**1. Introduction**

Plants combat pathogens by activating their innate immunity. Microbe/pathogen-associated molecular patterns (MAMPs/PAMPs), which have molecular structures that are conserved in fungi or bacteria, alert plants to pathogen attacks. Chitin (a backbone of fungal cell walls), peptidoglycan (a component of bacterial cell walls), lipopolysaccharide (LPS; a component of the outer membranes of Gram-negative bacteria), flagellin, and elongation factor-Tu (EF-Tu) are well-known MAMPs [1]. MAMPs are perceived by corresponding pattern-recognition receptors (PRRs) on host cell surfaces [2]. In rice (*Oryza sativa*), CHITIN ELICITOR RECEPTOR KINASE 1 (OsCERK1), a lysine motif (LysM)-receptor-like kinase (RLK), is well-characterized as a protein component of PRRs. OsCERK1 interacts with a receptor-like protein CHITIN ELICITOR BINDING PROTEIN (CEBiP) [3] to recognize the chitin oligomer [4–6] and with rice LYSM-CONTAINING PROTEIN 4/6 (OsLYP4/6) to recognize peptidoglycan [7]. OsCERK1 also functions as a receptor/co-receptor for LPS signaling [8]. Thus, the

central RLKs, like OsCERK1, function as hub receptors. PRR complexes activate pattern-triggered immunity (PTI) through intracellular signaling pathways. Protein phosphorylation signals originating in PRRs positively regulate early phase responses, such as oxidative burst and the activation of mitogen-activated protein kinase (MAPK) cascades, followed by the transcriptional activation of defense-related genes [9,10]. Oxidative burst is caused by a rapid production of reactive oxygen species (ROS) by plant NADPH oxidases, called RESPIRATORY BURST OXIDASE HOMOLOG (RBOH) proteins [11]. Host-derived ROS play various roles in PTI. They possess an antimicrobial activity that can kill microbes, and they enhance physical barrier production by promoting lignin synthesis and cross-linking of plant cell walls [12–15]. Hydrogen peroxide (H2O2), an ROS produced in oxidative bursts, acts as a signaling molecule to induce the transcriptional activation of defense-related genes, biosynthesis of phytoalexin, and programmed cell death [11,16].

Pathogens have adapted to suppress PTI through the secretion of effectors using type III secretion system (TTSS) and structural variations on MAMPs [17]. Molina and Kahmann (2007) reported that the detoxification of host-derived ROS is required for a biotrophic pathogen of maize *Ustilago maydis* to overcome PTI [18]. The deletion of *YAP*, encoding an oxidative stress-responsive transcription factor, in *U. maydis* increases the sensitivity to H2O2 and significantly decreases the pathogenicity. Rice blast fungus (*Pyricularia oryzae*), which causes a serious disease in rice, releases a catalase-peroxidase B (CPXB)-dependent ROS-degrading activity near conidia [19,20]. Enzymes that compose the glutathione and thioredoxin antioxidation system in *P. oryzae* are required for virulence as well as resistance to ROS [21,22]. *P. oryzae* mutant strains Δ*des1* and Δ*sir2*, which lack transcriptional regulators for extracellular peroxidases and superoxide dismutase, cannot form susceptible lesions because they induce defense responses, including the accumulation of host-derived ROS and the upregulation of defense-related genes [23,24]. These findings indicate that host-derived ROS is a crucial factor in host–microbe interactions.

Receptor-like cytoplasmic kinases (RLCKs) contribute to cytoplasmic phosphorylation signaling pathways in PTI. RLCKs are characterized as cytoplasmic proteins that contain a RLK-homologous kinase domain but not a transmembrane domain. *Arabidopsis thaliana* and rice have 147 and 379 RLCK-encoding genes, respectively [25,26]. RLCKs are classified into 17 subfamilies based on their sequence features. Several RLCKs belonging to subfamily-VII are involved in PTI [27]. In *A. thaliana*, BOTRYTIS-INDUCED KINASE 1 (BIK1) is phosphorylated by PRRs composed of BRI1-ASSOCIATED KINASE 1 (BAK1) and FLAGELLIN SENSING 2 (FLS2) or EF-Tu RECEPTOR (EFR) depending on the ligand (flagellin or EF-Tu, respectively)-binding [28,29]. Phosphorylated BIK1 further phosphorylates and activates RBOHD, which is responsible for the oxidative bursts in PTI [30,31]. In rice, OsRLCK176, an ortholog of BIK1, interacts with OsCERK1 to mediate chitin- and peptidoglycan-induced defense responses [7]. OsRLCK57, OsRLCK107, and OsRLCK118, which are highly homologous to OsRLCK176, also have similar functions [32]. OsRLCK118 directly phosphorylates OsRBOHB [33]. OsRLCK185 interacts with OsCERK1 and MAPKKK, connecting PRRs and the MAPK cascade [34,35].

BROAD-SPECTRUM RESISTANCE 1 (BSR1; OsRLCK278), a rice RLCK-VII protein, has unique disease control effects when overexpressed. It was identified in a screen for disease resistance in rice Full-length cDNA OvereXpressor (FOX) *Arabidopsis*lines [36,37]. The screening revealed that transgenic *A. thaliana* plants overexpressing BSR1 were highly resistant to *Pseudomonas syringae* pv. *tomato* DC3000 and *Colletotrichum higginsianum*. Furthermore, overexpression of BSR1 in rice conferred strong resistance against four rice pathogens: rice blast fungus, brown spot fungus (*Cochliobolus miyabeanus*), rice leaf blight bacteria (*Xanthomonas oryzae* pv. *oryzae*), and *Burkholderia glumae*, which is the causal agent of bacterial seedling rot and bacterial grain rot [36,38]. To our knowledge, among the many RLCKs, BSR1 is the only one that can enhance disease resistance when overexpressed. However, the mechanism underlying the broad-spectrum disease resistance conferred by the overexpression of BSR1 remains unknown.

The contribution of BSR1 to the innate immunity of wild-type rice has been analyzed. A knockout of *BSR1* caused significant suppression of chitin-induced defense-responses, including oxidative bursts and the transcriptional activation of defense-related genes [39]. BSR1 has an active protein kinase domain that phosphorylates serine/threonine and tyrosine residues [40]. These indicate that BSR1 should mediate the downstream phosphorylation signaling of OsCERK1, because the perception of chitin completely depends on OsCERK1 [4]. The silencing of *BSR1* decreased resistance to not only fungal but also bacterial diseases [40], suggesting that BSR1 is involved in the signaling pathway activated by bacterial MAMPs downstream of OsCERK1.

In this report, we investigated whether BSR1 contributes to defense responses elicited by bacterial MAMPs. The resulting resistance is almost independent of salicylic acid, a plant hormone related to immunity [40]. Therefore, we focused on the early phase of defense responses, like the oxidative bursts. Furthermore, to reveal the mechanisms underlying broad-spectrum disease resistance in the BSR1-overexpressing rice plants, we analyzed the early defense events using suspension-cultured cells and sliced leaf blades overexpressing BSR1.
