Summary Ralstonia solanacearum, the causal agent of bacterial wilt disease, is considered one of the most destructive bacterial pathogens due to its lethality, unusually wide host range, persistence and broad geographical distribution. In spite of the extensive research on plant immunity over the last years, the perception of molecular patterns from R. solanacearum that activate immunity in plants is still poorly understood, which hinders the development of strategies to generate resistance against bacterial wilt disease. The perception of a conserved peptide of bacterial flagellin, flg22, is regarded as paradigm of plant perception of invading bacteria; however, no elicitor activity has been detected for R. solanacearum flg22. Recent reports have shown that other epitopes from flagellin are able to elicit immune responses in specific species from the Solanaceae family, yet our results show that these plants do not perceive any epitope from R. solanacearum flagellin. Searching for elicitor peptides from R. solanacearum, we found several protein sequences similar to the consensus of the elicitor peptide csp22, reported to elicit immunity in specific Solanaceae plants. A R. solanacearum csp22 peptide (csp22Rsol) was indeed able to trigger immune responses in Nicotiana benthamiana and tomato, but not in Arabidopsis thaliana. Additionally, csp22Rsol treatment conferred increased resistance to R. solanacearum in tomato. Transgenic A. thaliana plants expressing the tomato csp22 receptor (SlCORE) gained the ability to respond to csp22Rsol and became more resistant to R. solanacearum infection. Our results shed light on the mechanisms for perception of R. solanacearum by plants, paving the way for improving current approaches to generate resistance against R. solanacearum.

Photodamage of the PSII reaction center (RC) is an inevitable process in an oxygen-rich environment. The damaged PSII RC proteins (Dam-PSII) undergo degradation via the thylakoid membrane-bound FtsH metalloprotease, followed by posttranslational assembly of PSII. While the effect of Dam-PSII on gene regulation is described for cyanobacteria, its role in land plants is largely unknown. In this study, we reveal an intriguing retrograde signaling pathway by using the Arabidopsis (Arabidopsis thaliana) yellow variegated2-9 mutant, which expresses a mutated FtsH2 (FtsH2 G267D) metalloprotease, specifically impairing its substrate-unfolding activity. This lesion leads to the perturbation of PSII protein homeostasis (proteostasis) and the accumulation of Dam-PSII. Subsequently, this results in an up-regulation of salicylic acid (SA)-responsive genes, which is abrogated by inactivation of either an SA transporter in the chloroplast envelope membrane or extraplastidic SA signaling components as well as by removal of SA. These results suggest that the stress hormone SA, which is mainly synthesized via the chloroplast isochorismate pathway in response to the impaired PSII proteostasis, mediates the retrograde signaling. These findings reinforce the emerging view of chloroplast function toward plant stress responses and suggest SA as a potential plastid factor mediating retrograde signaling. Chloroplast-generated reactive oxygen species (ROS) damage primarily the photosynthetic apparatus, thereby deleteriously affecting plant development. Among ROS, singlet oxygen (1 O 2), mainly generated by PSII, has been considered as a prime cause of PSII damage because of its proximity to PSII (Triantaphylidès et al., 2008; Kato and Sakamoto, 2009). Since 1 O 2 is a known by-product of photosynthesis, PSII reaction center (PSII RC) proteins undergo constant damage in a light-dependent manner (Ohad et al., 1984; Mishra et al., 1994). The damaged PSII (Dam-PSII) becomes repaired via a series of steps: (1) D1 oxidation and subsequent phosphorylation; (2) migration of the Dam-PSII from the grana core (appressed membranes) to the grana margin (nonappressed membranes); (3) dephosphorylation of D1; (4) degradation of the damaged D1 protein by the membrane-bound FtsH metalloprotease; (5) de novo synthesis of D1; and (6) reassembly and subsequent remigration to the grana core (Yamamoto et al., 2013; Yoshioka-Nishimura and Yamamoto, 2014). Accordingly, the impaired PSII repair in Arabidopsis (Arabidopsis thaliana) mutants lacking the membrane-bound FtsH protease leads to the failure of acclimation to a sublethal intensity of