Redundancy in Plant Defense Figure 7. Basal resistance in eds1 sid2 plants. (A) Growth of virulent P. syrinage on indicated genotypes. The error bars indicate SD. Asterisks indicate data statistically significant from wt (Col-0 or Ws) (P,0.05, n = 4). The eds1-1 and eds1-22 are in Ws and Col-0 ecotypic backgrounds, respectively. (B) ELISA showing levels of TCV CP in the inoculated leaves of indicated genotypes at 3 dpi. The error bars indicate SD (n = 4). doi:10.1371/journal.pgen.1000545.g007 feedback loop and EDS1 is widely considered an upstream effector of SA [16,19,23,45]. Recent data has shown that EDS1 signals resistance via both SA-dependent as well as SA-independent pathways [46]. Strikingly, EDS1-dependent but SA-independent branch of EDS1 pathway still requires SA pathway for full expression of resistance [46]. In this study, we have characterized the relationship between EDS1 and SA. We show that the two components act in a redundant, and not necessarily sequential manner to regulate R gene expression induced in response to a reduction in the levels of the FA 18:1. Furthermore, EDS1 and SA also function redundantly in R gene-mediated defense against viral, bacterial and oomycete pathogens. It appears that the redundant functions of EDS1 and SA may have prevented their identification as required components for signaling mediated by CC-NBS-LRR R proteins. Indeed, RPS2-mediated signaling is fully compromised only in eds1 sid2 and not in the single mutant PLoS Genetics | www.plosgenetics.org 11 July 2009 | Volume 5 | Issue 7 | e1000545