MLO genes and reproduction
NORTIA is a member of the MLO (Mildew Resistance Locus O) family of plant proteins that was originally discovered in the context of pathogen susceptibility in barley (Buschges et al., 1997). MLO proteins have seven predicted transmembrane domains and all members of the family have a calmodulin-binding domain in the C-terminal tail, suggesting that calcium signaling may be involved in MLO function (Devoto et al., 1999; Kim et al., 2002). Wild-type versions of MLO genes are necessary for powdery mildew (PM) infection in both monocots and dicots, while mutations in these genes lead to resistance (Elliott et al., 2002; Consonni et al., 2006; Bai et al., 2008). Arabidopsis has fifteen MLO family members (Chen et al., 2006), but functions have been identified for only six. AtMLO2, AtMLO6, and AtMLO12 are involved in PM infection (Consonni et al., 2006), AtMLO4 and AtMLO11 are implicated in touch-induced root tropism (Chen et al., 2009), and NTA (AtMLO7) has a specific role in PT reception (Kessler et al., 2010). These data indicate that MLOs help cells respond to external stimuli, however little is known about their biochemical functions in signaling pathways. Barley MLO has been shown to bind calmodulin (Kim et al., 2002), to form homodimers (Elliott et al., 2005) and related MLOs are predicted to be able to form heterodimers (Chen et al., 2009), but interactions with other proteins have not been studied.
Several of the Arabidopsis MLO genes are expressed in reproductive tissues. We are currently investigating the potential roles of these genes in reproduction.
Bai, Y., Pavan, S., Zheng, Z., Zappel, N.F., Reinstadler, A., Lotti, C., De Giovanni, C., Ricciardi, L., Lindhout, P., Visser, R., Theres, K., and Panstruga, R. (2008). Naturally occurring broad-spectrum powdery mildew resistance in a Central American tomato accession is caused by loss of mlo function. Mol Plant Microbe Interact 21, 30-39.
Buschges, R., Hollricher, K., Panstruga, R., Simons, G., Wolter, M., Frijters, A., van Daelen, R., van der Lee, T., Diergaarde, P., Groenendijk, J., Topsch, S., Vos, P., Salamini, F., and Schulze-Lefert, P. (1997). The barley Mlo gene: a novel control element of plant pathogen resistance. Cell 88, 695-705.
Chen, Z., Hartmann, H.A., Wu, M.J., Friedman, E.J., Chen, J.G., Pulley, M., Schulze-Lefert, P., Panstruga, R., and Jones, A.M. (2006). Expression analysis of the AtMLO gene family encoding plant-specific seven-transmembrane domain proteins. Plant Mol Biol 60, 583-597.
Chen, Z., Noir, S., Kwaaitaal, M., Hartmann, H.A., Wu, M.J., Mudgil, Y., Sukumar, P., Muday, G., Panstruga, R., and Jones, A.M.s. (2009). Two Seven-Transmembrane Domain MILDEW RESISTANCE LOCUS O Proteins Cofunction in Arabidopsis Root Thigmomorphogenesis. Plant Cell.
Consonni, C., Humphry, M.E., Hartmann, H.A., Livaja, M., Durner, J., Westphal, L., Vogel, J., Lipka, V., Kemmerling, B., Schulze-Lefert, P., Somerville, S.C., and Panstruga, R. (2006). Conserved requirement for a plant host cell protein in powdery mildew pathogenesis. Nat Genet 38, 716-720.
Devoto, A., Piffanelli, P., Nilsson, I., Wallin, E., Panstruga, R., von Heijne, G., and Schulze-Lefert, P. (1999). Topology, subcellular localization, and sequence diversity of the Mlo family in plants. J Biol Chem 274, 34993-35004.
Elliott, C., Zhou, F., Spielmeyer, W., Panstruga, R., and Schulze-Lefert, P. (2002). Functional conservation of wheat and rice Mlo orthologs in defense modulation to the powdery mildew fungus. Mol Plant Microbe Interact 15, 1069-1077.
Elliott, C., Muller, J., Miklis, M., Bhat, R.A., Schulze-Lefert, P., and Panstruga, R. (2005). Conserved extracellular cysteine residues and cytoplasmic loop-loop interplay are required for functionality of the heptahelical MLO protein. Biochem J 385, 243-254.
Kessler, S.A., Shimosato-Asano, H., Keinath, N.F., Wuest, S.E., Ingram, G., Panstruga, R., and Grossniklaus, U. (2010). Conserved molecular components for pollen tube reception and fungal invasion. Science 330, 968-971.
Kim, M.C., Panstruga, R., Elliott, C., Muller, J., Devoto, A., Yoon, H.W., Park, H.C., Cho, M.J., and Schulze-Lefert, P. (2002). Calmodulin interacts with MLO protein to regulate defence against mildew in barley. Nature 416, 447-451.
Predicted structure of MLO proteins