Upon pathogen infection, plants activate an array of defense responses, including reprogramming the expression of thousands of genes. However, a major challenge in the field is to determine which genes regulate defense as opposed to being diagnostic for infection. In order to discover novel defense genes, we performed a high throughput genetic screen, based on the unique Arabidopsis mutant called accelerated cell death 6-1 (acd6-1). ACD6 encodes a membrane protein with ankyrin repeats and was shown to be a major determinant of fitness in Arabidopsis (Lu et al., 2003; Todesco et al.). acd6-1 is a gain-of-function mutant that demonstrates constitutive defense, severe cell death, and extreme dwarfism and accumulates high levels of salicylic acid (SA), the key defense signaling molecule (Rate et al., 1999; Lu et al., 2003). Interestingly, second site mutations disrupting SA accumulation and/or SA signaling suppress the small size of acd6-1, suggesting that acd6-1-conferred dwarfism is inversely correlated with its defense levels (Lu et al., 2009). Therefore, the size change of acd6-1 can be used as a convenient readout in a large-scale screen of acd6-1 suppressors (sups), which may harbor mutations disrupting important defense genes. In addition, we have also exploited this unique feature of acd6-1 in elucidating functional relationships among components in defense signaling networks (Ng et al., 2011).

Figure panel A: ACD6 protein structure and the acd6-1 mutant.

Figure panel B: the size of acd6-1 is sensitized to the change of defense levels. 

1. Lu, H., Rate, D.N., Song, J.T., and Greenberg, J.T. (2003). ACD6, a novel ankyrin protein, is a regulator and an effector of salicylic acid signaling in the Arabidopsis defense response. Plant Cell 15, 2408-2420.
2. Lu, H., Salimian, S., Gamelin, E., Wang, G., Fedorowski, J., LaCourse, W., and Greenberg, J.T. (2009). Genetic analysis of acd6-1 reveals complex defense networks and leads to identification of novel defense genes in Arabidopsis. Plant J 58, 401-412.
3. Ng, G., Seabolt, S., Zhang, C., Salimian, S., Watkins, T.A., and Lu, H. (2011). Genetic dissection of salicylic acid-mediated defense signaling networks in Arabidopsis. Genetics, in press.
4. Rate, D.N., Cuenca, J.V., Bowman, G.R., Guttman, D.S., and Greenberg, J.T. (1999). The gain-of-function Arabidopsis acd6 mutant reveals novel regulation and function of the salicylic acid signaling pathway in controlling cell death, defenses, and cell growth. Plant Cell 11, 1695-1708.
5. Todesco, M., Balasubramanian, S., Hu, T.T., Traw, M.B., Horton, M., Epple, P., Kuhns, C., Sureshkumar, S., Schwartz, C., Lanz, C., Laitinen, R.A., Huang, Y., Chory, J., Lipka, V., Borevitz, J.O., Dangl, J.L., Bergelson, J., Nordborg, M., and Weigel, D. (2010). Natural allelic variation underlying a major fitness trade-off in Arabidopsis thaliana. Nature 465, 632-636.