabidopsis has 14 calcium pumps, 10 of which appear to be regulated by calmodulin. Whereas animals have a calmodulin-regulated pump at the plasma membrane, in plants, homologs of these pumps are found in multiple locations, including the endoplasmic reticulum, vacuoles, and chloroplasts.

To further understand the downstream effects of calcium signals, we are conducting global mRNA expression profiles under 3 stress conditions known to trigger calcium signals: drought, salt, and cold. We found 2600 genes that are regulated by these conditions. We plan to further investigate how calcium signals are decoded into a stimulus-specific response, allowing plants to survive in the extremes of the desert heat or the arctic cold. We anticipate that our genetic and biochemical studies will contribute to our ability to engineer crop plants to produce more nutritional foods, medicine, and materials to meet the needs of the world's growing population.

PUBLICATIONS

Chen, W., Provart, N.J., Glazebrook, J., Katagiri, F., Chang, H.S., Eulgem, T., Mauch, F., Luan, S., Zou, G., Whitham, S.A., Budworth, P.R., Tao, Y., Xie, Z., Chen, X., Lam, S., Kreps, J.A., Harper, J.F., Si-Ammour, A., Mauch-Mani, B., Heinlein, M., Kobayashi, K., Hohn, T., Dangl, J.L., Wang, X., Zhu, T. Expression profile matrix of Arabidopsis transcription factor genes suggests their putative functions in response to environmental stresses. Plant Cell 14:559, 2002.

Christodoulo, J., Hu, H., Chung, J., Rance, M., Harper, J.F., Chazin, W.J. ¹H, ¹⁵N and ¹³C assignments of the regulatory domains of calcium-dependent protein kinase (CDPK). J. Biomol. NMR 23:249, 2002.

Gribskov, M., Fana, F., Harper, J., Hope, D.A., Harmon, A.C., Smith, D.W., Tax, F.E., Zhang, G. PlantsP: a functional genomics database for plant phosphorylation. Nucleic Acids Res. 29:111, 2001.

Harmon, A.C., Gribskov, M., Gubrium, E., Harper, J.F. The CDPK superfamily of protein kinases. New Phytol. 151:175, 2001.

Kreps, J., Wu, Y., Wang, X., Zhu, T., Harper, J.F. Transcriptome changes for Arabidopsis in response to salt, osmotic and cold stress. Plant Physiol., in press.

Sanders, D., Pelloux, J., Brownlee, C., Harper, J.F. Calcium at the crossroads of signaling. Plant Cell 14(Suppl.):S401, 2002.

Veeraraghavan, S., Fagan, P.A., Hu, H., Lee, V., Harper, J.F., Huang, B., Chazin, W.J. Structural independence of the two EF-hand domains of caltractin. J. Biol. Chem. 28:28564, 2002.

Wu, Z., Liang, F., Hong, B., Young, J.C., Sussman, M.R., Harper, J.F., Sze, H. An endoplasmic reticulum-bound Ca²⁺/Mn²⁺ pump, ECA1, supports plant growth and confers tolerance to Mn²⁺ stress. Plant Physiol. 130:128, 2002.